phase behavior and photo-responsive studies of photoactive liquid crystalline hyperbranched...

Phase Behavior and Photo-Responsive Studies ofPhotoactive Liquid Crystalline Hyperbranched PolyethersContaining Benzylidene Moiety

V. SRINIVASA RAO, A. B. SAMUI

Polymer Division, Naval Materials Research Laboratory, Ambernath-E, Thane, Maharashtra 421506, India

Received 4 February 2009; accepted 23 February 2009DOI: 10.1002/pola.23360Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Two sets of hyperbranched polyether epoxies were synthesized to studythe effect of substituent, rigidity, and nature of photoactive unit on the thermal andphotoresponsive properties. Each set was comprised of one molecule with an acyclicmoiety in the repeating unit, and two molecules with a cyclic moiety of varying rigid-ity (cycle size) in the repeating unit. Two substituents on aromatic rings in therepeating unit were present in one set, and other set was without a substituent. Themesogenic and photoresponsive properties were studied and correlated to the variedstructural parameters. The effects of varied molecular structural parameters onphase behavior and photoresponsive properties were very prominent. Out of sixmonomeric diols, only four have exhibited liquid crystalline phase while the polymerscorresponding to all monomeric diols revealed mesophase. The findings in photore-sponsive properties were further supported by molecular modeling studies. Thechanges in refractive index, photoviscosity, and fluorescence intensity with irradia-tion time substantiated the spectral pattern observed in UV-Vis spectroscopy. VVC 2009

Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 2774–2786, 2009

Keywords: hyperbranched; hyperbranched polymers; liquid-crystalline polyethers(LCP); molecular modeling; phase behavior; photoresponsive polymers; polyethers;structure-property relationship

INTRODUCTION

Although the term ‘‘hyperbranched’’ was coinedby Kim and Webster in the late 1980s,1,2 the basicconcept is much older. Flory analyzed the polycon-densation of ABm monomers3 in the early 1950s.Since the initial report on this class of moleculesby Vogtle in 1978,4 many different structuralclasses of dendritic macromolecules have beenreported which include dendrimers,5,6 hyper-branched polymers,7 linear dendritic polymers,8

linear dendritic copolymers,9 star dendriticcopolymers,10 multi-arm star polymers,11 mainchain polymers with dendritic side groups,12 andso forth. Initially, a divergent synthetic approachwas used to prepare these polymers. Later, a con-vergent synthetic approach,13 ‘‘double-stage’’ con-vergent approach,14 and the combination of TER-MINI and metal catalyzed living radical polymer-izations15 were proposed. On the other hand, theuncontrolled chain growth propagation app-roach16 was also utilized in synthesizing highlybranched polymers. Significant conformationalchange occurs when dendrimer reaches specificgeneration, particularly for generations greaterthan 4 the structure assumes a densely packedglobular shape, which decrease chain

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 47, 2774–2786 (2009)VVC 2009 Wiley Periodicals, Inc.

Correspondence to: A. B. Samui (E-mail: [email protected])

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entanglements and molecular aspect ratio.17 Thisfeature imparts attractive solution and bulkproperties to dendrimers. The hyperbranchedmolecules attracted attention from academia andindustry because structural perfection may not bea strict prerequisite for many applications andalso because of limiting factors of dendrimerssuch as tedious multi step syntheses in a con-trolled manner. However, the limited availabilityof ABn monomers restricted researchers to syn-thesize hyperbranched polymers according to asimple route, that is, A2 þ B3 approach.

It is well known that even dendritic architec-tures allow mesogens to align to form LC phase ifthe spacers are indeed flexible enough. In princi-ple, two different strategies have been employedfor the incorporation of mesogenic structural ele-ments into cascade-branched architecture; eithermesogen can be part of each branching monomer,or it can be coupled to the end group of scaffold.The initial report of Percec et al.,18 on thermotropiclinear and dendritic19 polyethers having mesogenunit was based on conformational isomerism and arange of spacers in the repeating unit. The pioneer-ing concepts and their validation have opened newsynthetic, theoretical, and practical opportunitiesin the area of liquid crystalline dendrimers. An al-ternative approach is the use of amphiphilic self-assembling dendrons to generate supramoleculardendrimers.20 This group pioneered the use ofmesogenic repeat units to prepare dendrons anddendrimers19,21 and the use of amphiphilic self-assembling dendrons to generate supramoleculardendrimers20(b),22 and self-organizable dendronizedpolymers.23 They are involved in devisingelaborate strategies for synthesis of spherical,24

flat tapered,12(b),20(b),25 twin-tapered,26 coni-cal,20(b),24(a),25 mesogen-jacketed27 monodendriticbuilding blocks that self-assemble to cylindrical,28

spherical,28,29 or nonsperical21 supramoleculardendrimers and subsequently self-organize intotwo-dimensional hexagonal columnar, smectic B,and smectic A, as well as in three-dimensionalcubic lattice. They designed and synthesized a se-ries of libraries of monodendrons based on singlerepeat unit such as AB2 or AB3,

29,30 and ofhybrid dendrons composed of nondendritic ((AB)y)and dendritic (ABn) building blocks31 or ofdendritic (ABm) and dendritic (ABn) buildingblocks,22(a),26(c),28(b) to investigate the fundamentalcorrelation between molecular structure of the den-dron and the shape and the diameter of the supra-molecular dendrimers. These preliminary experi-ments are useful for the creation of simplest

nonbiological systems that mimic the role of biolog-ical functions. Ringsdorf and coworkers,32 Jin andcoworkers,33 and later, our group34 also reportedon dendritic polymers with mesogens in thebranching unit. There are numerous reports fromFrey and coworkers,35 Shibaev and coworkers,36

and others on liquid crystalline dendritic polymerswith mesogens coupled to the end groups.

Interest in polyethers is increasing because oftheir intrinsically high flexibility, which enablesliquid crystalline behavior even when the spacersare very short.37 The inert polyether scaffold rep-resents an ideal system for further functionaliza-tion due to its stability toward both chemical reac-tions and thermal stress.38 Further, the polyetherbackbone exhibits good elastic and adhesive char-acteristics, which enables the design of materialsfor advanced technologies.

Photoaddressable dendritic liquid crystallinepolymers (PADLCPs) are promising materials foroptical data storage applications because of theircombined physical properties of low molar masscompounds and macromolecules such as rapidresponse, strong anisotropic optical propertieseven with week external fields (optical, electric,and magnetic fields), stability of stored informationand possibility of data recording in thin films. Pho-tochromic event can bring about configurationaland conformational changes (isomerization, cycli-zation, crosslinking, etc.) in molecular level whichleads to structural changes in the macroscopiclevel accompanied by changes in other proper-ties,39 such as refractive index, birefringence, scat-tering, and absorption. All the changes undergoneby photoresponsive polymers are a function ofchemical structure of polymers as both thermody-namic and steric factors are related to those pa-rameters. The variation in the structure providesthe ideal way to optimize their properties toachieve best possible application advantage. Incontinuation to our previous work on linear andhyperbranched photoactive liquid crystalline poly-mers,34 we report here synthesis, characterizationand effect of substituent, rigidity and nature ofphotoactive mesogenic unit on phase behavior andphotoactive properties of benzylidene based hyper-branched polyethers.

EXPERIMENTAL

Materials

4-Hydroxybenzaldehyde (98%; Sigma Aldrich),Vanillin (98%; Sd Fine Chemicals, India),

PHOTOACTIVE LIQUID CRYSTALLINE HYPERBRANCHED POLYETHERS 2775

Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola

cyclopentanone (99%; Acros Organics, Belgium),cyclohexanone (98%; Acros Organics, Belgium),acetone (99%; Sd Fine Chemicals, India), tetra-n-butylammonium chloride (TBAC, 98%, Lancaster,India), and triphenylolmethane triglycidyl ether(TPM, Sigma Aldrich) were used as received with-out any further purification. Dimethylformamide(DMF) (Sd Fine Chemicals, India) and dimethylsulfoxide (DMSO) (Sd Fine Chemicals, India)were purified before use as reported.40

Techniques

FTIR spectra were recorded on Perkin–Elmer1600 series Fourier transform infrared spectropho-tometer using KBr pellets. UV-Visible spectrumwas taken on a Cary 500 Scan UV-VIS-NIR Spec-trophotometer. 1H and 13C NMR spectra wererecorded on a 500 MHz Bruker-FT NMR spec-trometer using DMSO-d6 as solvent. Chemicalshifts were measured using tetramethyl silane asan internal standard (TMS). Molecular weights ofpolymers were determined by gel permeation chro-matography (GPC-Waters, Polystyrene standards).Differential scanning calorimeter (DSC) (TAinstruments) was used to determine the thermaltransitions. Heating/cooling rate used for all DSCanalysis was 5 �C/min and sample weight wasaround 7 mg. Thermo gravimetric analyzer (TGA)(TA instruments His Res TGA 2950) was used forthermal analysis with a heating rate of 20 �C/minin N2 atmosphere. A Leica DMLD polarizing opti-cal microscope with image analyzer, equipped withLINKAM TMS 94 hot stage and LINKAM LNPcontrolling unit, was used to observe the thermaltransition and liquid crystalline state. Refractiveindices of polymer thin films were measured usingFilmetrics F20, thin-film analyzer. The inherentviscosities were measured with an Ubbelohde Vis-cometer at 30 �C (0.5 g/dL) in DMSO. Photolysis ofthe polymer was carried out in DMSO solution at30 �C. Irradiation of polymer samples was per-formed using a spectroline UV-lamp, model ENF-260/FE in discontinuous mode from a distance of10 cm for various time intervals. Irradiated solu-tions/films were subjected to spectral analysis, vis-cosity, and refractive index measurements. Molec-ular modeling studies were carried out using Acce-lyrs Materials Studio, version 4.2.

Synthesis of Photoactive Diol

Synthesis and characterization of three diols{1,3-bis(4-hydroxybenzylidene)acetone (Diol-A3,0);

2,5-bis(4-hydroxybenzylidene)cyclopetanone(Diol-C5,0); 2,6-bis(4-hydroxybenzylidene)cyclo-hexanone (Diol-C6,0)} were reported in our ear-lier communications.34(b,c) Another three diolswere synthesized by using vanillin in place of 4-hydroxybenzaldehyde. The diols are abbreviatedby incorporating number of carbon atoms incyclic/acyclic moiety (where C stands for cyclicand A stands for acyclic) and number of substitu-ents on the aromatic rings, for example, Diol-A3,2 represents a diol with three acyclic carbonatoms and two substituents on aromatic ring andso on.

1,3-Bis(3-methoxy-4-hydroxybenzylidene)-acetone (Diol-A3,2)

Yield: 70%. mp: 120–121 �C. ELEM. ANAL.(C19H18O5) (326.33): Calcd. C, 69.92%; H, 5.56%.Found: C, 69.40%; H, 5.48%. FTIR (KBr, cm�1):3219 (OH), 3015 (¼¼CAH), 2955 (CH3), 1638(C¼¼O, ketone), 1590 (C¼¼C, benzylidene), 1563,1511, 1460 (aromatic), 1268, 1028 (PhAOAC). 1HNMR (DMSO-d6, d ppm): d ¼ 9.54 (s, 2H,PhAOH), 7.65 (d, J ¼ 15.5 Hz, 2H, ¼¼CHAPh),7.35 (s, 2H, aromatic), 7.21 (d, J ¼ 8.0 Hz, 2H),7.14 (d, J ¼ 16.0 Hz, 2H, ¼¼CHA, a to C¼¼O), 6.84(d, J ¼ 8.0 Hz, 4H, aromatic), 3.86 (s, 6H, OCH3).13C NMR (DMSO-d6, d ppm): d ¼ 188.47 (C¼¼O),149.88, 148.46, 124.21, 123.69, 116.15, 112.15 (ar-omatic ring carbons), 143.12 (¼¼CHAPh), 126.86(ACH¼¼, a to C¼¼O), 56.28 (OCH3).

2,5-Bis(3-methoxy-4-hydroxy benzylidene)-cyclopentanone (Diol-C5,2)

Yield: 78%. mp: 215–216 �C. ELEM. ANAL.(C21H20O5) (352.37): Calcd. C, 71.57%; H, 5.72%.Found: C, 71.29%; H, 5.59%. FTIR (KBr, cm�1):3495 (OH), 3013 (¼¼CAH), 2930 (CH3), 1677(C¼¼O, ketone), 1620 (C¼¼C, benzylidene), 1580,1508, 1446 (aromatic), 1268, 1029 (PhAOAC). 1HNMR (DMSO-d6, d ppm): d ¼ 9.64 (s, 2H,PhAOH), 7.34 (s, 2H, ACH¼¼), 7.23 (s, 2H, aro-matic), 7.14 (d, J ¼ 8.5 Hz, 2H, aromatic), 6.88 (s,J ¼ 8.5 Hz, 2H, aromatic), 3.83 (s, 6H, OCH3),3.04 (s, 4H, CH2 b to C¼¼O). 13C NMR (DMSO-d6,d ppm): d ¼ 195.10 (C¼¼O), 148.99, 148.17, 133.16,125.16, 116.28, 115.12 (aromatic ring carbons),135.82 (¼¼CHAPh), 127.68 ([C¼¼, a to C¼¼O),56.09 (OCH3), 26.31 (CH2 b to C¼¼O).

2776 RAO AND SAMUI


2,6-Bis(3-methoxy-4-hydroxybenzylidene)-cyclohexanone (Diol-C6,2)

Yield: 73%. mp: 176–177 �C. ELEM. ANAL.(C22H22O5) (366.39): Calcd. C, 72.11%; H, 6.05%.Found: C, 71.90%; H, 5.90%. FTIR (KBr, cm�1):3374 (OH), 3075 (¼¼CAH), 2952 (CH3), 1638(C¼¼O, ketone), 1578 (C¼¼C, benzylidene), 1513,1466 (aromatic), 1256, 1036 (PhAOAC). 1H NMR(DMSO-d6, d ppm): d ¼ 9.50 (s, 2H, PhAOH), 7.54(s, 2H, ACH¼¼), 7.10 (s, 2H, aromatic), 7.02 (d, J¼ 8.5 Hz, 2H, aromatic), 6.83 (d, J ¼ 8.0 Hz, H,aromatic), 3.80 (s, 6H, OCH3), 2.87 (t, 4H, CH2 bto C¼¼O), 1.71 (m, 2H, CH2 c to C¼¼O). 13C NMR(DMSO-d6, d ppm): d ¼ 188.24 (C¼¼O), 148.38,148.34, 136.81, 134.56, 115.23, 109.28 (aromaticring carbons), 137.47 (¼¼CHAPh), 126.25 ([C¼¼, ato C¼¼O), 56.73 (OCH3), 28.04 (CH2 b to C¼¼O),23.28 (CH2 c to C¼¼O).

Synthesis of Hyperbranched Polymers

Photoactive liquid crystalline hyperbranched poly-ether epoxies were synthesized by polyadditionof six different photoactive bisbenzylidene alka-none diol monomers with triphenylolmethane tri-glycidyl ether using the method reported by Nish-ikubo and coworkers41 In a typical recipe, mixtureof 2.663 g (0.01 mol) of photoactive diol (Diol-A3,0), 4.605 g (0.01 mol) of triphenylolmethanetriglycidyl ether, 0.14 g of tetra-n-butyl ammo-nium chloride (5 mol %) and 100 mL of DMF werecharged into 250 mL 3-necked flask equippedwith stirrer and a condenser. While stirring (usingmagnetic stirring bar), the mixture was heated at110 �C for 24 h under N2 atmosphere. The result-ing polymer was precipitated in excess amount ofmethanol. It was purified by repeated precipita-tion from DMF into methanol and water, respec-tively. The polymer product was filtered, dried at60 �C in vacuum for 16 h. Structure of hyper-branched polyether is shown in Scheme 1.

Hyperpolyether-A3,0

Yield: 76%. FTIR (KBr, cm�1): 3398 (OH), 3032(¼¼CAH), 2928 (CH3), 1642 (C¼¼O), 1596 (C¼¼C),1507, 1449 (aromatic), 1241, 1031 (PhAOAC), 915(epoxy). 1H NMR (DMSO-d6, d ppm): 7.88–7.72,7.19–7.15, 7.00–6.89 (12H, aromatic protons fromTPM), 7.63 (d, J ¼ 16.0, 2H, PhACH¼¼), 7.59 (d, J¼ 8.5 Hz, 4H, aromatic diol unit), 7.02 (d, J ¼15.5, 2H, APhACH¼¼CHA), 6.78 (d, J ¼ 8.5 Hz,4H, aromatic diol unit), 5.73 [br s, 1H,

HC(PhAOA glycidyl unit)3], 5.43 (s, 3H, AOH),4.28 (dd, 3H, CH2 of glycidyl unit), 4.16–3.82 (m,15H, AOACH2ACH(OH)ACH2AOA), 3.78 (dd,3H, CH2 of glycidyl unit), 3.39 (m, 3H, CH of ep-oxy ring), 2.83 (m, 3H, CH2 of epoxy ring), 2.70(m, 3H, CH2 of epoxy ring). 13C NMR (DMSO-d6,d ppm): 191.69 (C¼¼O, Acetone), 160.94, 157.19,132.22, 130.30, 127.91, 124.83, 115.42, 114.65 (ar-omatic ring carbons), 142.57 (carbon a to ketone),117.00 (ACH¼¼ unit), 70.44, 67.86 (AOACH2ACH(OH)ACH2AOA), 69.31, 50.16, 44.20 (CH2 ofglycidyl unit, CH and CH2 of epoxy ring), 57.99[HC(PhAOA glycidyl unit)3].

Hyperpolyether-C5,0

Yield: 78%. FTIR (KBr, cm�1): 3398 (OH), 3060(¼¼CAH), 2928 (CH3), 1677 (C¼¼O), 1597 (C¼¼C),1509, 1449 (aromatic), 1241, 1024 (PhAOAC), 915(epoxy). 1H NMR (DMSO-d6, d ppm): 7.63, 7.20–6.89 (8H, aromatic protons from TPM), 7.54 (d, J¼ 8.5 Hz, 4H, aromatic diol unit), 7.33 (6H,ACH¼¼ coupled with aromatic protons of TPM),6.85 (d, J ¼ 8.5 Hz, 4H, aromatic diol unit), 5.73[br, 1H, HC(PhAOA glycidyl unit)3], 5.43 (s, 3H,AOH), 4.28 (dd, 3H, CH2 of glycidyl unit), 4.16–3.85 (m, 15H, AOACH2ACH(OH)ACH2AOA),3.80 (dd, 3H, CH2 of glycidyl unit), 3.34 (m, 3H,CH of glycidyl epoxy ring), 3.01 (s, 4H, b to C¼¼O),2.83 (m, 3H, CH2 of epoxy ring), 2.70 (m, 3H, CH2

of epoxy ring). 13C NMR (DMSO-d6, d ppm):195.18 (C¼¼O, cyclopentanone), 160.01, 156.92,132.88, 130.30, 128.63, 126.43, 120.90, 115.42,114.65 (aromatic ring carbons), 136.25 (carbon ato ketone), 116.65 (ACH¼¼ unit), 69.85, 67.86(AOACH2ACH(OH)ACH2AOA), 69.31, 50.15,44.20 (CH2 of glycidyl unit, CH and CH2 of epoxyring), 57.99 [HC(PhAOA glycidyl unit)3], 26.38(carbon b to C¼¼O).

Hyperpolyether-C6,0

Yield: 76%. FTIR (KBr, cm�1): 3377 (OH), 3060(¼¼CAH), 2928 (CH3), 1654 (C¼¼O), 1596 (C¼¼C),1507, 1451 (aromatic), 1243, 1031 (PhAOAC), 915(epoxy). 1H NMR (DMSO-d6, d ppm): 7.55 (6H,ACH¼¼ coupled with aromatic protons of TPM),7.46, 7.20–6.88 (8H, aromatic protons from TPM),7.40 (d, J ¼ 9.0 Hz, 4H, aromatic diol unit), 6.85(d, J ¼ 8.5 Hz, 4H, aromatic diol unit), 5.73 [br,1H, HC(PhAOA glycidyl unit)3], 5.43 (s, 3H,AOH), 4.27 (dd, 3H, CH2 of glycidyl unit), 4.12–3.85 (m, 15H, AOACH2ACH(OH)ACH2AOA),3.80 (dd, 3H, CH2 of glycidyl unit), 3.43 (m, 3H,



CH of epoxy ring), 3.01 (s, 4H, b to C¼¼O), 2.83(m, 3H, CH2 of epoxy ring), 1.72 (m,2H, c toC¼¼O). 13C NMR (DMSO-d6, d ppm): 195.00(C¼¼O, cyclohexanone), 159.46, 157.19, 132.59,130.29, 128.48, 126.29, 120.89, 115.08, 114.64(aromatic ring carbons), 136.34 (carbon a toketone), 116.23 (ACH¼¼ unit), 69.79, 67.85(AOACH2ACH(OH)ACH2A OA), 69.31, 50.14,44.19 (CH2 of glycidyl unit, CH and CH2 of epoxyring), 57.89 [HC(PhAOA glycidyl unit)3], 28.43(carbon b to C¼¼O), 22.94 (carbon c to C¼¼O).

Hyperpolyether-A3,2

Yield: 76%. FTIR (KBr, cm�1): 3446 (OH), 3057

(¼¼CAH), 2927 (CH3), 1642 (C¼¼O), 1588 (C¼¼C),

1507, 1455 (aromatic), 1250, 1027 (PhAOAC), 915

(epoxy). 1H NMR (DMSO-d6, d ppm): 7.67 (br, 2H,

PhACH¼¼), 7.38 (br, 2H, aromatic diol unit), 7.28

(br s, 2H, aromatic diol unit), 7.19 (br, 2H,

PhACH¼¼), 7.05, 6.97 (8H, aromatic protons of

TPM), 6.90–6.70 (br, 6H, aromatic protons of TPM

coupled with aromatic protons of diol unit), 5.75

Scheme 1. Structure of hyperbranched polyether with (a) acyclic alkanone in therepeating unit and (b) cyclic alkanone in the repeating unit.

2778 RAO AND SAMUI


[s, 1H, HC(PhAOA glycidyl unit)3], 5.33 (s, 3H,AOH), 4.28 (m, 3H, CH2 of glycidyl unit), 4.20–3.70 (m, 21H, AOACH2ACH(OH)ACH2AOA andAOCH3), 3.43 (m, 3H, CH of epoxy ring), 2.82 (m,3H, CH2 of epoxy ring), 2.69 (m, 3H, CH2 of epoxyring). 13C NMR (DMSO-d6, d ppm): 191.69 (C¼¼O,Acetone), 160.94, 157.19, 132.22, 130.30, 127.91,115.42 (aromatic ring carbons of TPM), 149.83,148.46, 126.81, 124.83, 123.74, 114.65 (aromaticring carbons diol unit), 142.57 (carbon a to ke-tone), 117.00 (ACH¼¼ unit), 70.44, 67.86(AOACH2ACH(OH)ACH2AOA), 69.31, 50.16,44.20 (glycidyl unit), 57.99 [HC(PhAOA glycidylunit)3].

Hyperpolyether-C5,2

Yield: 78%. FTIR (KBr, cm�1): 3422 (OH), 3060(¼¼CAH), 2927 (CH3), 1680 (C¼¼O), 1590 (C¼¼C),1508, 1454 (aromatic), 1238, 1028 (PhAOAC), 915(epoxy). 1H NMR (DMSO-d6, d ppm): 7.38 (s, 2H,ACH¼¼), 7.25 (s, 2H, aromatic diol unit), 7.19 (s,2H, aromatic diol unit), 7.10, 6.98 (8H, aromaticprotons of TPM), 6.88 (br, 6H, aromatic protons ofTPM coupled with aromatic protons of aromaticdiol unit), 5.74 [s, 1H, HC(PhAOA glycidylunit)3], 5.32 (s, 3H, AOH), 4.28 (m, 3H, CH2 ofglycidyl unit), 3.91 (m, 3H, glycidyl unit), 4.19–3.68 (m, 21H, AOACH2ACH(OH)ACH2AOA andAOCH3), 3.44 (m, 3H, epoxy), 3.07 (s, 4H, b toC¼¼O), 2.83 (m, 3H, CH2 of epoxy), 2.69 (m, 2H,CH2 of epoxy). 13C NMR (DMSO-d6, d ppm):195.18 (C¼¼O, cyclopentanone), 156.89, 156.19,130.28, 129.16, 120.89, 113.22 (aromatic ring car-bons of TPM), 149.98, 132.90, 127.81, 124.76,113.89 (aromatic ring carbons of diol unit), 136.12(carbon a to ketone), 114.81 (ACH¼¼ unit), 70.60,68.00 (AOACH2ACH(OH)ACH2AOA), 69.35,50.15, 44.20 [glycidyl unit), 57.99 (HC(PhAOAglycidyl unit)3], 56.14 (AOCH3), 26.30 (carbon b toC¼¼O).

Hyperpolyether-C6,2

Yield: 76%. FTIR (KBr, cm�1): 3424 (OH), 3058(¼¼CAH), 2928 (CH3), 1656 (C¼¼O), 1593 (C¼¼C),1508, 1454 (aromatic), 1245, 1030 (PhAOAC), 908(epoxy). 1H NMR (DMSO-d6, d ppm): 7.57 (s, 2H,ACH¼¼), 7.11 (s, 2H, aromatic diol unit), 7.04 (d, J¼ 8.0 Hz, 2H, aromatic diol unit), 7.19, 6.98 (8H,aromatic protons of TPM), 6.89 (br, 6H, aromaticprotons of TPM coupled with aromatic protons ofaromatic diol unit), 5.74 [br s, 1H, HC(PhAOAglycidyl unit)3], 5.32 (br s, 3H, AOH),

4.28 (m, 3H, CH2 of glycidyl unit), 4.18–3.69(m, 21H, AOACH2ACH(OH)ACH2AOA andAOCH3), 3.43 (m, 2H, CH of epoxy ring), 2.90 (s,4H, b to C¼¼O), 2.83 (m, 3H, CH2 of epoxy ring),2.69 (m, 3H, CH2 of epoxy ring), 1.73 (m, 2H, cto C¼¼O). 13C NMR (DMSO-d6, d ppm): 188.98(C¼¼O, cyclohexanone), 156.99, 156.16, 136.16,130.29, 127.81, 113.80 (aromatic ring carbons ofTPM), 149.31, 148.07, 133.62, 129.08, 124.19,116.19 (aromatic ring carbons of diol unit), 134.97(carbon a to ketone), 114.15 (ACH¼¼ unit), 70.61,68.02 (AOACH2ACH(OH)ACH2AOA), 69.35,50.13, 44.20 (glycidyl unit), 57.89 [HC(PhAOAglycidyl unit)3], 56.20 (AOCH3 ), 28.31 (carbon bto C¼¼O), 22.96 (carbon c to C¼¼O).

RESULTS AND DISCUSSION

Two sets of hyperbranched polyether epoxies weresynthesized by polyaddition of bisbenzylidenediols and triphenylolmethane triglycidyl ether.Each set comprised of one acyclic moiety and twocyclic moieties with varying rigidity (cycle size),respectively in the repeating unit. One set is withtwo substituents on aromatic ring in the repeat-ing unit and other one is without a substituent.Temperature, time, and concentration of reac-tants and catalyst were controlled carefullyto prevent gelation. Weight-average molecularweights of the resulting polymers determined byGPC are in the range of 7100–8800 with polydis-persity 1.7–2.1 and their inherent viscositiesfound in the range of 0.21–0.28 dL/g (Table 1).The structures of synthesized monomers and poly-mers were identified by FTIR, 1H, and 13C NMRspectroscopic techniques. The absorption peaks inFTIR spectrum around 3424, 1656, 1029 cm�1,and 908 cm�1 arising from hydroxyl, ketone,ether, and epoxy groups, respectively confirm theincorporation of both monomers into the polymerstructure. Disappearance of phenolic proton sig-nals while retaining all other signals of diol unitalong with the appearance of new signals aroundd 4.19–3.68 ppm in 1H NMR due to ring openingof epoxy (AOACH2ACH(OH)ACH2AOA) con-firms the formation of polymers. The broad peaksat d 5.32 and 5.74 ppm can be attributed to thehydroxyl group generated due to ring openingreaction of epoxy ring and methine proton ofTPM, respectively. 13C NMR provided additionalinsight into the structure of these hyperbranchedpolymers. The epoxide resonances are observed atd 50.13 (CH) and 44.20 (CH2) and methine carbon



of TPM at 57.89 ppm. All the signals up field fromthe epoxides (d 70.61 and 68.00 ppm) correspondto methyleneoxy and methineoxy groups resultingfrom incorporation of diepoxy units into the poly-mer backbone. Degree of branching (DB) was esti-mated by using the method described by Frechetand coworkers42 from 1H NMR. The integral val-ues of dendritic, linear, and terminal unit aromaticproton resonances of triphenylolmethane triglyci-dylether (around 7.10, 6.98, 6.89 ppm, respec-tively) were used for the estimation of DB. The DBvalues are found to be in the range of 0.55–0.59.

Thermal Properties

The thermal properties and phase behavior ofmonomers and polymers are characterized andsummarized in Table 1. The liquid crystallinephases determined by DSC were found to be con-sistent with POM results. By selecting two sets ofpolymers, variation in substituent, rigidity andphotoactive units were ensured. The effect ofthese variations on thermal and LC phase behav-ior was studied. The data from Table 1 revealedthat the effect of varied molecular structural pa-rameters on phase behavior is considerable. Fromthe first set of diols, that is, diols without sub-stituents, only Diol-C6,0 shows nematic phase.The other two, Diol-A3,0 and Diol-C5,0, exhibitonly crystalline phase without any mesophase.While from the second set of diols, that is, all the

diols with substituents show liquid crystallinephase along with crystalline phase. The texturesof Diol-A3,2 and Diol-C6,2 could be clearlyobserved as broken fan texture (Fig. 1), which is acharacteristic of smectic-A phase while Diol-C5,2 revealed nematic phase. The monomers withcyclic moieties show higher transition tempera-tures compared to the corresponding monomerswith acyclic moieties which is due to highly rigidnature of cyclic units. While the monomers withfive-membered ring show higher transition tem-peratures than that of molecules with six-mem-bered ring, the presence of substituents decreasedthe transition temperatures. It is expected thatthe steric hindrance due to presence of substitu-ents reduce the packing ability of molecules thatcauses the reduction of transition temperatures.The transition temperatures of polymers followedalmost similar trend as observed in the mono-mers, that is, both first and second order transi-tions show higher values, in the absence ofsubstituent and, in the presence of rigid cyclicmoieties. The only difference of polymers withmonomers is that both the polymers with five andsix membered rings in their repeating unit showalmost similar isotropization temperatures. Allthe polymers show liquid crystallinity eventhough two monomers are not liquid crystalline innature. It is supposed that these two monomericunits attain minimum balance between rigid andflexible units after polymerization that leads

Table 1. Thermal and Physical Properties of Polymers

Monomer/Polymer Thermal Transitionsa (�C) Mnb Mw

b PDIb ginhc (dL/g)

Diol-A3,0 K 245 Ti – – – –Diol-C6,0 K 283 N 295 Ti – – – –Diol-C5,0 K 320 Ti – – – –Diol-A3,2 K 124 Sm 140 Ti – – – –Diol-C6,2 K 130 Sm 177 Ti – – – –Diol-C5,2 K 180 N 215 Ti – – – –Hyperpolyether-A3,0 Tg 158 N 238 Ti 4300 7600 1.7 0.26Hyperpolyether-C6,0 Tg 167 N 262 Ti 3900 7600 1.9 0.24Hyperpolyether-C5,0 Tg 183 N 262 Ti 3900 7100 1.8 0.21Hyperpolyether-A3,2 Tg 126 N 235 Ti 4900 8800 1.8 0.28Hyperpolyether-C6,2 Tg 126 Sm 244 Ti 3600 7400 2.0 0.23Hyperpolyether-C5,2 Tg 161 N 246 Ti 3800 8100 2.1 0.22

aMelting temperature and glass transition temperatures are identified from 1st heating and 1st cooling cycles of DSC,respectively (at a heating/cooling rate of 5 �C/min under N2 atm).

bDetermined by GPC in THF with an RI detector. Mn, number average molecular weight; Mw, weight average molecularweight; PDI, polydispersity index.

c Inherent viscosity measured at 30 �C with a polymer solution (Concentration ¼ 0.5 g/dL in DMSO).

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Figure 1. POM photomicrographs of (a) Diol-A3,2 (142 �C, �400), (b) Diol-A3,2(135 �C, �400), (c) Diol-A3,2 (125 �C, �400), and (d) Diol-A3,2 (120 �C, �400).

Figure 2. POM photomicrographs of (a) Hyperpolyether-C6,2 (120 �C, �100), (b)Hyperpolyether-C6,2 annealed for 10 min (120 �C, �200), (c) Hyperpolyether-C6,2annealed for 15 min (120 �C, �400), and (d) Hyperpolyether-C6,2 (140 �C, �400).

them to exhibit liquid crystalline phases. Out ofsix polymers only one polymer (Hyperpolyether-C6,2) show battonet texture (smectic A) whileothers exhibit nematic texture (Fig. 2). Theseresults suggest that after polymerization the lat-eral interaction of mesogenic segments in otherpolyethers decreases which leads to nematic meso-phase. It is also noticed from the table that thebroadness in temperature range of liquid crystal-line regions increased very much compared to thecorresponding monomeric form. Thermogravimet-ric analysis (TGA) indicates that all polymers arestable up to 300 �C and the stability of polymerswithout substituents is higher (Fig. 3).

Photoresponsive Properties

The polymer solutions and films were exposed tothe radiation of wavelength close to the absorp-tion maximum, that is, 365 nm and subjected tospectral analysis by UV-Vis spectrophotometer tostudy the photoresponsive property. The changesin the spectral patterns of polymer solutions andfilms irradiated for different time intervals atroom temperature, is shown in Figure 4. A signifi-cant decrease in intensity as well as small blueshift in the wavelength is observed with increas-ing irradiation time due to rupture of olefinic dou-ble bond followed by 2p þ 2p cycloaddition reac-tion, which leads to formation of cyclobutaneunit.43 To distinguish inter and intramolecularcycloaddition, the irradiated polymer solutionswere studied for change in inherent viscosity. Theincrease in the inherent viscosity with irradiationtime confirms the intermolecular cycloaddition.

Intermolecular photocycloaddition was also con-firmed by 1H NMR of irradiated polymer solutionsin our previous report.35(a) A drastic reduction inthe absorption maxima in UV-Vis spectra isobserved for all polymer solutions in the first 30min (Fig. 5). It should be noted that long irradia-tion time is due to very low power of the UVsource used in the experiment (0.5 mW/cm2).Polymers (solutions and films) with no substitu-ent on the rigid repeating unit show faster rate ofphoto response compared to their analogues withsubstituents (Fig. 5). Polymer with acyclic moietyin the repeating unit show rapid response higherthan cyclic units while polymers with six-membered ring show faster response compared tothe corresponding polymers with five-memberedring (Fig. 5). It is expected that less steric hin-drance of acyclic moieties facilitate photocycload-dition while higher rigidity of five membered ringand steric hindrance of substituent prevent thephotoresponsive units to move close to each other.

Figure 3. TGA thermograms of polymers. [Color fig-ure can be viewed in the online issue, which is avail-able at www.interscience.wiley.com.].

Figure 4. UV-Vis spectra of (a) hyperpolyether-A3,2solution and (b) hyperpolyether-A3,2 film irradiatedfor different time intervals. [Color figure can beviewed in the online issue, which is available atwww.interscience.wiley.com.].

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These results suggest the significant role of stericfactors in photoresponsive properties of polymers.Change in the inherent viscosity and refractiveindex further supported the above findings. Thetrends in variation of inherent viscosity of poly-mer solutions (increase) and, in the refractiveindex of polymer films (decrease) with irradiationtime are similar to that observed in spectral stud-ies (Fig. 6). The change in the refractive index isobserved due to large change in the molecularpolarizability during photocrosslinking that leadsto large variation in contribution from bondrefraction to the total molar refraction accordingto Lorentz-Lorenz equation.44,45

Molecular Modeling Study of Model Compounds

Because oligomers have many striking similar-ities with polymers, a range of photoactive LCmonodendrons with 1-crosslink (as model com-pounds for polymers) have been constructed usingmolecule-building tools in the Accelrys MaterialsStudio. Conjugate gradient method of discovermolecular simulation program was used to opti-

mize the geometries (energy minimized) of the ini-tial structures of crosslinked monodendrons (Fig.7). Energies of crosslinked dendrons calculatedusing Universal Force field, Atom based Summa-tion method of ‘‘Forcite module’’ and the summaryof energy data of all molecules with varied struc-tural parameters are tabulated in Table 2. Aswith other molecular mechanics force fields, theUniversal Force field method calculates potential

Figure 5. Effect of nature of photoactive unit, cyclesize, and substituent on photocrosslinking rate of (a)polymer solutions and (b) polymer films. [Color figurecan be viewed in the online issue, which is availableat www.interscience.wiley.com.].

Figure 6. Effect of nature of photoactive unit, cyclesize, and substituent on (a) Photoviscosity of polymersolutions and (b) refractive index of polymer films.[Color figure can be viewed in the online issue, whichis available at www.interscience.wiley.com.].

Figure 7. 3D structure of crosslinked monodendronwith 1-crosslink point. [Color figure can be viewed inthe online issue, which is available at www.inter-science.wiley.com.].



energy on the basis of molecular geometry andthis energy comprises contributions from bonded(bond, angle, torsion and inversion) and non-bonded (van der waals and elctrostatic) interac-tions. It can be noticed from the Table 2 that themolecules with acyclic moieties show less totalenergies than the corresponding molecules withcyclic moieties, and the presence of substituentsincreases energy of molecules. However, the mole-cules with six-membered ring show less energycompared to the molecules with five-memberedring. It is also observed from Table 2 that thechanges in contributions from the bonds andangles to the total energy are more significant.The above findings suggest that steric factors andrigidity of the moieties play important role in thecrosslinking of photoactive units of molecules.These theoretical results can be correlated to

experimental photocrosslinking studies of UV-Vis spectroscopy, that is, faster photoresponsivebehavior of molecules with acyclic moieties isbecause of their steric freedom and the decreasein the rate of photocrosslinking with substitutentsoccur as stability of resulting molecules decreasesdue to the steric hindrance.

Fluorescence Study

Fluorescence spectra of polymers were recordedin DMSO at various time intervals under expo-sure to UV light (365). All hyperbranched poly-mers are fluorescent in nature and the fluores-cence maxima are observed in the range of 510–575 nm. The decrease in intensity with increasingirradiation time was explained in terms of compe-tition in the excited state to convert to dimer bycyclization and to deactivate to the relaxed stateby emitting fluorescence (Fig. 8).34 With anincrease in the time of irradiation the number ofreacted molecules increases and the number ofunreacted excited molecules decreases resultingin decrease of fluorescence intensity. The rate ofdecrease in fluorescence intensity of polymers fol-lowed the UV-Vis spectral patterns.

CONCLUSIONS

Two sets of hyperbranched polyethers, with struc-tural variations, were synthesized successfullyand characterized for their structural, mesogenic,and photoresponsive properties. The results sug-gested that the effects of substituent, rigidity, andnature of photoactive unit on phase behavior and

Table 2. Energy Calculation of Crosslinked Polyethers with 1-Generation and 1-Crosslink Point UsingMolecular Modeling Studies

Energy

Sample Name

Valence Energy(kcal/mol)

Nonbond Energy(kcal/mol)

Total Energy ¼Valence Energy þNonbond EnergyBond Angle Torsion Inversion

Van derWaals Electrostatic

Hyperpolyether-A3,0 75.99 188.05 23.58 0.03 411.85 2.97 702.49Hyperpolyether-A3,2 74.95 204.53 90.81 0.03 494.89 62.52 927.75Hyperpolyether-C6,0 93.87 170.42 100.83 0.30 474.91 57.42 897.77Hyperpolyether-C6,2 92.73 202.61 139.47 0.26 586.82 92.38 1114.29Hyperpolyether-C5,0 80.78 287.13 60.95 0.17 410.02 107.73 946.80Hyperpolyether-C5,2 78.81 309.04 135.63 0.22 547.25 159.74 1230.72

Figure 8. Rate of change of fluorescence intensity ofhyperpolyether-A3,2. [Color figure can be viewed inthe online issue, which is available at www.inter-science.wiley.com.].

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photoresponsive properties were quite appreci-able. Out of six monomeric diols, only four haveexhibited liquid crystalline phase while the poly-mers corresponding to all monomeric diolsrevealed mesophase. Two diols showed broken fantexture and other two diols showed nematic tex-tures. One polymer exhibited batonett texture(smectic-A) while remaining all showed nematicphase. The rigid cyclic moieties in the repeatingunit increased the transition temperatures whilethe presence of substituents decreased the transi-tion temperatures. The acyclic units facilitatedthe photocycloadition while the steric hindranceof substituents decreased the rate of photocy-cloaddition. Energies calculated from molecularmodeling studies of crosslinked monodendrons,and changes in the refractive index, photovis-cosity, and fluorescence intensity with irradia-tion time provided additional information sup-porting the spectral pattern observed in UV-Visspectroscopy.

The authors thank J. N. Das, Director of Naval Materi-als Research Laboratory, for his permission to publishthis article. They thank S. Srivastava, Tata Institute ofFundamental Research, India, for extending the highfield NMR facility. They also thank S. Das and V. Kanse,Pidilite Industries, India, for extending the GPC andDSC instruments facility.

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phase behavior and photo-responsive studies of photoactive liquid crystalline hyperbranched...

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