polymer networks containing reversibly associating side-groups · 2015. 12. 11. · the author was...
TRANSCRIPT
Polymer Networks Containing
Reversibly Associating Side-Groups
by
Jiahui Li
Submitted in Partial Fulfillment
of the
Requirements for the Degree
Doctor of Philosophy
Supervised by
Professor Mitchell Anthamatten
Department of Chemical Engineering Art, Sciences and Engineering
Edmund A. Hajim School of Engineering and Applied Sciences
University of Rochester Rochester, New York
2011
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Dedication
To my wife Yuxiu and our daughter Shangqing
For their love and support
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Curriculum Vitae
The author was born in Jimo, Shandong Province, China on January 26,
1978. In the Fall of 1995, he attended Beijing University of Aeronautics and
Astronautics and graduated with a Bachelor of Engineering degree in Polymer
Materials and Composites in 1999. He then went to Dalian University of
Technology and graduated with a Master of Science degree in Organic Chemistry
in 2002, thesis titled ―Study of 9,9-bis (methomethyl) fluorene synthesis‖. After
working at Yantai Wanhua Polyurethane Inc. for a year and half, the author came to
the United States to continue his education. In August 2004, he was admitted by
Louisiana State University with a teaching assistantship in the Department of
Chemistry. During his stay in LSU, he was working with Professor Evgueni E.
Nesterov on molecularly imprinted fluorescent conjugated polymer materials. In
January 2006, the author continued to pursue a Ph.D. Degree in the Department of
Chemical Engineering at the University of Rochester, under the supervision of
Professor Mitchell Anthamatten. His study of focus is polymer networks containing
reversibly associating side-groups. In 2006, he received A Technology Innovation
Award from both China Petroleum and Chemical Industry Federation and Liaoning
Department of Science and Technology because of his work at Dalian University of
Technology. From 2009 to the present, he received a Horton Fellowship from
Laboratory for Laser Energetics, at the University of Rochester.
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List of publications and patents
1. J. Li, C. L. Lewis, D. L. Chen, M. Anthamatten, Dynamic Mechanical Behavior
of Photo-crosslinked Shape-Memory Elastomers, Macromolecules, submitted.
2. J. Li, K. D. Sullivan, E. B. Brown, M. Anthamatten, Thermally Activated
Diffusion in Reversibly Associating Polymers, Soft Matter, 2010, 6, 235-238.
3. J. Li, J. A. Viveros, M. H. Wrue, M. Anthamatten, Shape Memory Effects in
Polymer Networks Containing Reversibly Associating Side-Groups, Advanced
Materials, 2007, 19, 2851-2855.
4. J. Li, C. E. Kendig, E. E. Nesterov, Chemosensory Performance of Molecularly
Imprinted Fluorescent Conjugated Polymer Materials. J. Am. Chem. Soc., 2007,
129, 15911-15918.
5. Z. Gao, J. Li et al., Preparation and Properties of 9,9-bis (methomethyl) fluorene,
Journal of Dalian University of Technology, 2007, 47, 639-642.
6. J. Li, Z. Gao, Diethers Used in Ziegler-Natta Catalysts and Their Synthesis
Research, Liaoning Chemical Industry, 2002, 31, 392-395.
7. M. Anthamatten, J. Li, Shape Memory Polymers, US Patent, US 7935131 B2,
May 3, 2011.
8. M. Anthamatten, J. Li et al., Use of Shape Memory Polymers for Implant
Devices, US Patent, S/N 61/170,604, Apr. 2009.
9. Z. Gao, Q. Dong, J. Li, Three-Phase Phase-transfer Catalytic Synthesis Process
of 9,9-bis (methomethyl) Fluorene, China Patent, No. 1336359, 2002.
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Acknowledgements
First of all, I would like to thank my advisor, Professor Mitchell
Anthamatten. I always felt so lucky to be his student and to pursue my Ph.D.
degree under his supervision. Throughout the past five years, I have learned a lot
from Professor Anthamatten not only for doing scientific research, but also for
being a better person. His broad knowledge in polymer science is always a reliable
source for me. His commitment in his own work often gives me inspiration to
perfect my research. Without the support from Professor Mitchell Anthamatten,
this thesis would not be a reality.
I also would like to express my thanks to Professor David Harding and
Professor Edward Brown for serving on my thesis committee. Professor Harding
supports me doing the thermal mechanical studies at LLE, and he always is the
person I would ask for help if I have any problem. Professor Brown teaches me
operating FRAP microscope, helps me on interpreting a lot FRAP data which
makes the diffusion study possible.
Throughout my five years of study in the University of Rochester, I have
been very fortunate to work with many talented fellow graduate students in our
research group. I want to thank Xichong Chen, Michelle Wrue, Lijun Zou,
Supacharee Roddecha, Zachary Green, Alexander Papastrat, Kavya Ramachandra,
Christopher Lewis and Ran Tao. They are always supportive to me in my research.
I want to express special thanks to Chris. Since Chris became my lab mate in 2009,
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he helps me a lot in my research project. We often work together and have a lot
helpful discussions. I also want to thank Chris for having all kinds of conversations
with me. It is a lot of fun in the middle of research work. Undergraduate students,
Chi (Suze) Ninh, Derek Smith, Andrew Hilmer, Helen Park and James Viveros all
helped me at different stages of my research. I want to express my thanks to them.
I want to thank Kelley Sullivan in Professor Brown’s lab for helping me
trouble shooting FRAP every time I run into problem. Thank Ku-Hsien Wei,
Lichang Zeng and Yong-Hsin Li in Professor Chen’s lab for helping me running
DSC and using Laser irradiation apparatus. My thanks also go to Dongxia Liu, Xue
Wei and Sherry Tsai in Professor Yates’ lab for assisting me running UV-vis
instrument.
I would also express my thanks to all of my friends for their friendship and
support. Finally, I would like to thank my wife Yuxiu and our daughter Shangqing
for their love and my parents Guishu Li, Chunling Yu, and my in-laws Chengyi
Liu, Caimei Sui for their long time support.
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Abstract
Supramolecular polymers consist of low molar mass subunits that non-
covalently bind together through hydrogen bonding or other non-covalent
interactions, forming macromolecular assemblies. Site-specific and reversible
hydrogen bonds and other non-covalent interactions are increasingly employed to
modify bulk polymer properties, enabling thermoplastic elastomers and self-healing
polymers. In this thesis, I investigate how hydrogen bonding groups directly
bonded onto an elastic polymer network affect material properties. A lightly
crosslinked covalent network containing hydrogen bonding side-groups
(ureidopyrimidinone, UPy) was synthesized. This architecture results in a novel
shape-memory effect, and the molecular events resulting in this behavior were
deduced. Further, to systematically evaluate how thermomechanical properties are
related to network architecture, a new photo-crosslinking route was developed to
prepare shape-memory elastomers. This method enables melt-processing of shape-
memory elastomers into complex permanent shapes, and samples can be prepared
with much higher UPy-content. Furthermore, the covalent and non-covalent
crosslink density can be accurately controlled. Dynamic mechanical analysis on
photo-crosslinked shape-memory elastomers revealed that dynamic crosslinks
behave nearly as effectively as permanent crosslinks below the UPy hydrogen bond
transition. Compared to linear polymers bearing identical hydrogen bonding
groups, the synthesized dynamic networks exhibit an enhanced temperature
dependence of mechanical properties. This indicates that the covalent network
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supports cooperative hydrogen bonding. This finding will guide researchers to
more effectively employ non-covalent interactions within bulk polymer materials.
Mass transport through dynamic networks was also studied using multi-photon
fluorescence recovery after photobleaching (MP-FRAP). In contrast to viscous
relaxation, small molecule mass transport through the dynamic networks is limited
by the density of hydrogen bonds instead of their exchange rate.
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Table of Contents
Foreword ............................................................................................................................ 1
Chapter 1: Introduction & Background ............................................................................. 3
1.1 Supramolecular polymers and supramolecular interactions in polymers ........... 3
1.2 Different type of supramolecular polymer systems .............................................. 5
Ionic interaction based supramolecular polymers ............................................... 8
Metal-ligand complex based supramolecular polymers ................................... 11
π-π stacking interaction based supramolecular polymers ................................. 13
Other supramolecular polymer systems ............................................................ 14
1.3 Hydrogen bonding in supramolecular polymers ................................................ 15
Thermal reversible polymers ............................................................................. 18
Thermoplastic elastomers .................................................................................. 19
Self healing materials ......................................................................................... 20
1.4 Some intellectual challenges in hydrogen bonding supramolecular polymer systems ........................................................................................................ 21
1.5 Objectives and overview of the thesis ................................................................. 24
References .................................................................................................................. 27
Chapter 2 Shape-Memory Effects in Polymer Networks Containing Reversibly Associating Side-Groups ........................................................................ 34
2.1 Introduction .......................................................................................................... 34
2.2 Experimental methods ......................................................................................... 38
Materials ............................................................................................................. 38
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Monomer and shape-memory polymer synthesis ............................................. 38
Thermal mechanical analysis experiments ........................................................ 39
2.3 Discussion ............................................................................................................ 40
Shape-memory effects studied via thermal mechanical analysis ..................... 40
Hydrogen bond dynamics in shape-memory elastomers .................................. 46
2.4 Summary ............................................................................................................. 52
References .................................................................................................................. 54
Chapter 3 Dynamic Mechanical Behavior of Photo-Crosslinked Shape-Memory Elastomers Containing Hydrogen Bonding Side-Groups ......................... 57
3.1 Introduction ......................................................................................................... 57
3.2 Methods ................................................................................................................ 61
Materials .............................................................................................................. 61
Monomer synthesis .............................................................................................. 61
Polymerization of linear macromers (2a, 2b) ..................................................... 63
Photo-crosslinking of macromer films ............................................................... 63
Linear Poly (butyl acrylate) (PBA) UV degradation .......................................... 64
Swelling and Gel fraction measurement ............................................................. 65
Dynamic Mechanical Testing ............................................................................. 65
Time-temperature superposition (TTS) to obtain master curves ....................... 66
3.3 Discussion ............................................................................................................ 66
3.3.1 Synthesis of photo-crosslinkable macromers ............................................ 66
3.3.2 Photo-crosslinking of macromer films ...................................................... 69
3.3.3 Dynamic Mechanical Analysis of Dynamic Networks ............................. 80
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3.4 Summary ............................................................................................................. 95
References .................................................................................................................. 98
Chapter 4 Thermally Activated Diffusion in Dynamic Polymers Containing Hydrogen Bonding Side-Groups ............................................................................. 101
4.1 Introduction ....................................................................................................... 101
4.2 Experimental methods ....................................................................................... 104
Materials ............................................................................................................ 104
Monomer and polymer synthesis ...................................................................... 104
Chemical Characterizations............................................................................... 106
Viscosity measurements .................................................................................... 107
Diffusion studies ................................................................................................ 107
4.3 Results and Discussion ...................................................................................... 112
Synthesis ............................................................................................................ 112
Dye molecule diffusion though polymers ......................................................... 113
The free volume theory ..................................................................................... 118
Viscous relaxation vs small molecule diffusion ............................................... 121
4.4 Summary ........................................................................................................... 126
References ................................................................................................................ 128
Chapter 5 Conclusions and Future Work ...................................................................... 130
5.1 Conclusions ........................................................................................................ 130
Discovery of shape-memory effect introduced by strong hydrogen bonding interactions .......................................................................................... 130
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New photo crosslinking route to produce shape-memory elastomers containing strong hydrogen bonding groups .................................................... 131
Impacts on material thermal mechanical properties by hydrogen bond dynamics ............................................................................................................ 133
Small molecule diffusion through reversibly associating polymers revealed a different mechanism from mechanical relaxation........................... 134
5.2 Future work ........................................................................................................ 135
Low temperature behavior of elastomers containing UPy-hydrogen bonding side-groups .......................................................................................... 135
Self-healing study of polymer networks containing strong hydrogen bonding interactions .......................................................................................... 137
Diffusion study of small molecule through dynamic polymer melts under shearing .............................................................................................................. 137
References ................................................................................................................ 138
Appendix 1 Supplementary Documents for Chapter 2 ................................................. 140
Appendix 2 Supplementary Documents for Chapter 3 ................................................. 145
Appendix 3 Supplementary Documents for Chapter 4 ................................................. 166
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List of Tables
Table 3.1 Molecular weight and composition of photo-crosslinkable macromers ........................................................................................................................ 68 Table 3.2 Gel fractions of UV-cured elastomers determined from swelling in isopropyl alcohol. ......................................................................................................... 78 Table 4.1 Molecular weight characteristics of synthesized reversibly associating copolymers (RACs) and control copolymers (CCPs) ................................ 113
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List of Figures
Figure 1.1 Schematic of supramolecular polymer made from secondary interactions. (Main difference from conventional polymer is the equilibrium of association and dissociation) ......................................................................................... 4 Figure 1.2 Three different growth mechanisms of supramolecular polymerization based on thermodynamics, 20 a) Isodesmic supramolecular polymerization, b) ring-chain supramolecular polymerization, c) cooperative supramolecular polymerization, Kr >> Ks ................................................... 7 Figure 1.3 the molecular scheme of PFSA ionomer ......................................................... 9 Figure 1.4 a) an example of Ruthenium (II) coordination polymer structure32, 33 and b) an example of typical terpyridine ligand34 ..................................... 12 Figure 1.5 Molecular structure of triphenylene ............................................................... 14 Figure 1.6 Different multiple hydrogen bonding groups in supramolecular polymers ........................................................................................................................... 16 Figure 2.1 A typical shape-memory cycle of a themoresponsive shape-memory polymer .............................................................................................................. 35 Figure 2.2 Synthesis scheme of lightly crosslinked shape-memory polymers (SMPs) containing pendent ureidopyrimidinone side-groups ........................................ 37 Figure 2.3 Typical shape-memory response curve of an elastomer containing 2 mol% UPy pendent side groups, 1.5 mol % TMP-TMA, and 96.5 mol % BA. The sample was equilibrated at 66 °C for 20 min., a 50 mN tensile load was applied for 30 min., temperature was decreased to 5 °C, and the load was removed ......................................................................................... 41 Figure 2.4 Cartoon of proposed shape-memory mechanism involving thermo-reversible hydrogen bonding. Colored side-groups represent hydrogen bonding groups in the hot (red) and cold (blue) states, and the darker lines represent the lightly crosslinked covalent network ..................................... 42 Figure 2.5 TMA shape-memory cycles to study strain-fixity and strain-recovery ratios .................................................................................................................. 45
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Figure 2.6 Strain-fixity and strain-recovery ratio of shape-memory elastomer as a function of thermal mechanical cycle ..................................................... 46 Figure 2.7 Analysis of mechanical creep data acquired on synthesized elastomer films containing UPy pendent groups. a) Isothermal (47 °C) creep data using a 50 mN tensile load on 2 mol% UPy sample (solid line) and least squares fit to constitutive model (dotted line). Creep data of other temperatures are available at Appendix 1 Figure A1.4 – A1.7 ...................................... 47 Figure 2.8 Diagram showing mechanical elements of constitutive model: spring element (E1) and Maxwell element (E2 η (T) ) in parallel ................................... 48 Figure 2.9 Arrhenius temperature-dependence of fitted viscosities obtained from creep data at different temperatures for 2 mol% and 1 mol% UPy content respectively.......................................................................................................... 49 Figure 2.10 Scheme showing that the activation energy barrier for hydrogen bond dissociation (Ea,r) equals of hydrogen bond energy (UHB) plus activation energy barrier for association (Ea,f) ................................................................ 49 Figure 2.11 DMTA scan (2 °C min–1) at constant frequency (0.1 Hz) of a shape-memory sample containing 2 mol% of UPy side-groups .................................... 51 Figure 3.1 Scheme illustrating photo-crosslinking of network precursors to form dynamic networks containing both covalent and reversibly-associating crosslinks .......................................................................................................................... 60 Figure 3.2 Photo-crosslinking of coumarin-containing macromers, UV-Vis spectra of coumarin containing macromer Cm1 thin film (~ 30 µm) during irradiation ......................................................................................................................... 70 Figure 3.3 Scheme showing photoreversible crosslinking of nearby coumarin side-groups ....................................................................................................... 70 Figure 3.4 Photo-reversibility of coumarin crosslinking in a macromer thin film, the absorbance peak at 312 nm was plotted against UV irradiation time where circles represent 302 nm irradiation, and dots represent 254 nm irradiation ......................................................................................................................... 72 Figure 3.5 Possible reaction scheme of benzophenone photo-reaction. ......................... 74 Figure 3.6 Photo-crosslinking of benzophenone-containing macromer Bp½-UPy2, UV-Vis spectra taken immediately after 10 minute periods of UV
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periods—the absorption at =333 nm indicates photo-crosslinking is incomplete ........................................................................................................................ 75 Figure 3.7 Photo-crosslinking of benzophenone-containing macromer Bp½-UPy2, the absorbance maximum (at 333 nm) of the LAT decays following the end of each UV exposure period ............................................................................... 76 Figure 3.8 Influence of cumulative UV irradiation on weight average molecular weight and polydispersity index (DPI) .......................................................... 77 Figure 3.9 Demonstration of shape-memory response in Bp½ –Upy2: a) permanent shape (30 2 1 mm); b) programmed, temporary shape immediately after forming; c) temporary shape after one minute in room temperature air; d) temporary shape immediately following immersion in hot water (ca. 65C); e) temporary shape after one second immersion in 65C water; and f) return to permanent shape occurs in approximately three seconds in 65C water ...................................................................................................... 79 Figure 3.10 DMA storage modulus of elastomer Bp1/2-UPy2, a) original data, b) TTS shifted master curve at reference temperature 60 °C................................. 81 Figure 3.11 Storage modulus master curves for photo-crosslinked elastomers: a) Bp½-UPy2, Bp1-UPy1, Bp1-UPy2, and Bp2-UPy2; and b) Bp1-UPy5. All data have been time-temperature superimposed to a 60 C reference temperature ....................................................................................................... 82 Figure 3.12 Bar chart comparison between strand densities corresponding to high and low plateaus of storage modulus for different compositions ........................... 84 Figure 3.13 Plot of strand density obtained from high temperature plateau modulus versus concentration of permanent crosslinks based on Bp content ............... 85 Figure 3.14 Plot of strand density obtained from low temperature plateau modulus versus total crosslink density based on Bp and UPy content .......................... 86 Figure 3.15 Plot of strand density attributed to UPy net-points versus UPy crosslink density ............................................................................................................... 87 Figure 3.16 DMA tan of elastomer Bp1/2-UPy2, a) original data, b) TTS shifted master curve at reference temperature 60 °C ...................................................... 89
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Figure 3.17 The influence of UPy-content on damping properties of photo-crosslinked elastomers: tan δ master curves (using a reference temperature of 60 C) of elastomers with different compositions ...................................................... 90 Figure 3.18 The influence of UPy-content on damping properties of photo-crosslinked elastomers: tan δ peak frequency (at 60 °C) vs. UPy content ..................... 91 Figure 3.19 The influence of UPy-content on damping properties of photo-crosslinked elastomers: the magnitude of the peak in tan δ versus the ratio of reversible to chemical crosslinks. ............................................................................... 92 Figure 3.20 Shift factors for UPy-containing elastomers determined from time-temperature superposition of storage modulus using a reference of 60 °C. Data sets were shifted vertically to avoid overlap ................................................... 93 Figure 3.21 Plot of activation energies calculated from storage modulus shift factor versus measured UPy-content in photo-crosslinked elastomers .................. 94 Figure 4.1 Synthesis schemes of copolymers containing a) hydrogen bonding (UPy) side-groups and b) non-hydrogen bonding (DMPU) side-groups ............................................................................................................................. 103 Figure 4.2 Scheme shown multi-photon fluorescence recovery after photobleaching setup ..................................................................................................... 108 Figure 4.3 Molecular structures of dye molecules used in FRAP experiments and their absorption and emission peaks, a) rhodamine 6G, b) fluorescein ...................................................................................................................... 109 Figure 4.4 MP-FRAP of copolymers: example of fluorescence recovery curve (RAC-1) with 20 µM rhodamine 6G at 60 °C. This sample was photobleached using 810 nm light (350 mW) for 10 ms, and subsequent fluorescence was monitored at a reduced power of 35 mW. The dark line is a non-linear regression, least squares fit, to Equation 4.1 by varying , F0
and D .............................................................................................................................. 114 Figure 4.5 Arrhenius plots of dye diffusivity through different polymers: poly(butyl acrylate) (Ea = 51.4 4.4 kJ/mol) and associating copolymers containing 0.8 and 1.7 mol% UPy side-groups (Ea = 47.8 2.1, 47.9 1.3 kJ/mol) Reported errors in Ea reflect 95% confidence intervals and were obtained from least-squares fitting ................................................................................ 115
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Figure 4.6 Arrhenius plots of dye diffusivity through control copolymers containing 1.1 mol% and 2.0 mol% DMPU side-groups (Ea = 50.7 3.6, 51.6 2.9 kJ/mol) .......................................................................................................... 116 Figure 4.7 Arrhenius plots of dye diffusivity through poly(butyl acrylate) and crosslinked poly (butyl acrylate) with 1 mol % of EGDA as crosslinker (Ea =47.4 2.8 kJ/mol) .................................................................................................. 117 Figure 4.8 Room temperature densities of synthesized polymers. Error bars are estimated from the uncertainty of the mass measurement (1 g) and the standard deviation of the profile volume measurements (see text and inset) ............... 120 Figure 4.9 Plot illustrating that reduction in measured diffusivity is consistent with Eqn. 4.5. The quantity ln(Dx/D0) was averaged at several different temperatures .................................................................................................... 120 Figure 4.10 a) Viscosities of poly(butyl acrylate), synthesized control copolymers (CCPs), and reversibly associating copolymers (RAPs) measured at a radial shear rate of 0.1 rad/s. b) Schematic illustrating viscous relaxation under shear stress () in a dynamic polymer network containing self-complementary, reversible hydrogen bonding groups .......................................... 123 Figure 4.11 Plot of kb / D6r versus 1/T for poly (butyl acrylate), synthesized control copolymers (CCPs), and reversibly associating copolymers (RAPs) ........................................................................................................ 125 Figure 5.1 Temperature sweep (- 100 °C to 100 °C) dynamic mechanical analysis of shape-memory elastomer with 2 mol% UPy side-groups .......................... 136 Figure A1.1 Synthesis of UPy-EMA monomer, a) synthesis scheme, b) 1H NMR spectrum in CDCl3 ............................................................................................... 140 Figure A1.2 Entropy elasticity comparing to thermal expansion of the shape-memory elastomer, a) entropy elasticity, b) thermal expansion, at this given load, entropy elasticity clearly outweighs thermal expansion ............................ 141 Figure A1.3 DSC scan of shape-memory elastomers studied by thermal mechanical analysis ....................................................................................................... 142 Figure A1.4 Shape-memory elastomers, creep data (blue solid) and model fitting (red dots) at 27 °C ............................................................................................... 143
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Figure A1.5 Shape-memory elastomers, creep data (blue solid) and model fitting (red dots) at 37 °C ............................................................................................... 143 Figure A1.6 Shape-memory elastomers, creep data (blue solid) and model fitting (red dots) at 57 °C ............................................................................................... 144 Figure A1.7 Shape-memory elastomers, creep data (blue solid) and model fitting (red dots) at 66 °C ............................................................................................... 144 Figure A2.1 Synthesis of benzophenone (Bp) monomer, a) synthesis scheme, b) 1H NMR spectrum in CDCl3 ....................................................................... 145 Figure A2.2 Synthesis of coumarin (Cm) monomer, a) synthesis scheme, b) 1H NMR spectrum in CDCl3 ......................................................................................... 146 Figure A2.3 1H NMR spectrum of macromer Bp1-UPy2 in CDCl3 ............................ 147 Figure A2.4 Macromer Cm1, a) 1H NMR spectrum, b) GPC trace ............................. 148 Figure A2.5 Macromer Cm1-UPy2, a) 1H NMR spectrum, b) GPC trace .................. 149 Figure A2.6 Macromer Bp1, a) 1H NMR spectrum, b) GPC trace .............................. 150 Figure A2.7 Macromer Bp1-UPy1, a) 1H NMR spectrum, b) GPC trace .................... 151 Figure A2.8 Macromer Bp1-UPy5, a) 1H NMR spectrum, b) GPC trace .................... 152 Figure A2.9 Macromer Bp1-UPy10, a) 1H NMR spectrum, b) GPC trace .................. 153 Figure A2.10 Macromer Bp1/2-UPy2, a) 1H NMR spectrum, b) GPC trace .............. 154 Figure A2.11 Macromer Bp2-UPy2, a) 1H NMR spectrum, b) GPC trace .................. 155 Figure A2.12 GPC trace of macromer Bp1-UPy2 ........................................................ 156 Figure A2.13 Volume fraction of polymer (ν2) as a function of strand density (n). Experimental data are compared to solution to the Flory-Rehner equation for various values of polymer – solvent interaction parameter () ................................................................................................................. 157 Figure A2.14 DMA storage modulus of elastomer Bp1-UPy1, a) original data, b) TTS shifted master curve at reference temperature 60 °C............................... 158
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Figure A2.15 DMA storage modulus of elastomer Bp1-UPy2 a) original data, b) TTS shifted master curve at reference temperature 60 °C............................... 159 Figure A2.16 DMA storage modulus of elastomer Bp2-UPy2, a) original data, b) TTS shifted master curve at reference temperature 60 °C............................... 160 Figure A2.17 DMA storage modulus of elastomer Bp1-UPy5, a) original data, b) TTS shifted master curve at reference temperature 60 °C............................... 161 Figure A2.18 DMA tan of elastomer Bp1-UPy1, a) original data, b) TTS shifted master curve at reference temperature 60 °C .................................................... 162 Figure A2.19 DMA tan of elastomer Bp1-UPy2, a) original data, b) TTS shifted master curve at reference temperature 60 °C .................................................... 163 Figure A2.20 DMA tan of elastomer Bp2-UPy2, a) original data, b) TTS shifted master curve at reference temperature 60 °C .................................................... 164 Figure A2.21 DMA tan of elastomer Bp1-UPy5, a) original data, b) TTS shifted master curve at reference temperature 60 °C .................................................... 165 Figure A3.1 Synthesis of DMPU control monomer, a) synthesis scheme, b) 1H NMR spectrum in CDCl3 .......................................................................................... 166 Figure A3.2 1H NMR spectrum for RAP-2 ................................................................... 167 Figure A3.3 1H NMR spectrum for CCP-2 ................................................................... 168 Figure A3.4 1H NMR spectrum for RAC-1 .................................................................. 168 Figure A3.5 1H NMR spectrum for CCP-1 ................................................................... 169 Figure A3.6 GPC trace of spectrum for PBA ............................................................... 169 Figure A3.7 GPC trace of spectrum for RAC-1 ............................................................ 170 Figure A3.8 GPC trace of spectrum for RAC-2 ............................................................ 170 Figure A3.9 GPC trace of spectrum for CCP-1 ............................................................ 171 Figure A3.10 GPC trace of spectrum for CCP-2 .......................................................... 171 Figure A3.11 MP-FRAP of linear polymer PBA, fluorescence recovery curve at 22 °C ................................................................................................................. 172
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Figure A3.12 MP-FRAP of linear polymer PBA, fluorescence recovery curve at 30 °C ................................................................................................................. 172 Figure A3.13 MP-FRAP of linear polymer PBA, fluorescence recovery curve at 40 °C ................................................................................................................. 173 Figure A3.14 MP-FRAP of linear polymer PBA, fluorescence recovery curve at 50 °C ................................................................................................................. 173 Figure A3.15 MP-FRAP of linear polymer PBA, fluorescence recovery curve at 60 °C ................................................................................................................. 174 Figure A3.16 MP-FRAP of linear polymer PBA, fluorescence recovery curve at 70 °C ................................................................................................................. 174 Figure A3.17 MP-FRAP of linear polymer PBA, fluorescence recovery curve at 80 °C ................................................................................................................. 175 Figure A3.18 MP-FRAP of copolymer RAC-1, fluorescence recovery curve at 30 °C ........................................................................................................................... 175 Figure A3.19 MP-FRAP of copolymer RAC-1, fluorescence recovery curve at 40 °C ........................................................................................................................... 176 Figure A3.20 MP-FRAP of copolymer RAC-1, fluorescence recovery curve at 50 °C ........................................................................................................................... 176 Figure A3.21 MP-FRAP of copolymer RAC-1, fluorescence recovery curve at 70 °C ........................................................................................................................... 177 Figure A3.22 MP-FRAP of copolymer RAC-1, fluorescence recovery curve at 80 °C ........................................................................................................................... 177 Figure A3.23 MP-FRAP of copolymer RAC-2, fluorescence recovery curve at 40 °C ........................................................................................................................... 178 Figure A3.24 MP-FRAP of copolymer RAC-2, fluorescence recovery curve at 50 °C ........................................................................................................................... 178 Figure A3.25 MP-FRAP of copolymer RAC-2, fluorescence recovery curve at 60 °C ........................................................................................................................... 179 Figure A3.26 MP-FRAP of copolymer RAC-2, fluorescence recovery curve at 70 °C ........................................................................................................................... 179
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Figure A3.27 MP-FRAP of copolymer RAC-2, fluorescence recovery curve at 80 °C ........................................................................................................................... 180 Figure A3.28 MP-FRAP of copolymer CCP-1, fluorescence recovery curve at 22 °C ........................................................................................................................... 180 Figure A3.29 MP-FRAP of copolymer CCP-1, fluorescence recovery curve at 30 °C ........................................................................................................................... 181 Figure A3.30 MP-FRAP of copolymer CCP-1, fluorescence recovery curve at 40 °C ........................................................................................................................... 181 Figure A3.31 MP-FRAP of copolymer CCP-1, fluorescence recovery curve at 50 °C ........................................................................................................................... 182 Figure A3.32 MP-FRAP of copolymer CCP-1, fluorescence recovery curve at 60 °C ........................................................................................................................... 182 Figure A3.33 MP-FRAP of copolymer CCP-1, fluorescence recovery curve at 70 °C ........................................................................................................................... 183 Figure A3.34 MP-FRAP of copolymer CCP-1, fluorescence recovery curve at 80 °C ........................................................................................................................... 183 Figure A3.35 MP-FRAP of copolymer CCP-2, fluorescence recovery curve at 22 °C ........................................................................................................................... 184 Figure A3.36 MP-FRAP of copolymer CCP-2, fluorescence recovery curve at 30 °C ........................................................................................................................... 184 Figure A3.37 MP-FRAP of copolymer CCP-2, fluorescence recovery curve at 40 °C ........................................................................................................................... 185 Figure A3.38 MP-FRAP of copolymer CCP-2, fluorescence recovery curve at 50 °C ........................................................................................................................... 185 Figure A3.39 MP-FRAP of copolymer CCP-2, fluorescence recovery curve at 60 °C ........................................................................................................................... 186 Figure A3.40 MP-FRAP of copolymer CCP-2, fluorescence recovery curve at70 °C ............................................................................................................................ 186
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Figure A3.41 MP-FRAP of copolymer CCP-2, fluorescence recovery curve at 80 °C ........................................................................................................................... 187 Figure A3.42 MP-FRAP of crosslinked PBA, fluorescence recovery curve at 30 °C ........................................................................................................................... 187 Figure A3.43 MP-FRAP of crosslinked PBA, fluorescence recovery curve at 40 °C .......................................................................................................................... 188 Figure A3.44 MP-FRAP of crosslinked PBA, fluorescence recovery curve at 50 °C ........................................................................................................................... 188 Figure A3.45 MP-FRAP of linear polymer PBA, fluorescence recovery curve at 60 °C ................................................................................................................. 189 Figure A3.46 MP-FRAP of linear polymer PBA, fluorescence recovery curve at 70 °C ................................................................................................................. 189 Figure A3.47 MP-FRAP of linear polymer PBA, fluorescence recovery curve at 80 °C ................................................................................................................. 190
1
Foreword
This thesis is all about my Ph.D. research work under the supervision of my
advisor Dr. Anthamatten during my stay at the Department of Chemical
Engineering, University of Rochester.
Chapter 1 is an introduction of the field in my research area. I did the
literature review and wrote the chapter.
Chapter 2 is a study of shape-memory properties for lightly crosslinked
polymer networks containing hydrogen associating groups. I synthesized and
prepared all the sample materials. I did the TMA tests, analyzed all obtained data,
and discussed the results. James A. Viveros helped me with the TMA tests and
some model fitting work. Dr. Michelle H. Wrue helped me with some of the
synthesis and discussion. Part of this work was published in Advanced Materials,
2007, 19, 2851-2855. I am the primary author of the paper. Proper copyright should
go to John Wiley & Sons, Inc.
Chapter 3 is a study of the dynamic mechanical behavior of photo-
crosslinked shape-memory elastomers. I synthesized, characterized, and prepared
all the sample materials. I did the photo-curing studies, DMA studies and TTS
fitting, analyzed all data obtained, and discussed the results. Christopher L. Lewis
helped me with swell studies, and photo-curing studies as well as DMA tests and
2
TTS model developing. Chris also provided a lot of useful discussios. Darcy L.
Chen helped me with TTA model fitting. Part of this work was submitted to
Macromolecules. I am the primary author of the paper. Future proper copyright
should go to American Chemical Society.
Chapter 4 is a study of thermally activated diffusion in reversibly
associating polymers. I synthesized and prepared all the sample materials. I did the
FRAP and rheology studies, analyzed all obtained data, and discussed the results.
Kelley D. Sullivan helped me with the initial FRAP experiment and interpreting
some FRAP data. Dr. Edward B. Brown helped me understand the FRAP technique
and provided a lot of useful discussion. Part of this work was published in Soft
Matter, 2010, 6, 235-238. I am the primary author of the paper. Proper copyright
should go to Royal Society of Chemistry.
Chapter 5 is the conclusion and future work. I summerized my PhD
research work. Based on the current research results, some future research
directions were suggested.
3
Chapter 1 Introduction & Background
1. 1 Supramolecular polymers and supramolecular interactions in polymers
In recent years, a lot of scientists have been studying and exploring the
ability to utilize noncovalent intermolecular forces to build controlled polymeric
structures and to tailor desirable properties. In particular, reversible binding
through cooperative hydrogen bonds1-4, ionic interactions5, 6, and metal-ligand
complexes7, 8 etc. can lead to aggregation, gelation, or sudden viscosity changes
that are triggered by changes in molecular concentration, pH, or temperature. The
assembled materials often exhibit polymeric material properties, both in solution or
in bulk states. Polymers based on this concept have unique advantages over
conventional polymers built up via covalent bonding because they combine the
properties of traditional polymeric materials with the reversibility of bonds which
hold monomer blocks. In their assembled states, they exhibit traditional polymeric
characteristics including a plateau modulus and a glass transition. However, since
the holding interactions are reversible, when they are removed, for example by
heating, their rigid or elastic networks can be transformed into low viscosity small
molecules. These unique features enable a new class of stimuli-responsive and
versatile polymers --- supramolecular polymers.
Generally speaking, supramolecular polymers are polymers held together by
secondary interactions instead of covalent bonds9, 10 Figure 1.1 depicts one example
of a supramolecular polymer consisting of end-to-end chaining of low molar mass
4
oligomers. Analogous to conventional polymers, supramolecular polymers show
similar macromolecular characteristics in solutions as well as in bulk. However,
because the connecting forces for their structures are reversible interactions,
supramolecular polymers still behave like small molecules when their reversible
interactions dissociate with changing conditions such as heating or solvent
switching.
Figure 1.1 Schematic of supramolecular polymer made from secondary
interactions. (The main difference from conventional polymer is the equilibrium of
association and dissociation).
Supramolecular interactions as noncovelent intermolecular forces were
discovered in the late 19th century. The famous Dutch physicist van der Waals is
credited for his finding of van der Waals force. As chemists continued to explore
the world of noncovalent forces, a new discipline in chemistry arose called
supramolecular chemistry.1, 11-14 The scope of supramolecular chemistry research
includes molecular self-assembly, molecular recognition, host-guest chemistry, etc.
As known in the polymer science community, supramolecular interactions are very
common inside polymer systems, and sometime they are significant in determining
Association
Dissociation
5
material structures and properties. Examples include the well known DNA double
helix structure, which is due to supramolecular interactions of enormous hydrogen
bonds. Despite its import role in polymer materials, supramolecular interactions did
not become an active research topic in the synthetic polymer field until recent years
as polymer researchers were inspired by some fascinating findings of
supramolecular systems.1, 2 As the studies progressed, supramolecular polymer
research expanded its scope to include not only monomer units connected together
by secondary interactions, but also polymeric materials that contain
structure/property governing secondary interactions. The quest to understand
structure/property relationships of polymers bearing reversibly associating
noncovalent interactions has opened a new research field at the interface of
polymers and supramolecular chemistry --- supramolecular polymer science. This
new supramolecular polymer concept has engendered some novel classes of
polymer materials including thermal reversible polymers2, thermoplastic
elastomers15, 16, and self-healing materials17-19 which will be discussed in greater
detail in sections 1.3.
1.2 Different type of supramolecular polymer systems
Supramolecular polymers can be named under their reversible secondary
bond nature. For example, supramolecular polymers built via hydrogen bonding
would be classified as hydrogen bonding supramolecular polymers. In a recent
6
review in supramolecular polymers by De Greef et al.20, supramolecular
polymerizations were classified into three different types, isodesmic, ring-chain,
and cooperative growth which are more or less like the three types of
polymerizations used to differentiate traditional polymers. (See Figure 1.2) This
classification is based on Gibb’s free energy thermodynamics which describes
different mechanisms of supramolecular polymerizations and shows how the
conversion is based on temperature, concentration, etc. Isodesmic supramolecular
polymerization is very much like step polymerization, its polymerization is
controlled by one thermodynamic equilibrium constant. On the other hand, for a
ring-chain supramolecular polymerization, polymerization is dictated by the ratio
between intermolecular chain growing equilibrium and intramolecular ring closing
equilibrium. As for cooperative growth supramolecular polymerization, the
supramolecular polymerization has two phases with two significant different
association constants and it has a critical concentration or temperature beyond
which the supramolecular polymerization starts growing rapidly. Like conventional
polymers, supramolecular polymers could also be classified into three categories by
their polymerization natures. This classification has its scientific merit by
reflecting different supramolecular polymers bonding dynamics. Moreover, it
easily differentiates supramolecular polymers from conventional polymers.
7
a)
b)
c)
Figure 1.2 Three different growth mechanisms of supramolecular polymerization
based on thermodynamics, 20 a) Isodesmic supramolecular polymerization, b) ring-
8
chain supramolecular polymerization, c) cooperative supramolecular
polymerization, Kr >> Ks.
On the other hand, it is not very intuitive to classify supramolecular
polymers by their polymerization dynamics. Some of them may go through
different dynamics in different stages. Moreover, the supramolecular material
concept can be expanded to include not only polymer like structures formed by
reversible secondary interactions, but also varieties of polymer networks containing
significant secondary interactions. Therefore, this classification is not appropriate
as it eliminates a lot of interesting supramolecular polymer systems. All in all, for
better discussion, it is better to go back to use the nature of secondary interactions
to classify different supramolecular systems. Some important secondary
interactions include ionic interactions, metal-ligand complex, π-π stacking,
hydrogen bonding, etc. Some example supramolecular polymer systems built via
different secondary interactions will be discussed. The hydrogen bonding
interaction bears a lot of significance in the supramolecular polymer field and
therefore it will be discussed in much more detail in the next section.
Ionic interaction based supramolecular polymers
Ionic interactions are well known as one of the most important noncovalent
bonds. Long before the supramolecular polymer concept came along, ion
containing polymers like ionomers have already been important materials and have
been studied extensively.21, 22 Inspired by a lot ionomer studies, scientists noticed
9
that the chain-end ionic interactions between telechelic polymers can lead to
gelation behavior in solution.5, 23, 24 In the early 1980s, Broze et al. synthesized
different telechelic ion ending linear polymers by neutralizing the carboxylic end
groups with different cations.6 Due to the ionic interaction between end groups, in
some non-polar solvents, gelation occurs at certain concentrations. This gelation
process is reversible, and is solvent, temperature and concentration dependent. It is
a typical supramolecular polymer-forming process due to ionic interactions.
Polymer scientist believed that the ion interactions between end ionic groups
formed clusters that bond linear polymer or oligomer chains together which led to
gelation.
In a broad scope, ionomeric materials as a whole could be classified as one
ion interaction based supramolecular polymer system. As mentioned above,
ionomer material came way before the new supramolecular polymer development.
A lot of ionomer materials already had their commercial success. One of most
important examples is the perfluorosulphonic acid ionomer (PFSA) material known
commercially as “Nafion” and developed by E. I. du Pont de Nemours and
Company.25 PFSA was widely used as a proton exchange membrane for Fuel Cell
applications as well as other applications like batteries, sensors, etc.26, 27
In a recent discovery by T. Xie, PFSA polymer showed a fascinating
tunable multiple stage shape-memory effect.28 The multiple shape-memory effect is
a perfect example of supramolecular properties due to sulphonic ionic interactions.
10
Figure 1.3 shows the simple molecular structure of PFSA ionomer. It possesses two
thermal transitions.26 One transition beyond 240 °C is due to its crystalline phase
of the polyfluorene backbone. The other one is a broad glass transition from ~
50 °C to ~ 135 °C. That transition is due to ionic cluster phase formed by sulphonic
side-group ionic interactions. On a micro level, these clusters have different domain
sizes with correspondingly different dissociation temperatures. Therefore, these
different micro cluster domains lead to a very broad glass transition. So for PFSA
ionomer material, the different ionic cluster domains are almost acting as they are a
blend of different materials with continuous yet distinguishable glass transition
temperatures. On a macro level, ionic clusters within a temperature interval have
the ability to “pin” a mechanical deformation when cooled, and to regain the
original form once heated. That unique feature ultimately results in a multiple stage
shape-memory effects for the PFSA ionomer material.
CF2CF2 CF2CF
O
CF2
FC
O
CF2
CF2
SO3H
CF3
m n
11
Figure 1.3 the molecular scheme of PFSA ionomer
In addition, a lot of polyelectrolyte biomaterials can be considered as ionic
interaction based supramolecular polymer systems. Some of them show typical
characteristics related to ionic interactions inside polymer network, and as the
supramolecular polymer concept continue to develop, it is probable that polymer
scientists will start looking at polyelectrolyte materials from a supramolecular
polymer perspective.
Metal-ligand complex based supramolecular polymers
Like the ionic interaction, the metal-ligand complex is also well known to
be a strong type of secondary interaction. Metal-ligand complexes played an
important role in supramolecular chemistry and are widely used in molecular self
assembly and host-guest recognition applications.11, 29-31 Recently, scientists
started to explore metal-ligand complexes as building blocks for supramolecular
polymer systems.
One early example of metal-ligand supramolecular polymers is the
Ruthenium (II) coordination polymer.32, 33 the Ruthenium (II) pyridine type ligand
complex is well known and relatively stable. By designing the right monomer
system, Ruthenium (II) coordination polymers can be formed. (Figure 1.4a)
One type of ligand of importance in metal-ligand complex based
supramolecular system is terpyridines (terpys).34-38 An example structure is shown
12
in Figure 1.4b. As one may draw analogy with Figure 1.4a, the terpy is able to bind
different metals like Fe2+, Zn2+, Ru2+, Co2+ , etc. to form bisterpy metal complexes.
Schmatloch et al. modified diethylene glycol end OH groups into terpys. By
utilizing terpy metal complex, they were able to get a reversible supramolecular
polymer soluble in water.35 Other scientists were able to synthesized telechelic
polymers with terpy end-groups and therefore they could produce different block
copolymers.34, 36, 39 This method to obtain block copolymer is rather robust and
versatile. Most notably it provides an alternative means of producing block
copolymers.
a)
N
N
N
N
N
N
N
N
N
N
N
N
N
N N
N N
N
NN
N N
Ru
Ru
2+
2+
(Cl-)2
(Cl-)2
n
b)
N
N
N
R
13
Figure 1.4 a) an example of Ruthenium (II) coordination polymer structure32, 33 and
b) an example of typical terpyridine ligand34
Another example of metal-ligand supramolecular polymer system is the
platinum phosphorus ligand coordinated polymer documented by Paulusse et al.40
Like ionic interactions, some of the complicated metal-ligand self assembly
structures, (e.g. helices) also can be considered as supramolecular polymer
systems.31, 41
π-π stacking interaction based supramolecular polymers
A lot of conjugated or aromatic molecular structures can go through -
stacking secondary interactions. π-π interaction usually results in discotic stacking
which leads to crystalline or liquid crystalline phase. Supramolecular materials can
be formed via π-π stacking.
Triphenylenes are common discotic like molecules studied by scientists to
build supramolecular structures.42-45 The common structure of triphenylene is
shown in Figure 1.5. By modifying the pendant R groups into proper side-chains,
the triphenylenes were able to form aggregated polymeric columns in either organic
solutions or water phase.42, 43 The degree of polymerization of these supramolecular
polymers can be tuned by temperature and concentration.44 Another example of π-π
stacking supramolecular polymer systems is a pyridine-pyridazine oligomer system
first described by Cuccia et al.46, 47 The strong π-π interactions folded the oligomers
14
into helices and then stacked them together into column-like supramolecular
structures. Recently, Burattini et al. discovered a novel healable polymer network
based on π-π stacking interactions between pyrenyl end-groups and chain folded
polyimides.19, 48, 49
OR
OR
ORRO
RO
RO
Figure 1.5 Molecular structure of triphenylene
π-π stacking is also very common in conjugated polymer or copolymer
systems. The cooperative π-π interactions inside some polymers leads to a lot of
new fascinating supramolecular morphologies and architectures which could
potentially open applications in areas like transistor electronics and light emitting
or photovoltaic devices..50-52
Other supramolecular polymer systems
Besides all the interactions discussed above, there are still other secondary
interactions which could lead to supramolecular structures. Examples include
supramolecular micelle structures built by hydrophobic forces.53, 54 In addition,
some supramolecular polymer systems may contain more than one form of
15
secondary interaction. Examples include some supramolecular systems where π-π
interactions work with a metal-ligand complex55, 56 or hydrogen bonding working
with π-π stacking57, 58. As supramolecular material research evolves, more and
more secondary interactions could be found valuable in constructing functional
supramolecular materials.
1.3 Hydrogen bonding in supramolecular polymers
Among all the reversible supramolecular interactions, hydrogen bonding is
the most important and most studied one due to their directionality and versatility.
Although hydrogen bonds are often considered weak, some cooperative multiple
hydrogen bonds can have energies as high as tens of kJ/mol. Plus, a variety of
hydrogen bonding motifs involving different multiple, cooperative bonding are
available readily via simple synthesis. Therefore, scientists have a wide range of
choices for different bonding strengths to begin with. Moreover, hydrogen bonds’
unique directional selectivity makes them ideal for molecular engineering of
desired polymer structures and material properties.
Hydrogen bonding strength is dependent on solvent, temperature, and other
environment. However, generally speaking, the strength of a hydrogen bonding
motif is determined by the number of individual hydrogen bonds involved. A
greater number of hydrogen bonds usually indicates stronger hydrogen bonding,
and the bond strength is nearly cumulative. Figure 1.6 shows examples of different
single and multiple hydrogen bonding systems.59 The strength of hydrogen bonding
16
can be easily tuned over a wide range from several kJ/mol to tens of kJ/mol.
hydrogen bonding association equilibrium was typically studied in solution using
solution NMR. In addition, IR spectroscopy and solid-state NMR were found to be
useful tools to study hydrogen bond dynamics60, 61
Single hydrogen bonds Double hydrogen bonds
Triple hydrogen bonds
Quadruple and multiple hydrogen bonds
O
N H O
NH
O
OH O
OH
N
N
N
N
NH H
N
N
N
N
NHH
NH
N
O
O
H
N
N
NH
H
N
O
O
N
N
NH
H
H
O
O
N H O
O
N HO
OH O
O H O
O
N
N
O
N
N
O
H3C
H
N
N
O
N
N
O
CH3
H
H
H
H
H
1
N
N
O
O
H
H
N
N
N
O
N
O
N
N
O
O
O
H
H
H
H
2
17
Figure 1.6 Different multiple hydrogen bonding groups in supramolecular
polymers
Another aspect of hydrogen bonding is its directionality and its
complementarity, especially for multiple hydrogen bonding systems. This feature
often leads to highly intermolecular ordering. For example, ureidopyrimidinone
(UPy) hydrogen bonding 1 and hydrogen bonding system 2 in Figure 1.6 are self-
complementary and hetero-complementary respectively. Self-complementary
means that same hydrogen bonding groups bond to each other. Hetero-
complementary means that hydrogen bonding groups only bond different
(complementary) groups. While self-complementary bonding promotes the overall
bonding efficiency, hetero-complementary bonding is more selective and can
produce more controlled intermolecular structures.
Although the impact of a single hydrogen bond on polymer morphology
might be small, a collective single hydrogen bonding interaction between polymer
chains can leads to liquid crystalline materials62 or phase segregation in polymer
blends63-65. On the other hand, some hydrogen bonds with multiple bonding sites
have relatively higher bonding energies and higher selectivity. Therefore, they
could lead to more drastic structure or property changes inside polymer systems. In
fact, in recent years, polymer scientists have been intensively studying polymer
networks containing multiple hydrogen bonding groups. A lot of novel
18
supramolecular materials have been exploited and some hydrogen bonding
interaction dynamics has been discovered as well.
Supramolecular polymers based on hydrogen bonding gave rise to a series
of novel materials. Several examples are discussed below.
Thermal reversible polymers
Lehn et al.1, 66 first reported a dynamic polymer formed by triple hydrogen
bonding interactions --- 2,6-dimino pyridine/uracile pairs (showed in Figure 1.6).
Rigid small molecules are bonded by hydrogen bond forming fiber like polymers.
Similar to rigid polymers, they exhibit liquid crystalline properties over a wide
temperature range. In order for small molecules association to occur to the extent
that polymeric properties are achieved (like polymer chain length), a higher
hydrogen bonding association constant K is needed.67 Meijer et al.68 developed the
ureidopyrimidinone (UPy) group which has a bonding constant K of 6 × 107 M-1.
Another major advantage of UPy hydrogen bonding groups over other hydrogen
bonding groups is its easy chemistry; it can be attached to many different building
blocks. By choosing proper building blocks, flexible thermal reversible polymers
have been obtained.2, 69 At room temperature, building blocks are held together by
UPy hydrogen bonds, and they behave like flexible polymers. While at higher
temperatures, UPy hydrogen bonds dissociate, the building blocks lose their
bonding and act like a viscous liquid. This dramatic phase and property transitions
introduced by hydrogen bond dynamics really make UPy groups unique. In fact,
19
UPy hydrogen interactions have drawn a great amount of interests as polymer
scientists have been studying the enhanced polymer mechanical properties as well
as phase behaviors introduced by UPy end-groups or side-groups.16, 70-72
Thermoplastic elastomers
Long et al. 3, 16 were able to couple UPy side-groups into linear poly(butyl
acrylate)s to obtain a new thermoplastic elastomer material. Pure poly(butyl
acrylate) is a polymer melt with a glass transition temperature (Tg) of -64 °C.
However, mechanical study showed that at room temperature, UPy containing
linear poly(butyl acrylate) melts behaved like a rubbery elastomer. They bear a
typical elastomer Young’s Modulus at ~ hundreds of kPa. However, at high
temperature (80 °C), this UPy containing polymer’s Young’s modulus dropped
significantly, acting like a viscous polymer melt. UPy hydrogen bond dynamics
were attributed to this novel effect. Long et al concluded that at low temperature,
UPy groups are bound together via hydrogen bonding, and they are acting as
crosslinking points for the polymer network. At elevated temperature, UPy
hydrogen bonds dissociate, the linear poly(butyl acrylate) becomes loose and
therefore regains its ability to flow like a melt. Feldman et al. have also studied
UPy hydrogen bond dynamics inside poly(butyl acrylate) systems.4, 72 They utilized
controllable synthesis and post polymerization modifications to systematically
produce poly(butyl acrylate)s with different UPy contents. In their recent studies,
dynamic mechanical analysis of different UPy content polymers showed that the
20
storage modulus increases as UPy content increases. In addition, calculated
mechanical activation energies based on were shown to increase along with higher
UPy content as well.72
Self healing materials
The self healing property is always desirable in polymer science and
engineering field as a lot of polymer materials have limited lifespan because of
irreversible damage. Supramolecular polymers have the potential to become good
self healing materials because of their reversible nature. Recently, Leibler et al
developed a self healing supramolecular elastomer based on multiple hydrogen
bonding interactions.17, 73, 74 They started with vegetable based fatty diacids and
triacids, first reacting with diethylene triamine, then further reacting with ureas to
get varieties of oligomers with multiple sites of self complementary hydrogen
bonds. The original resulting plastic like material has a Tg at about 28 °C. At
temperature above Tg, the material showed typical elastomer behaviors. It can be
deformed with stress and regain its shape once the force is removed. At even higher
temperature beyond 160 °C, the material could flow like a viscous liquid. In
addition, the material was shown to be soluble in some organic solvents which also
indicated that it was not covalently crosslinked. Moreover, by doping in some
dodecane as plasticizer, the material showed typical elastomer behavior even at
room temperature. This elastomer like properties shown by this material was
attributed to the multiple hydrogen bonding interactions between the oligomers. At
21
lower temperature, the hydrogen bonds with extremely slow dynamics served as
crosslinking points to hold the material together like an elastomer.
More interestingly, this material exhibited excellent self mending abilities.
Once the cut or fractured pieces were put together, the material is able heal itself
with time by recombining the broken hydrogen bonds. Leibler et al did show in
their studies that the healing ability is waiting time dependent. If waited too long to
put the pieces together, the free hydrogen bonding sites had the time to rearrange
finding other partners which leads to an unsuccessful healing process. Hydrogen
bond dynamics was perfectly demonstrated here as well, the slower hydrogen
bonding association at room temperature gives the material a chance to heal itself.
However, in their study, they did not show any study of self healing process in
higher temperatures. As hydrogen bond dynamics proceeds rapidly, one would
expect healing would occur as well with multiple hydrogen bonding sites
exchanging quickly.
1.4 Some intellectual challenges in hydrogen bonding supramolecular polymer
systems
As discussed above, hydrogen bonding based supramolecular polymer
systems have been extensively studied, and a lot scientific details about hydrogen
bond dynamics has been discovered recently. However, there are still some
fundamental understandings and property discoveries that need to be addressed.
22
A major challenge facing the hydrogen bonding associating polymer field is
the development of synthetic techniques that overcome reagent immiscibility and
enable a variety of associating groups to be incorporated into bulk polymers. To
study reversibly associating polymer materials, there are only a limited number of
hydrogen bonding functional groups to choose from. Those associating groups are
typically rigid moieties containing multiple hydrogen bonds which suffer from
compatibility problems with common reagents, solvents, and macromolecules. The
ureidopyrimidinone (UPy, see Figure 1.6) hydrogen bonding group, for example,
requires a strong polar solvent like DMSO to compensate for the incompatibility of
UPy groups with other reagents16. Currently, scientists strive to increase hydrogen
bonding groups’ compatibility. The traditional approach is to modify hydrogen
bonding groups by attaching some alkyl chains or other functional moieties to
increase miscibility with hydrocarbon reagents.
Along with the immiscibility of strong hydrogen bonding groups (i.e. UPy
groups), another challenge is to covalently couple them directly onto a three-
dimensional elastic network. Conventionally, a crosslinked elastic polymer network
is made from bulk monomers or polymer precursors (e.g. the vulcanization of
natural rubber). Due to the notorious insolubility of these strong hydrogen bonding
groups in common monomers, no attempt has been made to synthesize a
crosslinked network containing strong, reversibly associating hydrogen bonding
groups. On the other hand, this is a viable objective. Using solution synthesis,
scientists have demonstrated thermal-reversible polymers via hydrogen bond
23
containing monomers or oligomers as well as thermoplastic elastomers via
hydrogen bond containing linear polymers. One may wonder how an elastic
network that is coupled directly to strong hydrogen bonding groups would behave.
Based on the dynamic nature of hydrogen bonding, a shape-memory effect is
expected. The covalent network should provide a driving force to shape recovery,
and the reversible network may be leveraged to “pin” the material in a temporarily
deformed state.
A fundamental understanding of how reversible hydrogen bond dynamics in
the melt phase differs from bonding in solution is always desirable. Polymers and
elastomers typically find commercial applications in highly condensed liquid or
glassy states. In these condensed states, hydrogen bond dynamics are undoubtedly
slower, and one begins to question whether hydrogen bond interactions are really
reversible. Careful study of hydrogen bond association in condensed phases is
needed to better understand hydrogen bond dynamics and to develop new
applications. The majority of studies are performed in solution because dynamics
are fast and easy to follow using spectroscopy (e.g. NMR techniques). In the
condensed state, dynamics are slow and there are far fewer techniques available.
For example, FT-IR can sense hydrogen bonding in bulk polymers; however, the
signals are difficult to identify and quantify. Rheological and mechanical
measurements are indirect methods to assess hydrogen bonding association in
condensed polymer phases.16, 75-77 However, detailed relationships that connect
24
hydrogen bond dynamics with rheological and mechanical properties are clearly
needed.
In addition, it is desirable to understand how reversible hydrogen bonding
interactions occurring with supramolecular structures affect mass transport of
polymers and small molecules. The fact that mass transport through “solid”
materials is inherently slow has discouraged scientists form studying mass transport
through bulk polymers. On the other hand, polymer melts can be considered liquids
with high viscosities, and molecular transport is an intrinsic property. It is natural to
expect that transport phenomena will be affected by hydrogen bond dynamics in
associating polymers. Therefore, molecular transport could be a valuable tool to
better understand hydrogen bonding within bulk polymers. Moreover,
understanding and controlling mass transport through amorphous polymers is an
important aspect of emerging technologies including drug delivery, chemical
sensors, and barrier elastomers.
1.5 Objectives and overview of the thesis
The objective of this thesis study is to address some of the remaining
challenges in the field of hydrogen bonding supramolecular polymer fields. To be
specific, we would like to (1) effectively incorporate strong hydrogen bonding
groups inside an elastomeric polymer network, (2) study polymer properties
introduced by hydrogen bond dynamics, and develop new materials and
25
applications from the new properties, (3) relate polymer material mechanical,
rheological properties to hydrogen bond dynamics, and understand hydrogen bond
dynamics in bulk polymer network, (4) explore small molecule diffusion through
polymer melts containing strong hydrogen bonding groups, and study how
hydrogen bond dynamics affects mass transportation.
In Chapter 2, strong hydrogen bonding (2-ureido-4-pyrimidinone, UPy)
side-groups were successfully incorporated into a lightly crosslinked elastomeric
polymer network via conventional free radical polymerization. The synthesized
polymer showed good shape-memory properties, and UPy hydrogen bonding
interactions were able to stabilize mechanically strained states at low temperatures.
In addition, the materials' shape recovery rate exhibits Arrhenius-like temperature
dependence due to the dynamics of hydrogen bond dissociation.
In Chapter 3, a photo-crosslinking route to get shape-memory elastomer
containing strong hydrogen bonding (UPy) side-groups is presented. This method
enables melt-processing of shape-memory elastomers into complex permanent
shapes, and samples can be prepared with much higher UPy-content. Dynamic
mechanical analysis on photo-crosslinked shape-memory elastomers revealed that
dynamic UPy hydrogen bonding interactions behave nearly as effectively as
permanent crosslinks at low temperatures and high frequencies. The results also
showed that in addition to increasing elastomers’ toughness, UPy hydrogen
bonding interactions also significantly enhanced materials’ damping property. Time
26
temperature superposition analysis of storage modulus curves showed that UPy
hydrogen bond dynamics is working in a cooperative manner, and that the presence
of a covalent network supports cooperative binding of UPy side-groups.
Small molecule diffusion through polymer network containing UPy
hydrogen bonding side-groups is discussed in Chapter 4. Multi-photon fluorescence
recovery after photobleaching (FRAP) was demonstrated to be a good tool to study
mass transport inside polymer systems. UPy hydrogen bonding interactions greatly
reduced small molecule’s diffusivity inside polymers. In contrast to material’s
viscous relaxation which is kinetically controlled by dissociation rate of hydrogen
bonds, the small molecule diffusion is thermodynamically limited by the hydrogen
bonding association constant.
Chapter 5 summarizes what was accomplished in this thesis. Based on the
scientific findings I have learned, some future research directions are presented.
Supporting spectra, data and procedures are provided in Appendices 1 - 3.
27
References
1. J. M. Lehn, Angew. Chem.-Int. Edit. Engl., 1990, 29, 1304-1319.
2. R. P. Sijbesma, F. H. Beijer, L. Brunsveld, B. J. B. Folmer, J. H. K. K.
Hirschberg, R. F. M. Lange, J. K. L. Lowe and E. W. Meijer, Science, 1997,
278, 1601-1604.
3. K. Yamauchi, J. R. Lizotte and T. E. Long, Macromolecules, 2002, 35, 8745-
8750.
4. K. E. Feldman, M. J. Kade, T. F. A. de Greef, E. W. Meijer, E. J. Kramer and
C. J. Hawker, Macromolecules, 2008, 41, 4694-4700.
5. G. Broze, R. Jerome and P. Teyssie, Macromolecules, 1981, 14, 224-225.
6. G. Broze, R. Jerome and P. Teyssie, Macromolecules, 1982, 15, 920-927.
7. W. C. Yount, H. Juwarker and S. L. Craig, J. Am. Chem. Soc., 2003, 125,
15302-15303.
8. M. J. Serpe, M. Rivera, F. R. Kersey, R. L. Clark and S. L. Craig, Langmuir,
2008, 24, 4738-4742.
9. L. Brunsveld, B. J. B. Folmer, E. W. Meijer and R. P. Sijbesma, Chemical
Reviews, 2001, 101, 4071-4097.
10. J. M. Lehn, Pergamon-Elsevier Science Ltd, 2005, pp. 814-831.
11. J. M. Lehn, Angew. Chem.-Int. Edit. Engl., 1988, 27, 89-112.
12. J. M. Lehn, Science, 1985, 227, 849-856.
13. C. J. Pedersen, Angewandte Chemie International Edition in English, 1988, 27,
1021-1027.
28
14. D. J. Cram and J. M. Cram, Science, 1974, 183, 803-809.
15. K. Chino and M. Ashiura, Macromolecules, 2001, 34, 9201.
16. K. Yamauchi, J. R. Lizotte and T. E. Long, Macromolecules, 2003, 36, 1083-
1088.
17. P. Cordier, F. Tournilhac, C. Soulie-Ziakovic and L. Leibler, Nature, 2008,
451, 977-980.
18. M. Burnworth, L. Tang, J. R. Kumpfer, A. J. Duncan, F. L. Beyer, G. L. Fiore,
S. J. Rowan and C. Weder, Nature, 2011, 472, 334-337.
19. S. Burattini, H. M. Colquhoun, J. D. Fox, D. Friedmann, B. W. Greenland, P. J.
F. Harris, W. Hayes, M. E. Mackay and S. J. Rowan, Chemical
Communications, 2009, 6717-6719.
20. T. F. A. De Greef, M. M. J. Smulders, M. Wolffs, A. Schenning, R. P.
Sijbesma and E. W. Meijer, Chemical Reviews, 2009, 109, 5687-5754.
21. W. J. Macknight and T. R. Earnest, Macromolecular Reviews Part D-Journal
of Polymer Science, 1981, 16, 41-122.
22. J. F. Joanny, Polymer, 1980, 21, 71-76.
23. G. Broze, R. Jerome, P. Teyssie and C. Marco, Macromolecules, 1983, 16,
996-1000.
24. G. Broze, R. Jerome and P. Teyssie, Macromolecules, 1982, 15, 1300-1305.
25. D. J. Connolly, Longwood and W. F. Gresham, US Patent 3282875, 1966.
26. K. A. Mauritz and R. B. Moore, Chemical Reviews, 2004, 104, 4535-4585.
29
27. K. D. Kreuer, S. J. Paddison, E. Spohr and M. Schuster, Chemical Reviews,
2004, 104, 4637-4678.
28. T. Xie, Nature, 2010, 464, 267-270.
29. J. M. Lehn, Science, 1993, 260, 1762-1763.
30. D. S. Lawrence, T. Jiang and M. Levett, Chemical Reviews, 1995, 95, 2229-
2260.
31. C. Piguet, G. Bernardinelli and G. Hopfgartner, Chemical Reviews, 1997, 97,
2005-2062.
32. S. Kelch and M. Rehahn, Macromolecules, 1997, 30, 6185-6193.
33. R. Knapp, A. Schott and M. Rehahn, Macromolecules, 1996, 29, 478-480.
34. B. G. G. Lohmeijer and U. S. Schubert, Angewandte Chemie-International
Edition, 2002, 41, 3825-3829.
35. S. Schmatloch, M. F. Gonzalez and U. S. Schubert, Macromolecular Rapid
Communications, 2002, 23, 957-961.
36. H. Hofmeier and U. S. Schubert, Macromolecular Chemistry and Physics,
2003, 204, 1391-1397.
37. R. Dobrawa and F. Wurthner, Journal of Polymer Science Part a-Polymer
Chemistry, 2005, 43, 4981-4995.
38. J. B. Beck, J. M. Ineman and S. J. Rowan, Macromolecules, 2005, 38, 5060-
5068.
39. M. Chiper, M. A. R. Meier, D. Wouters, S. Hoeppener, C. A. Fustin, J. F.
Gohy and U. S. Schubert, Macromolecules, 2008, 41, 2771-2777.
30
40. J. M. J. Paulusse, J. P. J. Huijbers and R. P. Sijbesma, Macromolecules, 2005,
38, 6290-6298.
41. M. Albrecht, Chemical Society Reviews, 1998, 27, 281-287.
42. D. Markovitsi, H. Bengs and H. Ringsdorf, Journal of the Chemical Society,
Faraday Transactions, 1992, 88, 1275-1279.
43. H. Ringsdorf, B. Schlarb and J. Venzmer, Angewandte Chemie, 1988, 100,
117-162.
44. J. R. Henderson, J Chem Phys, 2000, 113, 5965-5970.
45. V. Percec, M. R. Imam, M. Peterca, D. A. Wilson, R. Graf, H. W. Spiess, V. S.
K. Balagurusamy and P. A. Heiney, J. Am. Chem. Soc., 2009, 131, 7662-7677.
46. L. A. Cuccia, J.-M. Lehn, J.-C. Homo and M. Schmutz, Angewandte Chemie
International Edition, 2000, 39, 233-237.
47. T. J. Katz, Angewandte Chemie International Edition, 2000, 39, 1921-1923.
48. S. Burattini, B. W. Greenland, W. Hayes, M. E. Mackay, S. J. Rowan and H.
M. Colquhoun, Chem. Mat., 2011, 23, 6-8.
49. S. Burattini, B. W. Greenland, D. H. Merino, W. G. Weng, J. Seppala, H. M.
Colquhoun, W. Hayes, M. E. Mackay, I. W. Hamley and S. J. Rowan, J. Am.
Chem. Soc., 2010, 132, 12051-12058.
50. L. Romaner, A. Pogantsch, P. Scandiucci de Freitas, U. Scherf, M. Gaal, E.
Zojer and E. J. W. List, Adv. Funct. Mater., 2003, 13, 597-601.
51. F. J. M. Hoeben, P. Jonkheijm, E. W. Meijer and A. Schenning, Chemical
Reviews, 2005, 105, 1491-1546.
31
52. D. M. Russell, A. C. Arias, R. H. Friend, C. Silva, C. Ego, A. C. Grimsdale
and K. Mullen, Appl. Phys. Lett., 2002, 80, 2204-2206.
53. J. Wang and M. Jiang, J. Am. Chem. Soc., 2006, 128, 3703-3708.
54. C. Zhong and P. Luo, Journal of Polymer Science Part B: Polymer Physics,
2007, 45, 826-839.
55. R. K. Kumar and I. Goldberg, Angewandte Chemie-International Edition, 1998,
37, 3027-3030.
56. M. Munakata, L. P. Wu, T. Kuroda-Sowa, M. Maekawa, Y. Suenaga, G. L.
Ning and T. Kojima, J. Am. Chem. Soc., 1998, 120, 8610-8618.
57. F. J. M. Hoeben, L. M. Herz, C. Daniel, P. Jonkheijm, A. P. H. J. Schenning, C.
Silva, S. C. J. Meskers, D. Beljonne, R. T. Phillips, R. H. Friend and E. W.
Meijer, Angewandte Chemie International Edition, 2004, 43, 1976-1979.
58. J. J. Miao and L. Zhu, Chem. Mat., 2010, 22, 197-206.
59. W. H. Binder and R. Zirbs, in Hydrogen Bonded Polymers, Springer-Verlag
Berlin, Berlin, 2007, pp. 1-78.
60. I. Schnell, B. Langer, S. H. M. Sontjens, R. P. Sijbesma, M. H. P. van
Genderen and H. W. Spiess, Phys. Chem. Chem. Phys., 2002, 4, 3750-3758.
61. H. W. Spiess, Macromolecular Symposia, 2003, 201, 85-88.
62. M. Shoji and F. Tanaka, Macromolecules, 2002, 35, 7460-7472.
63. S. Y. Liu, C. M. Chan, L. T. Weng and M. Jiang, J Polym Sci Pol Phys, 2005,
43, 1924-1930.
32
64. S. Y. Liu, C. M. Chan, L. T. Weng, L. Li and M. Jiang, Macromolecules, 2002,
35, 5623-5629.
65. S. Y. Liu, C. M. Chan, L. T. Weng and M. Jiang, Polymer, 2004, 45, 4945-
4951.
66. C. Fouquey, J.-M. Lehn and Anne-Marie Levelut, Adv Mater, 1990, 2, 254-257.
67. L. Brunsveld, B. J. B. Folmer and E. W. Meijer, MRS Bull., 2000, 25, 49-53.
68. K. Ojelund, T. Loontjens, P. Steeman, A. Palmans and F. Maurer,
Macromolecular Chemistry and Physics, 2003, 204, 52-60.
69. R. F. M. Lange, M. Van Gurp and E. W. Meijer, Journal of Polymer Science
Part a-Polymer Chemistry, 1999, 37, 3657-3670.
70. K. E. Feldman, M. J. Kade, E. W. Meijer, C. J. Hawker and E. J. Kramer,
Macromolecules, 2010, 43, 5121-5127.
71. M. H. Wrue, A. C. McUmber and M. Anthamatten, Macromolecules, 2009, 42,
9255-9262.
72. K. E. Feldman, M. J. Kade, E. W. Meijer, C. J. Hawker and E. J. Kramer,
Macromolecules, 2009, 42, 9072-9081.
73. D. Montarnal, F. Tournilhac, M. Hidalgo, J. L. Couturier and L. Leibler, J. Am.
Chem. Soc., 2009, 131, 7966-+.
74. D. Montarnal, P. Cordier, C. Soulie-Ziakovic, F. Tournilhac and L. Leibler,
Journal of Polymer Science Part a-Polymer Chemistry, 2008, 46, 7925-7936.
75. J. Hirschberg, F. H. Beijer, H. A. van Aert, P. Magusim, R. P. Sijbesma and E.
W. Meijer, Macromolecules, 1999, 32, 2696-2705.
33
76. K. Yamauchi, A. Kanomata, T. Inoue and T. E. Long, Macromolecules, 2004,
37, 3519-3522.
77. S. H. M. Sontjens, R. A. E. Renken, G. M. L. van Gemert, T. A. P. Engels, A.
W. Bosman, H. M. Janssen, L. E. Govaert and F. P. T. Baaijens,
Macromolecules, 2008, 41, 5703-5708.
57
Chapter 3 Dynamic Mechanical Behavior of Photo-Crosslinked
Shape-Memory Elastomers Containing Hydrogen Bonding Side-
Groups
3.1 Introduction
In the previous chapter, we demonstrated a novel shape-memory effect
introduced by hydrogen bond dynamics. However, due to the incompatibility of
strong hydrogen bonding groups in monomers, solvent was needed to conduct the
one step polymerization. This leads to some limitations, (1) it prevents higher
hydrogen bonding contents, (2) imperfection and internal stress may occur after
final solvent removal, (3) final elastomer structure or composition quantification is
inconvenient. So, a new synthetic approach is needed to overcome these
limitations. Ultimately, the goal is to engineer shape-memory elastomers with
defined compositions, especially with a range of different hydrogen bonding
contents, to study the elastomers’ thermal mechanical behaviors and to relate
thermal mechanical properties to hydrogen bond dynamics.
Elastomers consist of macromolecular chains that are bound together into a
network by covalent or non-covalent crosslinks. Crosslinks serve as permanent
entanglements, restricting long-range and irreversible chain slippage. When
deformed, chains are distorted from their most probable and preferable
configurations, giving rise to an entropic restoring force.
58
A thermoresponsive shape-memory polymer (SMP) is capable of fixing a
temporary shape when cooled, under elastic strain, beneath a well-defined shape-
memory temperature (TSM) that is often accompanied by crystallization or the
formation of a polymer glass.1-3 At temperatures beneath TSM, the deformed shape
is stabilized by the formation of crystalline or glassy domains, and this shape can
be maintained indefinitely, even in the absence of stress. However, upon
subsequent heating above TSM, the SMP can be triggered to revert to its original
shape as stored elastic strain-energy is recovered. Compared to shape-memory
alloys and ceramics, SMPs are lightweight, relatively inexpensive, and the shape-
recovery temperature can be adjusted through modification of polymer structure or
architecture. Consequently, SMPs have received a great deal of research attention
over the past decade.4, 5 Much effort has been devoted to improving shape fixity,
increasing the recovery stress and creating engineering materials that can recover
extremely large (several hundred percent) strain.6 Other notable developments
included SMPs with multi-staged recovery 7, 8, light-induced shape-recovery9, the
ability to inductively heat particle-loaded materials using oscillating magnetic or
electrical fields 10, 11, and the tailoring of biocompatible SMPs to meet specific
biomedical needs such as sutures, stents, and catheters.12
In addition to crystallization and vitrification, dynamic transitions can also
be used to stabilize mechanically deformed elastomers. We have previously shown
that reversible hydrogen bonding can stabilize elastically deformed states.
Specifically, poly(butyl acrylate) covalent networks containing 2-ureido-4-
59
pyrimidinone (UPy) side-groups were synthesized and studied.13, 14 The UPy group
contains a linear array of four hydrogen bonding groups and undergoes self-
dimerization with extraordinarily high solution dimerization constants (Kdim ~107
M-1 in CDCl3).15 A unique feature of these networks containing UPy side-groups is
that the rate of shape-recovery is adjustable and depends on temperature and the
density of associating side-groups. Moreover, the materials behave as elastomers
both above and below the shape-memory transition temperature. Creep and
rheology experiments of these and similar dynamic networks show Arrhenius-like
temperature dependence, suggesting that mechanical relaxation is controlled by the
rate of hydrogen bond dissociation.13, 16 The integration of hydrogen bonding
groups into soft materials has been extended to titin-mimicking modular
polymers17, 18 and the concept of introducing both reversible and covalent
crosslinks has been applied to improve stress relaxation of coatings below the glass
transition temperature.19, 20
Here we demonstrate a new method of preparing shape-memory networks
that utilize reversible association to temporarily stabilize mechanically deformed
states. A series of linear polymer melts containing different amounts of reversibly
associating UPy-groups and photo-crosslinkable side-groups were synthesized and
crosslinked using UV light (see Figure 3.1). This approach offers several
advantages over the previously reported reactive-casting method.13 Since linear
polymers are prepared in solution, a greater density of hydrogen bonding side-
groups can be introduced into polymer networks, and more pronounced shape-
60
memory effects are expected. Prior to crosslinking, linear polymers can be
chemically characterized to properly deduce the composition of resulting networks.
Network precursors can be molded into complex shapes that are defined by a
transparent mold or a light pattern. Furthermore, since solvent removal is avoided,
the network is formed near the stress-free state.21
Figure 3.1 Scheme illustrating photo-crosslinking of network precursors to form
dynamic networks containing both covalent and reversibly-associating crosslinks
In this study, two different photosensitive monomers were synthesized: one
containing coumarin (2a) and the other containing benzophenone (2b). Coumarin
dimerizes upon exposure to UV light to form covalent crosslinks (a bimolecular
process)22, whereas benzophenone undergoes photolysis upon irradiation (a
unimolecular process) followed by hydrogen extraction to form crosslinks.23, 24
This manuscript addresses the efficacy of photo-crosslinking and the physical
61
properties of formed networks. The resulting dynamic mechanical properties will
be discussed and related to the density of permanent and reversible crosslinks with
the goal of assessing the degree to which dynamic hydrogen bond interactions act
like permanent crosslinks.
3.2 Methods
Materials
All chemicals were reagent-grade and used without further purification.
Butyl acrylate, 2-isocyonatoethyl methacrylate, azobisisobutyronitrile (AIBN),
triethyl amine, and deuterated chloroform (CHCl3) were purchased from Aldrich.
2-amino-4-hydroxy-6-methylpyrimidine, 7-hydroxy-4-methyl coumarin and
methacryloyl chloride were purchased from Alfa Aesar. Solvents N-
methylpyrrolidone (NMP), methanol, and dimethyl sulfoxide (DMSO) were
purchased from Alfa Aesar. Solvent tetrahydrofuran (THF), CHCl3 were purchased
from J. T. Baker. THF and CHCl3 used in this synthesis were purified using a Pure
Solve PS-MD-3 solvent purification system from Innovative Technology.
Monomer synthesis
Ureidopyrimidinone ethyl methacrylate
UPy monomer was made according to literature25 and Chapter 2.
62
4-methacryloyloxy benzophenone
Bp monomer synthesis was adopted from literature.26 In a 50 ml flask, 1.98
g (10 mmol) of 4-hydroxyl benzophenone was dissolved in 30 ml of CHCl3; 1.11 g
(11 mmol) of triethylamine was also added. Under ice water bath, 1.07 ml (11
mmol) of methacryloyl chloride was added drop wise. The reaction was kept in ice
water bath for 6 hrs. The solution was passed through a short silica gel filter, and
the filtrate solution was rotavaperated to give light yellow color product, yield
about 90%. The raw product was then further purified via column
chromatography. 1H NMR (Bruker 400) was used to determine the purity of
synthesized monomer. Spectrum is available at Appendix 2, Figure A2.1.
7-acryloyloxy-4-methyl coumarin
Coumarin monomer synthesis was adopted from literature.27 Under N2
atmosphere, 3.52 g (20 mmol) of 7-hydroxy-4-methyl coumarin and 50 ml THF
was charged in a 100 ml flask. Sodium hydride 1.2 g (30 mmol, 60% dispersed in
oil) was also added. The mixture was stirred for 4 hrs at room temperature. The
THF was then evaporated via rotavapor. The remaining yellowish solid was
redissolved in 50 ml of CHCl3. Under ice water bath, 2.15 ml (22 mmol) of
methacryloyl chloride was added drop wise. The reaction was kept in ice water bath
for 2 hrs. After evaporation of CHCl3, the remaining white solid was recrystallized
by acetone, yield about 50%. 1H NMR (Bruker 400) was used to determine the
formation of synthesized monomer. See spectrum at Appendix 2, Figure A2.2.
63
Polymerization of linear macromers (2a, 2b)
A typical polymerization procedure to prepare macromer 2a will be briefly
described. In an air-free reaction flask, 73 mg (0.3 mmol) of 7-acryloyloxy-4-
methyl coumarin, 164 mg (0.6 mmol) of ureidopyrimidinone ethyl methacrylate
and 3.84 g (30 mmol) of butyl acrylate were charged. 4.8 mg of
azobisisobutyronitrile (AIBN) was added as initiator; and 30 ml of CHCl3 was
added as solvent. After the mixture was degassed by N2 bubbling for 0.5 hr, the
flask was immersed into a 75 °C oil bath and reacted overnight. The resulting
polymer solution was precipitated in methanol, yielding a viscous liquid, yield =
80%. An analogous procedure was followed to prepare macromers 2b using 4-
methacryloyloxy benzophenone as a reagent. 1H NMR (Bruker 400) was used to
determine the chemical compositions of the synthesized macromers. Molecular
weight and polydispersity were measured by GPC (Agilent 1100) using THF as an
eluent. Example of 1H NMR spectrum determining composition is available ar
Appendix 2, Figure A2.3, other NMR spectra and GPC traces are shown in
Appendix 2, Figure A2.4 – A2.12.
Photo-crosslinking of macromer films
Macromer solutions (~5 wt%, in chloroform) were cast onto quartz slides
using a Teflon spacer with a 40 × 10 mm window. After solvent removal, the
thickness of resulting films was about 0.5 mm. Films were placed into vacuum
oven and dried overnight at 60 C. Irradiation was conducted in a nitrogen glove
64
box to avoid oxygen inhibition. Samples were irradiated with a UV lamp (UVLM-
28) with a measured intensity of 5 mW/cm2 at 365 nm for 10 min increments. After
each exposure, samples were held for one hour to ensure free radical intermediates
were consumed. The procedure was repeated until free radical intermediates were
no longer observed using ultraviolet spectroscopy. Typically this required a total
exposure time of 60 min.
Linear Poly (butyl acrylate) (PBA) UV degradation
UV irradiation on linear PBA was conducted to determine whether the UV
irradiation procedure resulted in degradation of linear macromers. Poly (butyl
acrylate) (PBA) homopolymers were prepared in a manner consistent with that
used to produce copolymer samples. Disc-shaped samples, 11 mm in diameter and
2.3 mm thick, were solvent-cast (i.e. 150 mg PBA / ml THF) stage wise into glass
vials followed by vacuum drying. UV Irradiation was conducted using a
Spectroline 400 (400W lamp with 320 – 400 nm spectral distribution). The source-
to-sample distance was altered in order to achieve a flux of 18.0 +/-1.5 mJ/cm2 at
365 nm (measured through the top of the glass cylinder using a hand-held
radiometer). Specimens were placed in an evacuated glass cylinder that was
subsequently filled with argon gas. Samples were then exposed for a specified
period of time. Following each exposure, the vessel was removed from the UV
source, the glass chamber opened up, and a sample removed. The glass chamber
was resealed and evacuated as described above and placed back into the chamber
65
for continued irradiation. This process was repeated for several time intervals.
Sample molecular weight and polydispersity were measured using GPC at different
total dosage.
Swelling and Gel fraction measurement
Weight and volume change were measured in triplicate for 0.5 mm thick
specimens with an initial area measuring roughly 5 × 10 mm. Specimens were
individually exposed to 15 ml of isopropyl alcohol (23 °C/ 48 hours) and the
sample mass and volume change recorded. Volume-swell is calculated according to
the following relationship:
11
Dry
Swell
s
p
MM
Q
where Q is the volume swell
(i.e. ratio of volume of swollen polymer to volume of dry polymer), s and p are
the solvent and polymer densities, respectively, and MDry and MSwell are the
measured masses in the dry and swollen states, respectively. Specimens were
subsequently dried in a vacuum oven at 80 °C for 48 hours and weighed to
determine gel fraction, expressed as the ratio of the final dry mass to the initial
mass.
Dynamic Mechanical Testing
Photo-cured samples were cut into 6 mm × 10 mm films, and dynamic
mechanical analysis was performed using a solid-state rheometer (Rheometrics,
RSA-2). Frequency sweeps from 0.01 to 100 rad/s, at 2% strain, were acquired at
66
temperatures ranging from 30 C to 100 C at 10 C increments. Storage modulus
E’, loss modulus E” and tan were recorded at each experimental frequency and
temperature. Data were collected and analyzed using a commercial software
package (TA Orchestrator v7).
Time-temperature superposition (TTS) to obtain master curves
TTS enables properties measured over a range of temperatures and
frequencies to be shifted to represent a wider range of frequencies for a single
temperature. Typically, this is done by graphing the viscoelastic data taken at many
different temperatures versus frequency or time on one log-scale plot. Each curve at
a different temperature is then shifted horizontally by a shift factor aT along the
frequency axis until it overlaps with the curve at an arbitrary reference temperature.
The end result of this process is a single curve that represents viscoelastic behavior
of the polymer versus frequency or time at the chosen reference temperature, over a
much wider range of frequencies than originally measured. A program was written
(Igor Pro, 6.2) that employed a non-linear regression routine to yield shift-factors
for each of the provided temperature curves.
3.3 Discussion
3.3.1 Synthesis of photo-crosslinkable macromers
The characteristics of synthesized poly(butyl acrylate) macromers
containing both UPy and photo-crosslinkable side-groups are summarized in Table
67
3.1. The sample name specifies the mol % of the functional monomers in the feed:
coumarin (Cm), benzophenone (Bp), and ureidopyrimidinone (UPy). Macromers
could be prepared with UPy-contents exceeding 10 mol %, and this is significantly
higher than the UPy-content of prior shape-memory elastomers (~2 mol %).13
Statistical copolymer compositions determined using 1H NMR, agree fairly well
with experimental feed compositions. Molecular weights, determined using GPC,
ranged from 38 to 104 kDaltons—note that all copolymers exceed the entanglement
molecular weight (~ 25 kDaltons) of poly(butyl acrylate).28 The polydispersity
indexes shown in Table 1 are lower than expected. It is suspected that this results
from the removal of low molecular weight species during reprecipitation. For
samples with high UPy-content, the polydispersity index was generally higher. For
these samples, the presence of intermolecular UPy-UPy association may preclude
the removal of low molecular weight species.
For coumarin-containing macromers, on average, each copolymer chain
contains between 3-4 coumarin moieties; and, for Bp-containing macromers, each
copolymer chain contains between 2-6 benzophenone moieties. Only a fraction of
available functional groups must be crosslinked to form an incipient gel. In the
absence of cycle formation, gelation occurs when the average number of formed
crosslinks per primary chain exceeds pw/( pw -1) where pw is the weight average
degree of polymerization. For high molecular weight polymers, pw is large, and,
therefore, on average, each chain requires just over one crosslink to form an
incipient gel.29
68
Table 3.1 Molecular weight and composition of photo-crosslinkable macromers.
Crosslinker
content
UPy content
Macromer
Feed
(mol %)
Meas.a
(mol %)
Feed
(mol %)
Meas.a
(mol %)
xx
xUPy
Mn b
PDI b
Coumarin-containing
Cm1 1.0 1.20 0 0 3.5 0 38,000 1.70
Cm1-UPy2 1.0 1.30 2.0 2.2 4.0 6.7 40,000 1.65
Benzophenone-containing
Bp1 1.0 0.70 0 0 5.6 0 104,000 1.14
Bp1-UPy1 1.0 0.78 1.0 0.6 5.8 4.4 96,400 1.21
Bp1-UPy2 1.0 0.77 2.0 1.7 5.9 12 98,800 1.22
Bp1-UPy5 1.0 1.10 5.0 6.4 3.5 19 42,500 1.82
Bp1-UPy10 1.0 0.86 10.0 11.8 3.6 49 61,400 1.77
Bp½-UPy2 0.5 0.54 2.0 2.1 2.6 10 62,100 1.39
Bp2-UPy2 2.0 1.65 2.0 2.5 5.4 8.2 43,000 1.72
a) 1H NMR, b) GPC, c) xXL & xUPy refer to average number of photo-crosslinkable (Cm or Bp) and reversibly-
binding (UPy) side-groups per macromer chain, respectively.
69
Macromer Bp1-UPy10 appeared to be a glassy polymer after precipitation
and vacuum-drying. Solvent-casting resulted in stress accumulation and cracking
during solvent removal; and macromer Bp1-UPy10 was not photo-crosslinked.
Further efforts to prepare quality Bp1-UPy10 films for mechanical testing may
involve slow solvent removal and thermal annealing.
3.3.2 Photo-crosslinking of macromer films
Coumarin-containing macromers
The UV-Vis spectrum of a 30 µm-thick film of macromer Cm1 during
irradiation (302 nm light, 5 mW/cm2) is displayed in Figure 3.2. Initially, the film
exhibited characteristic absorptions at 275 nm and 312 nm. The intensity of both
peaks decreased during irradiation. This is consistent with the [2 + 2] photo-
cycloaddition of nearby coumarin side-groups to form cyclobutane dimers.22, 30
(scheme shown in Figure 3.3) The spectra further indicate that after four hours of
exposure more than 80 % of coumarin side-groups had reacted. In addition,
irradiated films were insoluble in chloroform, confirming that chemical crosslinks
had formed.
70
Figure 3.2 Photo-crosslinking of coumarin-containing macromers, UV-Vis spectra
of coumarin containing macromer Cm1 thin film (~ 30 µm) during irradiation
O
O
O
O
O
O
O
O
O
O
O
O
300 nm
254 nm
Figure 3.3 Scheme showing photoreversible crosslinking of nearby coumarin side-
groups.
71
A thin film (~10 m thick) was irradiated to test the reversibility of the
coumarin photo-reaction. Figure 3.4 shows two coumarin addition-cleavage cycles
using 302 nm and 254 nm light, respectively. Upon irradiation with 302 nm light,
photo-addition occurred, and the coumarin absorption peak decreased. Subsequent
irradiation with 254 nm UV light resulted in photo-cleavage of dimers and a
recurrence of the coumarin absorption peak. The efficiency of photo-cleavage
appeared to decrease from one cycle to the next, with ~ 60% of the coumarin
groups cleaving as compared to the previous cycle.
While thin films could be easily crosslinked by UV exposure, efforts to
photo-crosslink thick films were unsuccessful. For example, a 500 m thick film
was still not completely crosslinked even after a high dose of UV irradiation (6 hrs
at 300-320 nm, 12 mW/cm2). The absorption at 312 nm was nearly that of the
uncrosslinked film, and most of the film remained soluble in chloroform.
Incomplete curing is attributed to the relatively high molar attenuation coefficient
of coumarin (6 105 m2/mol, at 312 nm) and, hence, the inability of desired
wavelengths to penetrate far enough into the film. An insoluble skin-layer was
observed, and this is consistent with incomplete penetration of UV light through the
film. In principle, higher doses should fully cure the films, however, at the
intensities used in this study, overdosing leads to chemical degradation. Future
efforts to photo-cure thick films may involve irradiation at lower intensities over
longer times, or the use of higher molecular weight macromers with just enough
coumarin side-groups to ensure the formation of a connected network.
72
Figure 3.4 Photo-reversibility of coumarin crosslinking in a macromer thin film,
the absorbance peak at 312 nm was plotted against UV irradiation time where
circles represent 302 nm irradiation, and dots represent 254 nm irradiation
Nagata et al. performed similar studies on poly(-caprolactone)s containing
pendent coumarin groups, and reported that photo-crosslinking of thicker films
(200 m) resulted in higher gel fractions, more pronounced shape-memory
properties and lower degradation rates.30 The present study raises the possibility
that these films are also incompletely crosslinked; the high gel fractions reported
could be explained by the presence of a skin layer that blocks mass transport of
polymer out of the photocured film. This possibility is also consistent with the
rather low modulus (~ MPa) observed by Nagata et al. in their crosslinked films.
0.65
0.60
0.55
0.50
0.45
Absorb
ance a
t 312 n
m
20151050
Time (min.)
302 nm irradiaiton 254 nm irradiation
73
The incorporation of both photo-crosslinkable coumarin and UPy side-
groups into butyl acrylate elastomers represents an important step in developing
light tunable shape-memory elastomers. Lendlein et al. showed that polymers
containing both cinnamic side-groups and permanent covalent crosslinks can be
deformed and fixed using UV light and, subsequently, shape-recovered using a
different wavelength.9 If coumarin or other photoreversible dimers are used as the
only source of covalent crosslinks, and the UPy groups are used as reversible
crosslinks, then the photo-reversibility of coumarin dimerization offers a way to
change a shape-memory elastomer’s ―permanent‖ shape. For example, irradiation
with the longer wavelength can be used to define a material’s permanent shape, and
irradiation with the shorter wavelength (254 nm) can be used to convert the
elastomer back to a deformable melt. If, on the other hand, light-insensitive
crosslinks are also present, then the use of both coumarin and UPy side-groups
offers two independent, non-interfering mechanisms for shape pinning: temperature
and light. This will enable triple-stage shape-memory elastomers that can be fixed
by cooling or irradiation and can be recovered by heating or irradiation at a
different wavelength.
Benzophenone-containing macromers
Macromers containing benzophenone side-groups were cast into films and
irradiated with a lamp measuring 5 mW/cm2 at 365 nm. Hydrogen abstraction of
benzophenone in polymer matrices has been well studied.23, 24, 31 Through a two-
74
photon process, the benzophenone group is excited to an n* triplet state that
abstracts a proton from the surrounding polymer matrix, forming a radical pair.
The resulting ketyl radical recombines with the formed radical on the polymer
matrix, resulting in a light-absorbing transient (LAT). The possible and simplified
reaction scheme is shown in Figure 3.5. The LAT has a distinct, long-lived UV
absorption signature, and it further reacts to form stable photo-products that are
covalently bound to the surrounding polymer matrix.
O
O
365 nm
H OH
O
*
*
O
OH
LAT
Figure 3.5 Possible reaction scheme of benzophenone photo-reaction
Absorption spectra of macromer films, displayed in Figure 3.6, were
obtained before and after UV exposures. Exposures were conducted in sequential
1, 2, 5 and 10-minute periods. A characteristic absorption band at 333 nm was
observed following exposure and is attributed to the LAT. The molar attenuation
coefficient corresponding to the LAT band is 1.2 × 105 m2/mol, and this is about 4
times that of the initial Bp coefficient. The observed band is similar to earlier
75
reports of free benzophenone in substituted poly(methacrylate)s.23, 24 Following
each exposure, films were held for a minimum of one hour to allow photochemical
intermediates to decay into stable photo-products. The decay of the LAT band at
333 nm was studied, and the observed absorbance is plotted against time in Figure
3.7. The half-life of this decay is about 8 minutes, and the rather high rate of decay
is attributed to the low rigidity and low microviscosity of the poly(butyl acrylate)
network.24 The initial absorbance of the LAT band decreased following each
irradiation period, indicating a reduction in the number of unreacted benzophenone
side-groups. UV exposures (ten minute periods) were repeated until the LAT band
was no longer observed, suggesting the film is fully cured. This typically required
about eight exposure-decay cycles.
Figure 3.6 Photo-crosslinking of benzophenone-containing macromer Bp½-UPy2,
UV-Vis spectra taken immediately after 10 minute periods of UV periods—the
absorption at =333 nm indicates photo-crosslinking is incomplete
76
Figure 3.7 Photo-crosslinking of benzophenone-containing macromer Bp½-UPy2,
the absorbance maximum (at 333 nm) of the LAT decays following the end of each
UV exposure period
To ensure that UV exposure does not damage the poly(butyl acrylate)
backbone, homopolymers were exposed to doses (10-100 J/cm2) well beyond those
used to cure benzophenone-containing polymers (~10-30 J/cm2). Irradiated
samples’ molecular weight and polydispersity were measured using GPC as a
function of total dosage, and the results are presented in Figure 3.8. As can be seen
from Figure 3.8, even after an irradiation dose of 100 J/cm2, the number average
molecular weight of poly (butyl acrylate) decreased by only about 10%. This result
indicated that the UV crosslinking procedure should not cause a severe damage to
the macromer chain.
77
Figure 3.8 Influence of cumulative UV irradiation on weight average molecular
weight and polydispersity index (DPI).
Equilibrium volume-swell and gel fraction of crosslinked elastomers were
measured by swelling in isopropanol followed by vacuum drying, and the results
are presented in Table 3.2. Gel fractions all exceeded 90 %, indicating the majority
of chains were fixed to the crosslinked network. As expected, the gel fraction
tended to be higher for samples with a high number of benzophenone side-groups
per chain (xx). For example, for Bp1-UPy1, xx = 5.8, and for Bp½-UPy2, xx = 2.6.
The volume swell did not appear to differ significantly for most of the samples
investigated. A plot of volume swell versus strand density is provided in Appendix
2, Figure A2.13.
10000
8000
6000
4000
2000
0
Mw
(D
alton)
100806040200
Dosage (J/cm2)
2.4
2.2
2.0
1.8
1.6
DP
I
78
Table 3.2 Gel fractions of UV-cured elastomers determined from swelling in
isopropyl alcohol.
Data reported as mean one standard deviation.
The shape-memory response of a typical sample (Bp½-UPy2) is shown in
Figure 3.9. A flat, ribbon-like specimen (Figure 3.9a) was first wrapped around a
mandrel at a sufficiently high temperature (65 °C) followed by submerging both the
sample and mandrel into an ice water bath for approximately one minute. The
sample, now programmed into a corkscrew shape (Figure 3.9b), was then removed
from the mandrel and blotted dry. At room temperature the programmed sample
could be sustained over short times (Figure 3.9c), and the permanent shape
appeared fully recovered after 48 hours. Further, it was demonstrated that the
shape-recovery time could be dramatically reduced by submersion of the
Gel fraction Q
Bp1 98+1.9 2.5+0.17
Bp1-UPy1 100+1.7 2.5+0.02
Bp1-UPy2 98+1.5 2.4+0.09
Bp1-UPy5 93+1.5 1.7+0.04
Bp½-UPy2 91+2.0 2.5+0.04
Bp2-UPy2 96+2.0 1.7+0.06
79
programmed sample into hot water (65 °C) where the recovery process occurred in
less than five seconds (Figures 3.9d-f). The strong temperature dependence of the
shape-recovery process is not surprising and is a direct consequence of hydrogen
bond exchange rate between temporary crosslinks.13 Dynamic mechanical analysis
(DMA) will be discussed next to more fully characterize the thermo-mechanical
behavior that gives rise to shape-memory effects.
Figure 3.9 Demonstration of shape-memory response in Bp½ –Upy2: a)
permanent shape (30 2 1 mm); b) programmed, temporary shape immediately
after forming; c) temporary shape after one minute in room temperature air; d)
temporary shape immediately following immersion in hot water (ca. 65C); e)
temporary shape after one second immersion in 65C water; and f) return to
permanent shape occurs in approximately three seconds in 65C water.
80
3.3.3 Dynamic Mechanical Analysis of Dynamic Networks
Elastic Energy Storage
Photo-crosslinked benzophenone films were subjected to dynamic
mechanical analysis. Frequency sweeps were performed at different temperatures,
and resulting storage modulus measurements were shifted to yield the master
curves. Figure 3.10 showed a sample original DMA storage modulus data and its
time-temperature superposition (TTS) shifted master curve. All other individual
original storage modulus data and shifted master curves for different samples are
available in Appendix 2, Figure A2.14 - 2.17.
All TTS shifted master curves were shown in Figure 3.11. Samples with up
to 2 mol % UPy content displayed two distinct plateaus (Figure 3.11a). The lower
plateau modulus observed at low frequencies (10-4 to 10-2 Hz), corresponds to a
network of covalent bonds created during photo-crosslinking. Evidently, the
modulus of this plateau is proportional to the benzophenone content. The upper
plateau, observed at higher frequencies (10-102 Hz) corresponds to the overall
crosslink density—including both photo-crosslinked net points and reversible net
points arising from the UPy dynamic network (Figure 3.11a). The highest storage
modulus plateau of about 2.5 MPa was observed for Bp2-UPy2, bearing ~2 mol %
of UPy and Bp side-groups.
81
a)
b)
Figure 3.10 DMA storage modulus of elastomer Bp1/2-UPy2, a) original data, b)
TTS shifted master curve at reference temperature 60 °C
4
5
6
7
8
9
106
E' (
Pa)
0.01 0.1 1 10
Freq (Hz)
30°C--------------100°C
4
5
6
7
8
9
106
E' (
Pa)
10-4 10
-2 100 10
2
Freq (Hz)
82
a)
b)
Figure 3.11 Storage modulus master curves for photo-crosslinked elastomers: a)
Bp½-UPy2, Bp1-UPy1, Bp1-UPy2, and Bp2-UPy2; and b) Bp1-UPy5. All data
have been time-temperature superimposed to a 60 C reference temperature.
6
106
2
3
4
5
6
107
2
E' (
Pa)
10-6 10
-4 10-2 10
0 102
Freq (Hz)
BP1-UP5
83
At higher UPy-contents, samples exhibit significantly higher storage
modulus at all frequencies examined (see Figure 3.11b). The storage modulus of
Bp1-UPy5 at 1 Hz was about 8 MPa, nearly an order of magnitude higher than
samples with lower UPy content at the same conditions. Moreover, the storage
modulus for Bp1-UPy5 increased strictly with frequency, over the range studied,
and plateaus were not observed. The same hydrogen-bond transition is observed,
however it occurs at lower frequencies and it spans over a much broader range of
frequencies. At low frequencies the data appear to approach a plateau modulus
similar to other samples with similar Bp content. The shift and broadening of the
hydrogen-bond transition is attributed to cooperative dynamics which will be
discussed later.
The classical theory of rubber elasticity can be applied to further understand
how covalent and reversible crosslinks affect storage modulus. Affine deformation
of an ideal, incompressible elastomer results in a stress given by:
21
11
nRT (3.1)
where n is the number of strands per unit volume, R is the gas constant, T is
temperature on an absolute scale, and is strain. In the limit of small strain,
Hooke’s law is valid, and the ratio of stress to strain (Young’s modulus) becomes:
nRTE 3 (3.2)
84
Thus, neglecting temperature dependence and chain-ends, each plateau modulus in
Figure 3.11 corresponds to a different strand density n (shown in Figure 3.12).
Figure 3.12 Bar chart comparison between strand densities corresponding to high
and low plateaus of storage modulus for different compositions
The strand density corresponding to the lower plateau is plotted against the
measured benzophenone concentration, i.e. the maximum possible crosslink
density, in Figure 3.13. The data are fairly linear, and a least-squares fit through the
origin results in a slope of ~ 1.4. According to the classical theory of rubber
elasticity, if every benzophenone formed a tetrafunctional crosslink, then each
crosslink would contribute two new strands to the network, and therefore a slope of
two in Figure 3.13 is expected. The difference between the observed and
85
theoretical slopes may arise from the following factors: (1) chain ends, which do
not contribute to stored elastic energy, are present but are neglected; (2) chain
connectivity is ignored; and (3) not all benzophenone side-groups may have reacted
to form interchain crosslinks. In light of these considerations, the data confirm
that, upon irradiation, benzophenone groups successfully form chemical crosslinks
that influence mechanical properties in an expected and predictable way.
Figure 3.13 Plot of strand density obtained from high temperature plateau modulus
versus concentration of permanent crosslinks based on Bp content
To examine whether reversible crosslinks also behave as net points, the
strand density corresponding to the upper plateau modulus is plotted against the
overall crosslink density in Figure 3.14. Since two UPy groups can dimerize to
86
form a single interchain crosslink, the overall crosslink density is taken as the sum
of the density of Bp side-groups and one-half the density of UPy side-groups. This
choice of the abscissa results in a least-squares slope of 1.22 and a coefficient of
determination of r2 = 0.87. Plotting the same strand density against UPy density
alone yields a poor correlation (r2 = 0.46). Thus, the data indicate that both
permanent and reversible crosslinks contribute to the modulus at high frequency.
Figure 3.14 Plot of strand density obtained from low temperature plateau modulus
versus total crosslink density based on Bp and UPy content
The fact the slope in Figure 3.14 is less than that in Figure 3.13 indicates
that UPy dynamic crosslinks are not as effective as Bp permanent crosslinks. To
87
further isolate the effectiveness of UPy crosslinks, the data in Figure 3.14 were
corrected by subtracting strand density contributions from permanent crosslinks.
Figure 3.15 displays the corrected strand density that is attributed to only UPy net-
points versus UPy crosslink density.
Figure 3.15 Plot of strand density attributed to UPy net-points versus UPy
crosslink density.
Least squares fitting results in a slope of 1.02, and, comparing this to the
slope in Figure 3.13 suggests that, UPy crosslinks are about 70% as effective as
covalent crosslinks at these concentrations. Furthermore, the data in Figure 3.15 are
not linear. The slope increases with increasing UPy-content, indicating that UPy
crosslinks become more effective by increasing their concentration. This effect is
attributed to cooperative dynamics, as will be discussed later.
88
Viscous Dissipation of Energy
In addition to the enhancement of the storage modulus, UPy side-groups
also significantly impact the materials’ damping properties. Like the storage
modulus, sample DMA original data for tan and TTS shifted master curve were
shown in Figure 3.16. All other individual original storage modulus data and
shifted master curves for different samples are available in Appendix 2, Figure
A2.18 - 2.21.
Again, tan master curves for all photo-crosslinked samples containing
UPy side-groups were put in Figure 3.17. As one can see from Figure 3.17, both the
magnitude and peak frequency of the loss-tangent depend on UPy-content. As
UPy-content increases, the damping peak shifts to lower frequencies (Figure 3.18).
At high frequencies less damping occurs because UPy dissociation events are too
slow compared to the imposed strain frequency. At low frequencies the opposite
occurs, and UPy bond dynamics are too fast to store and dissipate energy. The
maximum viscous dissipation of energy occurs when the rate of chain relaxation,
influenced by hydrogen bond dynamics, nearly matches the experimental
frequency. This frequency is nearly coincident with the inflection point of the
storage modulus curves in Figure 3.11.
89
a)
b)
Figure 3.16 DMA tan of elastomer Bp1/2-UPy2, a) original data, b) TTS shifted
master curve at reference temperature 60 °C
0.4
0.3
0.2
0.1
Ta
n
0.01 0.1 1 10
Freq (Hz)
30°C--------------100°C
0.4
0.3
0.2
0.1
Ta
n
10-4
10-3
10-2
10-1
100
101
Freq (Hz)
90
Prior studies have indicated that hydrogen bonding events in transient
networks are correlated, resulting in slower dynamics for networks with higher
concentrations of associating groups.13, 16, 32 The correlated dynamics of UPy
binding may explain the shift in the loss-tangent to lower frequencies with
increasing UPy content observed in this study .
Figure 3.17 The influence of UPy-content on damping properties of photo-
crosslinked elastomers: tan δ master curves (using a reference temperature of 60
C) of elastomers with different compositions
91
Figure 3.18 The influence of UPy-content on damping properties of photo-
crosslinked elastomers: tan δ peak frequency (at 60 °C) vs. UPy content
Figure 19 indicates that the magnitude of the loss-tangent peak is
proportional to the molar ratio of UPy to Bp functional groups. UPy hydrogen
bond dynamics influence the rate of chain relaxation by providing an additional
mechanism to absorb energy. Enhanced frictional energy loss is attributed to
continuous breaking and reforming of hydrogen bonds during the chain relaxation
process giving rise to the observed relationship between tan and the number
density of UPy groups. On the other hand, an increase in Bp concentration will
yield an increase in storage modulus, which serves to reduce tan δ at a given UPy
concentration. The ability of the UPy-groups to increase material stiffness while
92
also increasing the level of viscous energy dissipation provides an exception to the
engineering trade-off between material stiffness and loss.33
Figure 3.19 The influence of UPy-content on damping properties of photo-
crosslinked elastomers: the magnitude of the peak in tan δ versus the ratio of
reversible to chemical crosslinks.
Cooperativity of UPy Dynamics
UPy bond dissociation leading to mechanical stress relaxation is a
thermally-activated process that exhibits an Arrhenius-dependence on
temperature.13, 14, 16 Figure 3.20 shows how the shift factor, obtained when
superimposing storage modulus curves, depends on inverse temperature for each
dynamic network. As expected, the data are linear, confirming that UPy-
93
dissociation leads to a loss of elastically stored energy and is thermally-activated.
The observed linearity is consistent with that observed in melts of random and
triblock copolymers containing UPy side-groups.16
Figure 3.20 Shift factors for UPy-containing elastomers determined from time-
temperature superposition of storage modulus using a reference of 60 °C. Data sets
were shifted vertically to avoid overlap.
The activation energies obtained from least-squares fits to the data in Figure
3.20 are plotted versus UPy-content in Figure 3.21. The activation energy depends
linearly on UPy-content indicating that UPy dissociation dynamics are correlated.
As a gauge for interpretation, the activation energy of UPy dissociation in
4
2
0
-2
-4
log
(a
T )
3.3x10-33.23.13.02.92.82.7
1/T (K-1
)
Bp1-UPy1 Bp1-UPy2 Bp½-UPy2 Bp2-UPy2 Bp1-UPy5
94
chloroform determined using temperature-dependent NMR Exchange Spectroscopy
is 70 ± 2 kJ/mol.15 The energies in Figure 3.21 can be rescaled by this experimental
value to yield a cooperativity factor, z (right-hand ordinate) that represents the
average number of cooperative dissociation events required for an incremental loss
in stored elastic energy. As a comparison, activation energies of UPy-containing
random copolymers are also included in Figure 3.21.16 The comparison shows that
the activation energy is more sensitive to UPy-content in crosslinked networks than
in linear copolymers. Thus, covalent crosslinking is an effective way to support the
cooperative dynamics and bonding of reversibly associating groups.
Figure 3.21 Plot of activation energies calculated from storage modulus shift factor
versus measured UPy-content in photo-crosslinked elastomers.
95
3.4 Summary
A photo-crosslinking approach to preparing shape-memory elastomers
bearing reversibly associating groups was demonstrated. Unlike solution-based
approaches, photo-crosslinking is advantageous because: (i) macromer precursors
can be thoroughly characterized using NMR and GPC techniques; (ii) the
crosslinking process is solventless—avoiding stress accumulation that arises from
solvent removal steps; (iii) a much greater fraction of hydrogen bonding side-
groups can be achieved due to favorable solubility of the macromer, and (iv) the
technique provides the ability to tune the number density of both permanent and
reversible crosslinks. Synthesis involved conventional free radical co-
polymerization of butyl acrylate, a monomer containing a photo-reactive coumarin
or benzophenone group, and a monomer containing the UPy side-group.
Benzophenone-containing macromers are readily crosslinked upon UV exposure,
and 500 micron-thick films were nearly completely crosslinked. Coumarin-
containing macromers could only be crosslinked to form thin elastomer films (~ 30
um) because coumarin’s high extinction coefficient the majority of irradiation was
absorbed near the surface. Upon exposure of thin coumarin-containing crosslinked
films to higher energy (254 nm) UV light, partial (~60%) photo-cleavage of
coumarin dimers was observed.
To understand how network architecture and reversible binding affects
mechanical properties, photo-crosslinked elastomers containing benzophenone
96
side-groups were prepared with varying number density of permanent and dynamic
crosslinks. Dynamic mechanical analysis revealed two plateaus in the storage
modulus master curves. A high-temperature plateau was attributed only to the
permanent network, and the low-temperature plateau was attributed to both
permanent and reversible crosslinks. Moreover, a maximum in the loss tangent
was observed that depends strongly on the UPy content. Higher UPy contents, and
lower Bp contents, increased the magnitude of the damping (tan δ) peak, while also
enhancing the materials stiffness. Activation energies could be calculated from the
temperature-dependence of shift factors obtained from time-temperature
superposition of storage modulus curves. The activation energy was found to
increase with UPy content, and this is consistent with cooperative dynamics of UPy
binding. Finally, by comparing measured activation energies to those of linear
UPy-containing polymers, the UPy binding effectiveness is clearly enhanced by
covalent crosslinks. Thus, in addition to providing mechanical support, covalent
networks may be engineered to reinforce internal, complementary binding, and this
idea may open new approaches to engineering shape-memory, self-healing, and
other stimuli-responsive materials.
In this chapter, a new photo-crosslinking approach to make shape-memory
elastomers containing UPy hydrogen bonding side-groups was presented. This
method enables melt-processing of shape-memory elastomers into complex
permanent shapes, and samples can be prepared with much higher UPy-content.
Moreover, compositions can be well characterized in linear polymer stage.
97
Therefore, this new synthetic approach really solves the difficulty of incorporating
strong hydrogen bonding groups inside polymer network. Furthermore, it makes
the systematic study of how hydrogen bonding dynamic affects mechanical
behavior of elastomers possible. Some findings include (1) dynamic crosslinks
behave nearly as effectively as permanent crosslinks below the UPy hydrogen bond
transition, (2) UPy hydrogen interactions are working in a cooperative fashion and
the presence of a covalent network enhances this cooperativity, (3) the addition of
UPy side-groups to polymer network not only enhances the material toughness, but
improves its damping properties. In a broad sense, those new understandings of
hydrogen bond dynamics could provide guidelines for new supramolecular material
development.
98
References
1. C. Liu, H. Qin and P. T. Mather, J Mater Chem, 2007, 17, 1543-1558.
2. P. T. Mather, X. F. Luo and I. A. Rousseau, Annu Rev Mater Res, 2009, 39,
445-471.
3. Q. H. Meng and J. L. Hu, Compos Part a-Appl S, 2009, 40, 1661-1672.
4. M. Behl, M. Y. Razzaq and A. Lendlein, Adv Mater, 2010, 3388-3410.
5. P. T. Mather, X. Luo and I. A. Rousseau, 2009, 39, 445-471.
6. W. Voit, T. Ware, R. R. Dasari, P. Smith, L. Danz, D. Simon, S. Barlow, S. R.
Marder and K. Gall, Adv. Funct. Mater., 2010, 20, 162-171.
7. X. F. Luo and P. T. Mather, Adv. Funct. Mater., 2010, 20, 2649-2656.
8. T. Xie, Nature, 2010, 464, 267-270.
9. A. Lendlein, H. Y. Jiang, O. Junger and R. Langer, Nature, 2005, 434, 879-882.
10. H. Koerner, G. Price, N. A. Pearce, M. Alexander and R. A. Vaia, Nature
Materials, 2004, 3, 115-120.
11. R. Mohr, K. Kratz, T. Weigel, M. Lucka-Gabor, M. Moneke and A. Lendlein,
Proceedings of the National Academy of Sciences of the United States of
America, 2006, 103, 3540-3545.
12. W. Small, P. Singhal, T. S. Wilson and D. J. Maitland, J Mater Chem, 2010, 20,
3356-3366.
13. J. Li, J. A. Viveros, M. H. Wrue and M. Anthamatten, Adv Mater, 2007, 19,
2851-2855.
99
14. J. Li, K. D. Sullivan, E. B. Brown and M. Anthamatten, Soft Matter, 2010, 6,
235-238.
15. S. H. M. Soentjens, R. P. Sijbesma, M. H. P. van Genderen and E. W. Meijer, J.
Am. Chem. Soc., 2000, 122, 7487-7495.
16. K. E. Feldman, M. J. Kade, E. W. Meijer, C. J. Hawker and E. J. Kramer,
Macromolecules, 2009, 42, 9072-9081.
17. A. M. Kushner, J. D. Vossler, G. A. Williams and Z. B. Guan, J. Am. Chem.
Soc., 2009, 131, 8766-8768.
18. A. M. Kushner, V. Gabuchian, E. G. Johnson and Z. B. Guan, J. Am. Chem.
Soc., 2007, 129, 14110-14111.
19. J. L. Wietor, A. Dimopoulos, L. E. Govaert, R. A. T. M. van Benthem, G. de
With and R. P. Sijbesma, Macromolecules, 2009, 42, 6640-6646.
20. A. Dimopoulos, J. L. Wietor, M. Wubbenhorst, S. Napolitano, R. A. T. M. van
Benthem, G. de With and R. P. Sijbesma, Macromolecules, 2010, 43, 8664-
8669.
21. L. F. Francis, A. V. McCormick, D. M. Vaessen and J. A. Payne, J Mater Sci,
2002, 37, 4717-4731.
22. S. R. Trenor, A. R. Shultz, B. J. Love and T. E. Long, Chemical Reviews, 2004,
104, 3059-3077.
23. C. Braeuchle, D. M. Burland and G. C. Bjorklund, J. Phys. Chem., 1981, 85,
123-127.
24. F. W. Deeg, J. Pinsl and C. Braeuchle, J. Phys. Chem., 1986, 90, 5715-5719.
100
25. K. Yamauchi, J. R. Lizotte and T. E. Long, Macromolecules, 2003, 36, 1083-
1088.
26. R. Toomey, D. Freidank and J. Ruhe, Macromolecules, 2004, 37, 882-887.
27. Y. Chen and J. L. Geh, Polymer, 1996, 37, 4473-4480.
28. A. Zosel and G. Ley, Macromolecules, 1993, 26, 2222-2227.
29. K. Dusek, in Polymer Networks: Principles of Their Formation Structure and
Properties, ed. R. F. T. Stepto, Blackie Academic & Professional, London,
1998, pp. 64-92.
30. M. Nagata and Y. Yamamoto, Journal of Polymer Science Part a-Polymer
Chemistry, 2009, 47, 2422-2433.
31. J. Chilton, L. Giering and C. Steel, J. Am. Chem. Soc., 1976, 98, 1865-1870.
32. D. M. Loveless, S. L. Jeon and S. L. Craig, J Mater Chem, 2007, 17, 56-61.
33. R. S. Lakes, J Compos Mater, 2002, 36, 287-297.
130
Chapter 5 Conclusions and Future Work
5.1 Conclusions
Discovery of shape-memory effect introduced by strong hydrogen bonding
interactions
Strong hydrogen bonding interactions were shown to be capable of “pinning”
the material into a temporarily deformed state, giving rise to a novel shape-memory
effect inside of crosslinked elastomers. Through a one-step solution process,
ureidopyrimidinone (UPy) side-groups were successfully incorporated into a
covalently crosslinked poly (butyl acrylate) network. Following removal of the
solvent, the resulted elastomer exhibited shape-memory properties due to UPy
hydrogen bond dynamics.
By comparing thermomechanical behavior of UPy-containing networks to
elastomer samples without UPy side-groups, shape-memory effects were confirmed
to result from hydrogen bonding interactions. At higher temperatures (above 65 °C),
UPy hydrogen bonds dissociate very fast, and, when stressed, the elastomer easily
deforms. On the other hand, at lower temperatures (~ 0 °C), the UPy hydrogen
bond dynamics are very slow, and when the elastomer is cooled while under strain,
it remains deformed even after stress is removed. Here, the dynamic network holds
the mechanically-strained state by forming strong hydrogen bonds. Upon
subsequently heating the elastomer, it returns to its original shape spontaneously
due to the fast hydrogen bond dissociation and rearrangement. Thermomechanical
131
cycling experiments showed a strain fixity of about 90 % and a strain recovery of
about 100 % for elastomers containing roughly 2 mol % UPy side-groups. The
excellent strain recovery indicates that the new material memorizes its shape well,
while the lack of complete strain fixity is due to the elasticity of the material itself.
Comparing to traditional thermoresponsive shape-memory polymers, one
unique feature of the new shape-memory elastomers is the dynamics of its shape-
memory response. In other words, the rate of shape recovery is tunable based on
different temperatures due to hydrogen bond dynamics. Creep experiments indicate
a mechanical activation energy that is the same order of magnitude as the activation
energy of chemical dissociation measured by others. This suggests that hydrogen
bond dissociation dynamics strongly influence mechanical relaxation. In addition,
dynamic mechanical studies clearly show a transition around 65 °C which is caused
by hydrogen bonding interactions.
New photo crosslinking route to produce shape-memory elastomers containing
strong hydrogen bonding groups
In order to overcome some of the limitations of one-step polymerization, a
photo-crosslinking approach to prepare shape-memory elastomers bearing
reversibly hydrogen bonding groups was developed. The new photo-crosslinking
route is advantageous because: (i) macromer precursors can be thoroughly
characterized using NMR and GPC techniques to obtain chemical composition and
polymer chain length; (ii) the crosslinking process does not involve solvent, which
132
avoids stress accumulation that arises during solvent removal; (iii) a much greater
fraction of hydrogen bonding side-groups can be achieved due to favorable
solubility of the macromer, and (iv) the technique provides the ability to tune the
number density of both permanent and reversible crosslinks.
Photo-crosslinkable macromers were synthesized via conventional free
radical polymerization. The monomer system consisted of butyl acrylate, a
monomer containing a photo-reactive coumarin or benzophenone group, and a
monomer containing the UPy side-group. Macromers with varieties number
densities of UPy side-group and coumarin (Cm) or benzophenone (Bp) side-groups
were successfully obtained and subsequently characterized. Coumarin-containing
macromers were crosslinked to form thin elastomer films (~ 30 um) using 310 nm
UV light. However, because of coumarin’s high extinction coefficient, the majority
of irradiation was absorbed near the surface, and UV crosslinking of thick films
was difficult to accomplish. Moreover, upon exposure of thin coumarin-containing
crosslinked films to higher energy (254 nm) UV light, partial (~60%) photo-
cleavage of coumarin dimers was observed. This illustrates that photocrosslinking
is, in part, photo-reversible. Benzophenone-containing macromers are readily
crosslinked upon UV exposure at wavelength 365 nm. The photo crosslinking
process involves a light absorbing transient; therefore, a procedure of irradiation-
relaxation cycles was applied to ensure the completion of the photo reaction. Films
around 500 µm were demonstrated to be nearly completely crosslinked. The
resulted photo-crosslinked films are ideal for dynamic mechanical studies.
133
Impacts on material thermal mechanical properties by hydrogen bond
dynamics
The success of photo-crosslinked shape-memory elastomer synthesis
provide the opportunity to systematically study how network architecture and
reversible hydrogen bonding affects material thermal mechanical properties. Photo-
crosslinked elastomers with varying number density of permanent (Bp) and
dynamic (UPy) crosslinks were studied via dynamic mechanical analysis (DMA).
Time-temperature superposition analysis was applied to original DMA data; master
curves for different elastomer samples were obtained. The results revealed two
plateaus in the storage modulus master curves. A low-frequency plateau was
attributed only to the permanent network, and the high-frequency plateau was
attributed to both permanent and reversible crosslinks. Moreover, a maximum in
the loss tangent was observed that depends strongly on the UPy content. Higher
UPy contents increased the magnitude of the damping (tan δ) peak, while also
enhancing the materials stiffness.
Activation energies could be calculated from the temperature-dependence of
shift factors obtained from time-temperature superposition of storage modulus
curves. The activation energy was found to increase with UPy content, and this is
consistent with cooperative dynamics of UPy binding. Furthermore, by comparing
measured activation energies to those of linear UPy-containing polymers, the UPy
cooperative binding effectiveness is clearly enhanced by covalent crosslinks
134
because the activation energy goes up significantly. Thus, in addition to providing
mechanical support, covalent networks may be engineered to reinforce internal,
complementary binding, and this idea may open new approaches to engineering
shape-memory and other stimuli-responsive supramolecular materials.
Small molecule diffusion through reversibly associating polymers revealed a
different mechanism from mechanical relaxation
Small molecule diffusion through dynamic polymer networks containing
reversibly hydrogen bonding side-groups was studied via multi-photon
fluorescence recovery after photobleaching (MP-FRAP) technique. Rhodamine 6G
was chosen to be the monitoring dye molecule. Different linear copolymers
containing UPy side-groups (RACs) and controlled non hydrogen bonding side-
groups (CCPs) were synthesized to undergo FRAP experiments. By fitting original
data from FRAP experiments, diffusion coefficients could be obtained. Direct
comparison between diffusions between RACs and CCPs revealed that UPy
hydrogen bonding interactions greatly reduced mass diffusivity through polymer
melts.
Arrhenius plots of molecular diffusivity indicated that hydrogen bond
association of side-groups affects diffusion in a fundamentally different way other
than it impacts on mechanical relaxation. Small molecule diffusion through UPy-
containing PBA appears to be limited more by thermal equilibrium, i.e. the value of
the association constant K. On the other hand, mechanical viscous relaxation is
135
limited by the hydrogen bonding dissociation rate, i.e. the value of the rate constant
kd. With the study of small molecule diffusion through dynamics polymers,
hydrogen bond dynamics inside polymer networks has become more complete.
Furthermore, the understanding of small molecule through dynamic polymers may
open up some other applications such as drug delivery, chemical sensors for
hydrogen bonding supramolecular polymers.
5.2 Future work
Low temperature behavior of elastomers containing UPy-hydrogen bonding
side-groups
As we already demonstrated, strong hydrogen bonding could impact the
materials mechanical properties in polymer networks at their elastomeric state
(beyond Tg). However, some studies1, 2 showed that even at temperatures near or
under Tg, the hydrogen bonding interactions still could be effective and enhance the
material’s toughness. However, in their study, the polymer system has a glass
transition temperature of around 30 °C. In poly (butyl acrylate) system, the pure
poly (butyl acrylate) has a Tg of – 64 °C, some studies3, 4 on linear poly (butyl
acrylate) containing UPy side-groups already indicated a Tg increase along with
UPy-content. There is a clear need for elastomers that are capable of performing at
low temperatures or even at cryogenic conditions without cracking or brittle
failure.5, 6
136
Preliminary dynamic mechanical analysis was performed on a sample
elastomer prepared using one-step synthesis (Figure 5.1). The results showed the Tg
transition at around -20 °C, a clear increase over linear poly (butyl acrylate). Also
from Figure 5.1, one can also observe that the elastomer exhibits two distinct
transitions. We already have explored the hydrogen dynamics related to the second,
higher temperature transition. Future studies should focus on understanding the
lower temperature transition (corresponding to Tg) and understanding why the
material properties below Tg are different from linear poly(butyl acrylate). This
study will show (i) how both covalent crosslinking and hydrogen bonding dynamic
crosslinking impact on elastomers’ Tg, (ii) whether and how hydrogen bonding
interactions has an impact on elastomer’s mechanical properties.
Figure 5.1 Temperature sweep (- 100 °C to 100 °C) dynamic mechanical analysis
of shape-memory elastomer with 2 mol% UPy side-groups
137
Self-healing study of polymer networks containing strong hydrogen bonding
interactions
Self-healing materials are increasingly sought after in the polymer science
community. Polymeric materials often experience irreversible damage due to aging
and degradation. Some researchers have investigated promising self-healing
supramolecular polymers based on systems that have a high concentration of
hydrogen bonding interactions.7-9 With its high association constant and self-
complimentary nature, the UPy hydrogen bonding group is a good candidate to
develop self-healing supramolecular polymers. A good adhesive property was
already reported.4 Suprapolix company also showed that UPy hydrogen bonding
could be used to engineer self-healing materials .10 However, no fundamental study
of self-healing properties of UPy hydrogen bonding interactions has been reported.
Self-healing study would benefit us to understand hydrogen bonding dissociation
and recombination and to study how hydrogen bond dynamics and polymer
architecture affects the self-healing process.
Diffusion study of small molecule through dynamic polymer melts under
shearing
Small molecule diffusion through polymers containing strong hydrogen
bonding interactions shows a different mechanism from polymers’ mechanical
relaxation. However, the diffusion study was performed at steady state. It is natural
to wonder whether the diffusion might be impacted while the dynamic polymer is
138
under mechanical stress. If under stress, hydrogen bonding interactions should react
accordingly, and it is expected that this effect might influence small molecule
diffusion as well. The hypothesis is that the small molecule diffusion is going to
show higher Arrhenius temperature dependence under shear stress. The higher the
stress is, the higher the diffusion activation energy would be. Therefore, to study
diffusion through dynamic polymers under shearing become desirable because it
will enable to us to test the hypothesis and to reveal some fundamental aspects
about hydrogen bond dynamics. Furthermore, it could potentially provide some
new insights in some mass transport applications like chemical sensor and drug
delivery.
References
1. J. L. Wietor, A. Dimopoulos, L. E. Govaert, R. A. T. M. van Benthem, G.
de With and R. P. Sijbesma, Macromolecules, 2009, 42, 6640-6646.
2. A. Dimopoulos, J. L. Wietor, M. Wubbenhorst, S. Napolitano, R. A. T. M.
van Benthem, G. de With and R. P. Sijbesma, Macromolecules, 2010, 43,
8664-8669.
3. K. E. Feldman, M. J. Kade, E. W. Meijer, C. J. Hawker and E. J. Kramer,
Macromolecules, 2009, 42, 9072-9081.
139
4. K. Yamauchi, J. R. Lizotte and T. E. Long, Macromolecules, 2003, 36,
1083-1088.
5. M. S. Kumar, N. Sharma and B. C. Ray, Journal of Reinforced Plastics and
Composites, 2008, 27, 937-944.
6. V. T. Bechel, J. D. Camping and R. Y. Kim, Composites Part B-
Engineering, 2005, 36, 171-182.
7. D. Montarnal, F. Tournilhac, M. Hidalgo, J. L. Couturier and L. Leibler, J.
Am. Chem. Soc., 2009, 131, 7966-+.
8. D. Montarnal, P. Cordier, C. Soulie-Ziakovic, F. Tournilhac and L. Leibler,
Journal of Polymer Science Part a-Polymer Chemistry, 2008, 46, 7925-
7936.
9. P. Cordier, F. Tournilhac, C. Soulie-Ziakovic and L. Leibler, Nature, 2008,
451, 977-980.
10. Suprabolix, Suprapolix Self-Healing Brochure,
http://www.suprapolix.com/SupraPolix_Self_Healing.pdf, Accessed May
16, 2011.
140
Appendix 1 Supplementary Documents for Chapter 2
a)
b)
Figure A1.1 Synthesis of UPy-EMA monomer, a) synthesis scheme, b) 1H NMR
spectrum in CDCl3
NH2
N
HN
O
H3C +
O
ODMSO
150 °C
O
O
NH
NH
O
N
HN
O
H3C
NCO
14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 ppm
1.726
1.936
2.286
3.559
3.573
3.587
3.601
4.258
4.272
4.286
4.322
5.546
5.549
5.598
5.785
6.184
7.269
10.518
11.956
12.977
3.039
3.120
2.145
2.123
1.038
0.947
1.057
0.997
1.000
1.002
UPy monomer 1.16.10
UPy monomer 1.16.10
141
a)
b)
Figure A1.2 Entropy elasticity comparing to thermal expansion of the shape-
memory elastomer, a) entropy elasticity, b) thermal expansion, at this given load,
entropy elasticity clearly outweighs thermal expansion.
30
25
20
15
perc
ent
str
ain
[%
]
300250200150100500
time [min]
80
60
40
20
tem
pera
ture
[°C]
force: constant 50mNtemperature: cooling steps of 20°C
10x10-3
5
0
-5
str
ain
10008006004002000
time [min]
80
60
40
20
tem
pera
ture
[°C]
Coefficient of Thermal Expansion0mN forceTemperature is from 0 to 80°C
142
Figure A1.3 DSC scan of shape-memory elastomer studied in thermal mechanical
analysis
143
Figure A1.4 Shape-memory elastomers, creep data (blue solid) and model fitting
(red dots) at 27 °C.
Figure A1.5 Shape-memory elastomers, creep data (blue solid) and model fitting
(red dots) at 37 °C.
144
Figure A1.6 Shape-memory elastomers, creep data (blue solid) and model fitting
(red dots) at 57 °C.
Figure A1.7 Shape-memory elastomers, creep data (blue solid) and model fitting
(red dots) at 66 °C.
145
Appendix 2 Supplementary Documents for Chapter 3
a)
O
HO
O
Cl+
O
O
O
TEA
CHCl3
b)
Figure A2.1 Synthesis of benzophenone (Bp) monomer, a) synthesis scheme, b) 1H
NMR spectrum in CDCl3
8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 ppm
1.469
3.221
1.012
1.000
2.430
1.258
4.778
Benzo 5 03.02.10
Benzo 5 03.02.10
146
a)
HO
O
O
NaH
THF 25 °C
NaO
O
O
C
O
Cl
CHCl3 0 °C
O
O
O
C
O
b)
Figure A2.2 Synthesis of coumarin (Cm) monomer, a) synthesis scheme, b) 1H
NMR spectrum in CDCl3
9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 ppm
0.888
1.263
1.467
1.708
1.734
1.758
1.767
2.088
2.455
2.458
5.832
5.835
5.839
6.289
6.292
6.402
7.108
7.114
7.130
7.136
7.160
7.165
7.270
7.623
7.644
3.020
3.252
0.999
0.881
0.999
1.759
1.046
coumarin monomer 110410
coumarin monomer 110410
147
Example of 1H NMR spectra to determine copolymer composition
Figure A2.3 shows a typical macromer NMR spectrum. UPy-content was
determined from comparing the UPy group’s signature peaks at 5.8 (aromatic CH),
10.5 (NH), 11.9 (NH), 13.0 (NH) ppm to the -OCH2- peak around 4.0 ppm.
Likewise, Bp-content was derived using its signature aromatic CH peaks around
7.5 and 7.8 ppm.
Figure A2.3 1H NMR spectrum of macromer Bp1-UPy2 in CDCl3
CH
O
H2C
OH2C
C
O
O
H2C
HN
HN
O
N
NH
O
CH3
C
O
H2C
O
H2C
a b c
CH3
O
CH3
13 12 11 10 9 8 7 6 5 4 3 2 1 0 ppm
0.875
0.922
0.940
0.958
1.346
1.364
1.382
1.398
1.475
1.494
1.597
1.614
1.633
1.903
1.919
2.119
2.284
4.023
4.039
4.050
7.270
7.281
2.000
0.017
0.025
0.029
0.015
0.018
0.016
1bp2upy_082510
1bp2upy_082510
148
a)
b)
Figure A2.4 Macromer Cm1, a) 1H NMR spectrum, b) GPC trace
8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 ppm
1.381
1.397
1.474
1.493
1.596
1.615
1.900
1.916
2.019
2.131
2.138
2.140
2.141
2.142
2.143
2.144
2.146
2.147
2.148
2.149
2.151
2.152
2.153
2.154
2.156
2.157
2.158
2.159
2.160
2.162
2.163
2.164
2.165
2.280
2.337
2.441
4.037
4.050
7.271
2.000
0.012
0.016
PBA 1Cou 111110
PBA 1Cou 111110
120
110
100
90
80
70
RI
6050403020100
Ret. Volume (ml)
Mn = 38000DPI = 1.70
149
a)
b)
Figure A2.5 Macromer Cm1-UPy2, a) 1H NMR spectrum, b) GPC trace
13 12 11 10 9 8 7 6 5 4 3 2 1 0 ppm
2.016
2.041
2.055
2.060
2.075
2.113
2.128
2.139
2.140
2.141
2.144
2.145
2.146
2.148
2.149
2.150
2.151
2.152
2.154
2.155
2.156
2.157
2.159
2.160
2.161
2.162
2.163
2.165
2.170
2.207
2.209
2.277
2.333
2.439
4.022
4.035
4.047
7.270
2.000
0.027
0.013
0.009
0.020
0.022
0.018
PBA 1COU2UPY 111110
PBA 1COU2UPY 111110
120
115
110
105
100
95
90
RI
6050403020100
Ret. Volume (ml)
Mn = 40000DPI = 1.65
150
a)
b)
Figure A2.6 Macromer Bp1, a) 1H NMR spectrum, b) GPC trace
8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 ppm
0.866
0.885
0.915
0.951
0.999
1.320
1.338
1.356
1.374
1.392
1.440
1.468
1.487
1.554
1.571
1.575
1.590
1.607
1.714
1.765
1.894
1.910
2.274
3.958
4.003
4.015
4.031
4.044
4.087
7.270
2.001
0.014
0.007
0.027
1bpPBA_082510
1bpPBA_082510
120
110
100
90
80
RI
6050403020100
Ret. Volume (ml)
Mn = 104000DPI = 1.14
151
a)
b)
Figure A2.7 Macromer Bp1-UPy1, a) 1H NMR spectrum, b) GPC trace
13 12 11 10 9 8 7 6 5 4 3 2 1 ppm
1.095
1.258
1.327
1.344
1.362
1.381
1.398
1.475
1.493
1.578
1.582
1.596
1.613
1.627
1.900
1.915
2.279
4.021
4.038
4.051
7.270
2.000
0.006
0.025
0.030
0.006
0.004
0.004
1bp1UpyPBA_082510
1bp1UpyPBA_082510
110
100
90
80
70
60
RI
6050403020100
Ret. Volume (ml)
Mn = 96400DPI = 1.21
152
a)
b)
Figure A2.8 Macromer Bp1-UPy5, a) 1H NMR spectrum, b) GPC trace
13 12 11 10 9 8 7 6 5 4 3 2 1 ppm
0.919
0.955
1.073
1.092
1.110
1.218
1.343
1.360
1.378
1.394
1.594
1.693
1.898
2.007
2.026
2.046
2.064
2.267
2.850
4.036
4.048
5.797
7.271
2.000
0.071
0.035
0.043
0.057
0.064
0.062
1bp5UpyPBA_082510
1bp5UpyPBA_082510
150
140
130
120
110
100
90
RI
6050403020100
Ret. Volume (ml)
Mn = 42500DPI = 1.82
153
a)
b)
Figure A2.9 Macromer Bp1-UPy10, a) 1H NMR spectrum, b) GPC trace
23456789101112131415 ppm
0.920
0.938
1.239
1.344
1.361
1.577
1.883
2.249
4.019
5.783
7.250
2.000
0.134
0.019
0.033
0.101
0.110
0.107
PBA10UPy1BP_6.22.10
PBA10UPy1BP_6.22.10
110
100
90
80
70
60
RI
6050403020100
Ret. Volume (ml)
Mn = 61400DPI = 1.77
154
a)
b)
Figure A2.10 Macromer Bp1/2-UPy2, a) 1H NMR spectrum, b) GPC trace
13 12 11 10 9 8 7 6 5 4 3 2 1 0 ppm
1.090
1.255
1.341
1.359
1.377
1.394
1.470
1.491
1.503
1.592
1.609
1.704
1.896
1.913
2.006
2.025
2.044
2.119
2.274
2.378
2.399
2.848
4.018
4.034
4.047
7.270
2.000
0.024
0.017
0.021
0.021
0.021
0.020
0.5bp2UpyPBA_082510
0.5bp2UpyPBA_082510
150
140
130
120
110
100
RI
6050403020100
Ret. Volume (ml)
Mn = 62100DPI = 1.39
155
a)
b)
Figure A2.11 Macromer Bp2-UPy2, a) 1H NMR spectrum, b) GPC trace
14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 ppm
1.042
1.073
1.093
1.108
1.177
1.221
1.256
1.343
1.360
1.378
1.394
1.430
1.508
1.594
1.607
1.685
1.770
1.898
2.011
2.075
2.273
4.020
4.035
4.048
7.270
7.488
7.782
7.798
2.000
0.024
0.059
0.074
0.018
0.020
0.019
PBA10UPY1BP7.1.10
PBA10UPY1BP7.1.10
120
110
100
90
80
RI
6050403020100
Ret. Volume (ml)
Mn = 43000DPI = 1.72
156
Figure A2.12 GPC trace of macromer Bp1-UPy2
Analysis of volume swell data
The classical theory of polymer swelling is often viewed in the context of
the Flory-Rehner equation (Flory, P.J. and Rehner, J., Journal Chem. Phy., 1943),
which suggests that the degree to which a polymer swells in a solvent depends on
the net energy resulting from the competition between the entropy of mixing, which
favors swelling, and the enthalpy of mixing and reduction in configuration entropy,
both of which oppose swelling. Mathematically, this is expressed as follows:
2])1[ln( 23
1
212
2122vvnvvvv
140
130
120
110
100
90
80
RI
6050403020100
Ret. Volume (ml)
Mn = 98800DPI = 1.22
157
where v1 is the molar volume of the solvent, v2 is the volume fractions of the
polymer, 1 is the polymer-solvent interaction parameter, and n is the number of
strands in tension (molar basis) per unit volume.
Solutions to the Flory-Rehner equation are plotted as a function of n for
various values of 1 in Figure S2. Experimental values of ν2, arising from
permanent crosslinks, are also included in the figure. The majority of samples fall
on a line corresponding to 1 0.67 in the Flory-Rehner equation. However, the
volume swell of Bp2-UPy2 corresponds to a somewhat higher value of the
interaction parameter (1 0.83).
Figure A2.13 Volume fraction of polymer (ν2) as a function of strand density (n).
Experimental data are compared to solution to the Flory-Rehner equation for
various values of polymer – solvent interaction parameter ()
158
a)
b)
Figure A2.14 DMA storage modulus of elastomer Bp1-UPy1, a) original data, b)
TTS shifted master curve at reference temperature 60 °C
5
6
7
8
9
106
E' (
Pa)
0.001 0.01 0.1 1 10
Freq (Hz)
5
6
7
8
9
106
E' (
Pa)
0.01 0.1 1 10
Freq (Hz)
30°C--------------100°C
159
a)
b)
Figure A2.15 DMA storage modulus of elastomer Bp1-UPy2 a) original data, b)
TTS shifted master curve at reference temperature 60 °C
6
7
8
9
106
E' (
Pa)
0.01 0.1 1 10
Freq (Hz)
30°C--------------100°C
6
7
8
9
106
E' (
Pa)
0.001 0.01 0.1 1 10
Freq (Hz)
160
a)
b)
Figure A2.16 DMA storage modulus of elastomer Bp2-UPy2, a) original data, b)
TTS shifted master curve at reference temperature 60 °C
2x106
3
4
E' (
Pa)
0.01 0.1 1 10
Freq (Hz)
30°C--------------100°C
2x106
3
4
E' (
Pa)
0.001 0.01 0.1 1 10
Freq (Hz)
161
a)
b)
Figure A2.17 DMA storage modulus of elastomer Bp1-UPy5, a) original data, b)
TTS shifted master curve at reference temperature 60 °C
7
106
2
3
4
5
67
107
2
3
E' (
Pa)
0.01 0.1 1 10
Freq (Hz)
30°C--------------100°C
7
106
2
3
4
5
67
107
2
3
E' (
Pa)
10-6 10
-4 10-2 10
0 102
Freq (Hz)
162
a)
b)
Figure A2.18 DMA tan of elastomer Bp1-UPy1, a) original data, b) TTS shifted
master curve at reference temperature 60 °C
0.15
0.10
0.05
0.00
Ta
n
0.01 0.1 1 10
Freq (Hz)
30°C--------------100°C
0.15
0.10
0.05
0.00
Ta
n
10-4
10-3
10-2
10-1
100
101
Freq (Hz)
163
a)
b)
Figure A2.19 DMA tan of elastomer Bp1-UPy2, a) original data, b) TTS shifted
master curve at reference temperature 60 °C
0.25
0.20
0.15
0.10
0.05
Ta
n
0.01 0.1 1 10
Freq (Hz)
30°C--------------100°C
0.25
0.20
0.15
0.10
0.05
0.00
Ta
n
10-4
10-3
10-2
10-1
100
101
Freq (Hz)
164
a)
b)
Figure A2.20 DMA tan of elastomer Bp2-UPy2, a) original data, b) TTS shifted
master curve at reference temperature 60 °C
0.25
0.20
0.15
0.10
0.05
Ta
n
0.01 0.1 1 10
Freq (Hz)
30°C--------------100°C
0.25
0.20
0.15
0.10
0.05
Ta
n
10-4
10-3
10-2
10-1
100
101
Freq (Hz)
165
a)
b)
Figure A2.21 DMA tan of elastomer Bp1-UPy5, a) original data, b) TTS shifted
master curve at reference temperature 60 °C
0.5
0.4
0.3
0.2
Ta
n
0.01 0.1 1 10
Freq (Hz)
30°C--------------100°C
0.5
0.4
0.3
0.2
Ta
n
10-4
10-3
10-2
10-1
100
101
Freq (Hz)
166
Appendix 3 Supplementary Documents for Chapter 4
a)
b)
Figure A3.1 Synthesis of DMPU control monomer, a) synthesis scheme, b) 1H NMR
spectrum in CDCl3
OH
+
O
O
NCO
O
O
NH
O
OTHF
DMAP
1.01.52.02.53.03.54.04.55.05.56.06.57.07.58.0 ppm
0.890
1.235
1.252
1.270
1.618
1.915
1.928
1.956
1.984
2.311
3.492
3.506
3.577
3.591
3.604
3.618
3.763
4.297
4.311
4.323
5.280
5.631
5.635
5.639
6.169
6.745
6.848
7.269
7.304
2.925
5.551
2.169
2.000
0.687
0.925
0.914
1.640
0.851
DMPU monomer
DMPU monomer
167
1H NMR analysis to determine UPy or DMPU side-group content in
synthesized copolymers
1H NMR was taken using CDCl3 as solvent. Examples of RAC-2 and CCP-
2 1H NMR spectra are shown in Figure A3.2 and Figure A3.3. For RAC
copolymers, the content of UPy was determined by comparing three N-H signals on
the UPy ring (present around 10-13 ppm) to the O-CH2 signal. For CCP
copolymers, the content of DMPU was determined by comparing the C-H signal of
the aromatic ring at around 6.8 ppm to the O-CH2 signal.
Figure A3.2 1H NMR spectrum for RAP-2
14 13 12 11 10 9 8 7 6 5 4 3 2 1 ppm
0.917
0.953
1.359
1.377
1.393
1.593
1.694
1.897
2.278
2.761
4.034
4.046
7.269
119.146
1.000
0.814
1.008
0.724
RAP-2
RAP-2
111213 ppm
168
Figure A3.3 1H NMR spectrum for CCP-2
Figure A3.4 1H NMR spectrum for RAC-1
12345678910111213 ppm
1.218
1.253
1.340
1.358
1.376
1.393
1.487
1.592
1.608
1.692
1.896
1.910
2.112
2.277
4.017
4.032
4.046
7.269
2.000
0.008
0.008
0.010
0.007
RAP-1
RAP-1
7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 ppm
0.862
0.872
0.877
0.881
0.889
0.923
0.941
0.959
1.234
1.252
1.270
1.347
1.364
1.382
1.398
1.438
1.478
1.503
1.522
1.900
1.917
1.956
2.299
2.362
3.495
3.505
4.039
4.051
7.270
102.439
3.000
CCP-2
CCP-2
169
Figure A3.5 1H NMR spectrum for CCP-1
Figure A3.6 GPC trace of spectrum for PBA
265
260
255
250
245
RI
6050403020100
Ret. Volume (ml)
Mn = 14100DPI = 1.53
8 7 6 5 4 3 2 1 0 ppm
0.924
0.942
0.960
1.349
1.367
1.385
1.402
1.591
1.616
2.298
4.042
4.051
7.269
182.838
3.001
CCP-1
CCP-1
170
Figure A3.7 GPC trace of spectrum for RAC-1
Figure A3.8 GPC trace of spectrum for RAC-2
265
260
255
250
245
RI
6050403020100
Ret. Volume (ml)
Mn = 16500DPI = 2.00
265
260
255
250
245
RI
6050403020100
Ret. Volume (ml)
Mn = 12600DPI = 2.11
171
Figure A3.9 GPC trace of spectrum for CCP-1
Figure A3.10 GPC trace of spectrum for CCP-2
300
295
290
285
280
RI
6050403020100
Ret. Volume (ml)
Mn = 13500DPI = 2.36
300
295
290
285
280
RI
50403020100
Ret. Volume (ml)
Mn = 14900DPI = 2.45
172
Figure A3.11 MP-FRAP of linear polymer PBA, fluorescence recovery curve at 22 °C
Figure A3.12 MP-FRAP of linear polymer PBA, fluorescence recovery curve at 30 °C
6500
6000
5500
5000flu
ore
sce
nce
sig
na
l [c
oun
ts]
1612840time [s]
raw data fit
6500
6000
5500
5000
4500
4000
flu
ore
sce
nce
sig
na
l [c
oun
ts]
86420time [s]
raw data fit
173
Figure A3.13 MP-FRAP of linear polymer PBA, fluorescence recovery curve at 40 °C
Figure A3.14 MP-FRAP of linear polymer PBA, fluorescence recovery curve at 50 °C
6500
6000
5500
5000
4500
4000
flu
ore
sce
nce
sig
na
l [c
oun
ts]
43210time [s]
raw data fit
6500
6000
5500
5000
4500
4000
3500
flu
ore
sce
nce
sig
na
l [c
oun
ts]
2.01.51.00.50.0time [s]
raw data fit
174
Figure A3.15 MP-FRAP of linear polymer PBA, fluorescence recovery curve at 60 °C
Figure A3.16 MP-FRAP of linear polymer PBA, fluorescence recovery curve at 70 °C
6500
6000
5500
5000
4500
4000
3500
flu
ore
sce
nce
sig
na
l [c
oun
ts]
1.51.00.50.0time [s]
raw data fit
6000
5500
5000
4500
4000
3500
3000
flu
ore
sce
nce
sig
na
l [c
oun
ts]
1.20.80.40.0time [s]
raw data fit
175
Figure A3.17 MP-FRAP of linear polymer PBA, fluorescence recovery curve at 80 °C
Figure A3.18 MP-FRAP of copolymer RAC-1, fluorescence recovery curve at 30 °C
6000
5500
5000
4500
4000
3500
3000
flu
ore
sce
nce
sig
na
l [c
oun
ts]
1.20.80.40.0time [s]
raw data fit
5800
5600
5400
5200
5000
flu
ore
sce
nce
sig
na
l [c
oun
ts]
302520151050time [s]
raw data fit
176
Figure A3.19 MP-FRAP of copolymer RAC-1, fluorescence recovery curve at 40 °C
Figure A3.20 MP-FRAP of copolymer RAC-1, fluorescence recovery curve at 50 °C
6600
6400
6200
6000
5800
5600
5400
flu
ore
sce
nce
sig
na
l [c
oun
ts]
20151050time [s]
raw data fit
7000
6500
6000
5500
flu
ore
sce
nce
sig
na
l [c
oun
ts]
20151050time [s]
raw data fit
177
Figure A3.21 MP-FRAP of copolymer RAC-1, fluorescence recovery curve at 70 °C
Figure A3.22 MP-FRAP of copolymer RAC-1, fluorescence recovery curve at 80 °C
8000
7500
7000
6500
6000
5500
5000
flu
ore
sce
nce
sig
na
l [c
oun
ts]
543210time [s]
raw data fit
6000
5500
5000
4500
4000
flu
ore
sce
nce
sig
na
l [c
oun
ts]
543210time [s]
raw data fit
178
Figure A3.23 MP-FRAP of copolymer RAC-2, fluorescence recovery curve at 40 °C
Figure A3.24 MP-FRAP of copolymer RAC-2, fluorescence recovery curve at 50 °C
6600
6400
6200
6000
5800
5600
flu
ore
sce
nce
sig
na
l [c
oun
ts]
3020100time [s]
raw data fit
7400
7200
7000
6800
6600
6400
6200
6000
flu
ore
sce
nce
sig
na
l [c
oun
ts]
20151050time [s]
raw data fit
179
Figure A3.25 MP-FRAP of copolymer RAC-2, fluorescence recovery curve at 60 °C
Figure A3.26 MP-FRAP of copolymer RAC-2, fluorescence recovery curve at 70 °C
7000
6500
6000
5500
flu
ore
sce
nce
sig
na
l [c
oun
ts]
20151050time [s]
raw data fit
7000
6500
6000
5500
flu
ore
sce
nce
sig
na
l [c
oun
ts]
14121086420time [s]
raw data fit
180
Figure A3.27 MP-FRAP of copolymer RAC-2, fluorescence recovery curve at 80 °C
Figure A3.28 MP-FRAP of copolymer CCP-1, fluorescence recovery curve at 22 °C
7500
7000
6500
6000
5500
5000
flu
ore
sce
nce
sig
na
l [c
oun
ts]
86420time [s]
raw data fit
4200
4000
3800
3600
3400
3200
flu
ore
sce
nce
sig
na
l [c
oun
ts]
20151050time [s]
raw data fit
181
Figure A3.29 MP-FRAP of copolymer CCP-1, fluorescence recovery curve at 30 °C
Figure A3.30 MP-FRAP of copolymer CCP-1, fluorescence recovery curve at 40 °C
4600
4400
4200
4000
3800
3600
3400
flu
ore
sce
nce
sig
na
l [c
oun
ts]
14121086420time [s]
raw data fit
4500
4000
3500
3000
flu
ore
sce
nce
sig
na
l [c
oun
ts]
121086420time [s]
raw data fit
182
Figure A3.31 MP-FRAP of copolymer CCP-1, fluorescence recovery curve at 50 °C
Figure A3.32 MP-FRAP of copolymer CCP-1, fluorescence recovery curve at 60 °C
5000
4500
4000
3500
flu
ore
sce
nce
sig
na
l [c
oun
ts]
86420time [s]
raw data fit
5000
4500
4000
3500
3000
flu
ore
sce
nce
sig
na
l [c
oun
ts]
2.01.51.00.50.0time [s]
raw data fit
183
Figure A3.33 MP-FRAP of copolymer CCP-1, fluorescence recovery curve at 70 °C
Figure A3.34 MP-FRAP of copolymer CCP-1, fluorescence recovery curve at 80 °C
5500
5000
4500
4000
3500
3000
flu
ore
sce
nce
sig
na
l [c
oun
ts]
1.20.80.40.0time [s]
raw data fit
5000
4500
4000
3500
3000flu
ore
sce
nce
sig
na
l [c
oun
ts]
1.21.00.80.60.40.20.0-0.2time [s]
raw data fit
184
Figure A3.35 MP-FRAP of copolymer CCP-2, fluorescence recovery curve at 22 °C
Figure A3.36 MP-FRAP of copolymer CCP-2, fluorescence recovery curve at 30 °C
4400
4200
4000
3800
3600
flu
ore
sce
nce
sig
na
l [c
oun
ts]
20151050time [s]
raw data fit
4600
4400
4200
4000
3800
3600
flu
ore
sce
nce
sig
na
l [c
oun
ts]
20151050time [s]
raw data fit
185
Figure A3.37 MP-FRAP of copolymer CCP-2, fluorescence recovery curve at 40 °C
Figure A3.38 MP-FRAP of copolymer CCP-2, fluorescence recovery curve at 50 °C
4800
4600
4400
4200
4000
3800
3600
3400
3200
flu
ore
sce
nce
sig
na
l [c
oun
ts]
1612840time [s]
raw data fit
4500
4000
3500
3000
flu
ore
sce
nce
sig
na
l [c
oun
ts]
1086420time [s]
raw data fit
186
Figure A3.39 MP-FRAP of copolymer CCP-2, fluorescence recovery curve at 60 °C
Figure A3.40 MP-FRAP of copolymer CCP-2, fluorescence recovery curve at70 °C
4500
4000
3500
3000flu
ore
sce
nce
sig
na
l [c
oun
ts]
6420time [s]
raw data fit
5000
4500
4000
3500
3000
2500
flu
ore
sce
nce
sig
na
l [c
oun
ts]
1.51.00.50.0time [s]
raw data fit
187
Figure A3.41 MP-FRAP of copolymer CCP-2, fluorescence recovery curve at 80 °C
Figure A3.42 MP-FRAP of crosslinked PBA, fluorescence recovery curve at 30 °C
5000
4500
4000
3500
3000
2500
flu
ore
sce
nce
sig
na
l [c
oun
ts]
1.20.80.40.0time [s]
raw data fit
2400
2300
2200
2100
2000
flu
ore
sce
nce
sig
na
l [c
oun
ts]
20151050time [s]
raw data fit
188
Figure A3.43 MP-FRAP of crosslinked PBA, fluorescence recovery curve at 40 °C
Figure A3.44 MP-FRAP of crosslinked PBA, fluorescence recovery curve at 50 °C
2600
2400
2200
2000
flu
ore
sce
nce
sig
na
l [c
oun
ts]
151050time [s]
raw data fit
2600
2400
2200
2000
1800
flu
ore
sce
nce
sig
na
l [c
oun
ts]
14121086420time [s]
raw data fit
189
Figure A3.45 MP-FRAP of linear polymer PBA, fluorescence recovery curve at 60 °C
Figure A3.46 MP-FRAP of linear polymer PBA, fluorescence recovery curve at 70 °C
2400
2200
2000
1800
1600
flu
ore
sce
nce
sig
na
l [c
oun
ts]
86420time [s]
raw data fit
2400
2200
2000
1800
1600
flu
ore
sce
nce
sig
na
l [c
oun
ts]
6420time [s]
raw data fit
190
Figure A3.47 MP-FRAP of linear polymer PBA, fluorescence recovery curve at 80 °C
2600
2400
2200
2000
1800
1600
flu
ore
sce
nce
sig
na
l [c
oun
ts]
543210time [s]
raw data fit