Electrothermal Hybrids for Soft Robotic Applications

by

Yu-Chen Sun

A thesis submitted in conformity with the requirements

for the degree of Doctor of Philosophy

Mechanical and Industrial Engineering

University of Toronto

© Copyright by Yu-Chen Sun 2019

ii

Electrothermal Hybrids for Soft Robotic Applications

Yu-Chen Sun

Doctor of Philosophy

Mechanical and Industrial Engineering

University of Toronto

2019

Abstract

Throughout the evolution of human technologies, electronic-based actuators have been

unreplaceable. These actuators can provide superior power and speed in comparison to biological

tissue. However, complexity and bulkiness are two of the major weaknesses that prevent

electronic-based actuators from being implemented in biomimicking and biomedical applications.

Recent advances in material sciences have suggested the possibility of flexible material-based

actuators. These “smart material” actuators can be constructed by combining different composite

materials, and motion or deformation can be observed once the actuator is subjected to a proper

stimulus. This thesis presents a series of studies that focus on the development of novel active

hybrid composites. Starting with a well-known polymeric blend shape memory polymer (SMP)

and electrothermal actuator (ETA) system, the composites were first modified with nanoparticles

to achieve room temperature deformability. The plasticizing effect was then utilized to enhance

the actuation temperature, while conductive filler was implemented to enable electroactive ability.

Lastly, a novel 4D printing fabrication process was presented, which demonstrated the possibility

of using the material-based actuators in artificial muscles and soft robotic applications.

iii

Acknowledgments

I would like to use this opportunity to express my gratitude to everyone who supported me

throughout the course of my PhD program. I cannot thank enough to my parents, friends, and the

rest of my family for enormous amount of support they have provided.

I would also like to acknowledge my PhD supervisor, Professor Hani Naguib for the tremendous

amount of guidance, inspirations, and opportunities he has provided. As a truly dedicated

supervisor, his encouragement in research is one of the essential elements for the complementation

of this work.

Special thanks to my lifelong friends, Nazanin Khalili, Harvey Shi, Eunji In, and Marco Chu. I am

deeply grateful for all their supports and encouragement. This work would not have been possible

without you. Acknowledgement also goes to my past and present colleagues from the Smart

Polymers can Composites Lab (SAPL) for providing me with endless help in research.

Financially this work was supported by the Natural Sciences and Engineering Research Council

(NSERC) of Canada, Ontario Graduate Scholarship (OGS), Ontario Centres of Excellence (OCE),

Mitacs agency, Governments of Ontario and Canada.

iv

Table of Contents

Table of Contents

Acknowledgments ............................................................................................................... iii

Table of Contents ................................................................................................................. iv

List of Tables .....................................................................................................................viii

List of Figures ....................................................................................................................... x

List of Symbols, Abbreviations and Nomenclature .............................................................. xv

Chapter 1 Introduction .......................................................................................................... 1

1.1 Preamble..................................................................................................................... 1

1.2 Definition of soft robotics smart active materials ....................................................... 2

1.3 Design of electrically and thermally active materials ................................................. 5

1.3.1 Shape Memory Polymer .................................................................................... 5

1.3.2 Electrothermal actuator ..................................................................................... 7

1.3.3 4D printing and 4D active materials .................................................................. 9

1.4 Motivation and Objectives ........................................................................................ 10

1.4.1 Objectives ....................................................................................................... 12

1.5 Organization of the Thesis ........................................................................................ 13

1.6 Contribution ............................................................................................................. 15

1.7 References ................................................................................................................ 16

Chapter 2 Room Temperature Deformable Shape Memory Composite with Fine-tuned

Crystallization Induced via Nanoclay Particles .............................................................. 22

2.1 Introduction .............................................................................................................. 23

2.2 Experimental ............................................................................................................ 26

2.2.1 Materials ......................................................................................................... 26

2.2.2 Composite Fabrication .................................................................................... 26

2.2.3 Composite Characterization ............................................................................ 27

v

2.3 Results and Discussion ............................................................................................. 29

2.3.1 SEM & Morphologies ..................................................................................... 29

2.3.2 Energy-dispersive X-ray Spectroscopy (EDS) ................................................. 31

2.3.3 FTIR ............................................................................................................... 34

2.3.4 DSC ................................................................................................................ 36

2.3.5 X-ray Diffraction ............................................................................................ 41

2.3.6 TGA ............................................................................................................... 43

2.3.7 Storage Modulus and Tan Delta ...................................................................... 45

2.3.8 Shape Memory Recovery ................................................................................ 47

2.4 Conclusion................................................................................................................ 51

2.5 References ................................................................................................................ 52

Chapter 3 Toward the Low Actuation Temperature of Flexible Shape Memory Polymer

Composites with Room Temperature Deformability via Induced Plasticizing Effect ...... 57

3.1 Introduction .............................................................................................................. 58

3.2 Experimental ............................................................................................................ 62

3.2.1 Materials ......................................................................................................... 62

3.2.2 Composite Fabrication .................................................................................... 62

3.2.3 Composite Characterization ............................................................................ 63

3.3 Results and Discussion ............................................................................................. 65

3.3.1 SEM & Morphology ....................................................................................... 65

3.3.2 FTIR ............................................................................................................... 66

3.3.3 DSC ................................................................................................................ 67

3.3.4 TGA ............................................................................................................... 70

3.3.5 Storage Modulus and Tan Delta ...................................................................... 71

3.3.6 Rheology ........................................................................................................ 73

3.3.7 Shape Memory Properties ............................................................................... 74

vi

3.4 Conclusion................................................................................................................ 79

3.5 References ................................................................................................................ 80

Chapter 4 Hybrid Electroactive Shape Memory Polymer Composites with Room

Temperature Deformability ........................................................................................... 84

4.1 Introduction .............................................................................................................. 85

4.2 Materials & Experimental ......................................................................................... 90

4.3 Results and Discussion ............................................................................................. 91

4.3.1 Polymer Blend Morphologies and SWCNT Dispersions.................................. 91

4.3.2 Chemical Bonding Verification ....................................................................... 94

4.3.3 Thermal Properties and Degradation Behaviours of SMP Composites ............. 95

4.3.4 Rheological Behaviour .................................................................................... 99

4.3.5 Mechanical Property Characterization ........................................................... 101

4.3.6 Shape Memory Characterization – High Temperature Deformation............... 104

4.3.7 Room Temperature Deformability................................................................. 107

4.3.8 Electrical Conductivity, Joule heating, and Electroactive Demonstration ....... 109

4.4 Conclusion.............................................................................................................. 111

4.5 References .............................................................................................................. 112

Chapter 5 Shape Programming of Polymeric-based Electrothermal Actuator (ETA) via

Artificially Induced Stress Relaxation .......................................................................... 117

5.1 Introduction ............................................................................................................ 118

5.2 Materials & Experimental ....................................................................................... 121

5.2.1 Fabrication of Dual Layer ETA ..................................................................... 121

5.2.2 Characterization ............................................................................................ 122

5.3 Results and Discussion ........................................................................................... 123

5.3.1 ETA Material Properties ............................................................................... 123

5.3.2 ETA Morphology .......................................................................................... 126

5.3.3 Actuation of ETA.......................................................................................... 127

vii

5.3.4 Shape Programming of ETA ......................................................................... 131

5.3.5 PDMS post-curing and thermo-mechanical verification................................. 135

5.3.6 Nuclear magnetic resonance analysis ............................................................ 139

5.3.7 ETA Applications with Different Programmed Shape ................................... 140

5.4 Conclusion.............................................................................................................. 143

5.5 References .............................................................................................................. 144

Chapter 6 4D Printed Hybrids with Localized Shape Memory Behaviour: Implementation in

Functionally Graded Structure ..................................................................................... 148

6. Introduction .............................................................................................................. 149

6.2 Materials & Experimental ....................................................................................... 153

6.3 Results and Discussion ........................................................................................... 155

6.3.1 Plasticizer Induced Microscopic Morphological Changes .............................. 155

6.3.2 Chemical Composition Verification .............................................................. 157

6.3.3 Thermal Behaviour of Plasticized PLA Composites ...................................... 158

6.3.4 Viscoelastic and Rheological Characterizations ............................................. 161

6.3.5 Temperature Dependent Mechanical Properties............................................. 162

6.3.6 Characterization for Tensile Shape Memory Properties ................................. 164

6.3.7 Plasticized 4D Materials and Their Recovery Behaviours.............................. 166

6.3.8 Functionally Graded 4D Material with Localized Recovery Ability .............. 168

6.4 Conclusion.............................................................................................................. 170

6.5 References .............................................................................................................. 171

Chapter 7 Conclusions and Recommendations .................................................................. 177

7.1 Conclusions ............................................................................................................ 177

7.2 Recommendations for Future Works ....................................................................... 180

viii

List of Tables

Table 2-1. Isothermal crystallization half time under different isothermal temperature conditions.

................................................................................................................................................ 37

Table 2-2. Thermal properties of PLA/TPU/MMT composites ................................................. 40

Table 2-3. Effect of annealing on thermal properties of PLA/TPU/MMT composites ............... 40

Table 2-4. Summary of 2θ and d-spacing of different PLA/TPU/MMT composites .................. 42

Table 2-5. Thermal degradation of different PLA/TPU composites .......................................... 44

Table 3-1. Summary of thermal properties of PLA/TPU/PEG composites ................................ 69

Table 3-2. Summary of initial degradation temperatures and maximum weight loss (PLA/TPU)

................................................................................................................................................ 70

Table 3-3. Summary of initial degradation temperatures and maximum weight loss (PEG) ...... 70

Table 3-4. Summary of tan Delta peak temperature of different PEG content ........................... 72

Table 4-1. Electroactive SMP performance from literature ....................................................... 86

Table 4-2. Summary of thermal properties of PLA/TPU/PEG/CNT composites ....................... 97

Table 4-3. Degradation temperature of PLA/TPU/PEG and PLA/TPU/CNT composites .......... 98

Table 4-4. Degradation temperature of PLA/TPU/PEG/CNT composites ................................. 98

Table 5-1. Thermal properties of PDMS ..................................................................................125

Table 5-2. ETA performance comparison chart ...................................................................130

Table 5-3. Programming and actuation behaviour comparison of ETA and SMP .....................133

ix

Table 6-1. Summary of thermal properties of PLA/PEG composites........................................159

Table 6-2. Summary of initial degradation temperatures and maximum weight loss ................160

Table 6-3. Mechanical properties under room temperature testing condition ............................163

Table 6-4. Mechanical properties under 70oC testing condition ...............................................163

x

List of Figures

Figure 1-1. Smart active materials and their corresponding mechanics. ...................................... 4

Figure 2-1. SEM images of (a, b) PLA/TPU, (c, d) PLA/TPU/1 wt% MMT, and (e, f)

PLA/TPU/3 wt% MMT ............................................................................................................ 30

Figure 2-2. EDS spectrum of PLA/TPU/MMT composites ...................................................... 31

Figure 2-3. EDS mapping of PLA/TPU (a) SEM image, (b) carbon/nitrogen overlay, (c) carbon

mapping, and (d) nitrogen mapping .......................................................................................... 32

Figure 2-4. EDS mapping of PLA/TPU/3MMT (a) SEM image, (b) carbon/nitrogen overlay, (c)

carbon mapping, (d) nitrogen mapping, (e) oxygen mapping, and (f) silicon mapping .............. 33

Figure 2-5. FTIR Absorption spectra of PLA, TPU, PLA/TPU, and PLA/TPU/MMT .............. 35

Figure 2-6. (a) Complete FTIR data of different PLA/TPU/MMT sample and (b) close up view

of TPU characterization peaks .................................................................................................. 35

Figure 2-7. Isothermal DSC curve for (a) 100°C, (b) 110°C, (c) 120°C, and (d) 130°C ............ 37

Figure 2-8. Crystallization half time (a) effect of MMT and (b) effect of isothermal temperature

................................................................................................................................................ 38

Figure 2-9. DSC heating curve of (a) PLA/TPU/MMT and (b) annealed PLA/TPU/MMT ....... 39

Figure 2-10. XRD measurement of different PLA/TPU/MMT composites ............................... 41

Figure 2-11. Thermal degradation of PLA/TPU/MMT composites .......................................... 43

Figure 2-12. (a, b) Storage modulus and tan delta of PLA/TPU/MMT composites, and (c, d)

annealed composites................................................................................................................. 46

Figure 2-13. Shape memory cycle for (a) PLA/TPU, (b) PLA/TPU/1MMT, (c)

PLA/TPU/2MMT, and (d) PLA/TPU/3MMT composites ......................................................... 48

xi

Figure 2-14. Shape recovery ratio (Rr) and shape fixing ratio (Rf) of different wt% MMT in

PLA/TPU composites ............................................................................................................... 49

Figure 3-1. SEM images of (a, b) PLA/TPU; (c, d) PLA/TPU/5%PEG; (e, f)

PLA/TPU/10%PEG; (g, h) PLA/TPU/15%PEG; and (i, j) PLA/TPU/20%PEG ........................ 65

Figure 3-2. FTIR absorption spectra of PLA/TPU and PLA/TPU/PEG .................................... 66

Figure 3-3. (a) DSC curves of pristine materials, (b) DSC curves of TPU/PEG & PLA/PEG

composites, (c)DSC curves of PLA/TPU/PEG composites and (d) thermal degradation of

PLA/TPU/PEG composites ...................................................................................................... 68

Figure 3-4. (a) Storage modulus and (b) tan delta of different PLA/TPU/PEG composites ....... 72

Figure 3-5. Viscosity of different PLA/TPU/PEG composites.................................................. 73

Figure 3-6. Shape memory cycles of (a) 0PEG, (b) 5PEG, (c) 10PEG, (d) 15PEG, (e) 20PEG,

and (f) Rf under two different shape fixing conditions ............................................................... 75

Figure 3-7. (a) Recovery ratio (Rr) and (b) recovery time of PLA/TPU/PEG composites under

two different recovery temperatures environment ..................................................................... 77

Figure 3-8. Shape recovery of PLA/TPU/PEG composites in 50°C water bath: top to bottom: 0,

10, 20 wt% PEG; left to right: 0, 5, 10, 20 seconds ................................................................... 78

Figure 3-9. Shape recovery of PLA/TPU/PEG composites in 70°C water bath: top to bottom: 0,

10, 20 wt% PEG; left to right: 0, 5, 10, 20 seconds ................................................................... 78

xii

Figure 4-1. Plasticizing effect enhanced CNT dispersion and stretchability enhancement ........ 88

Figure 4-2. Design of PEG/CNT hybrid SMP composites ........................................................ 89

Figure 4-3. SEM images of PLA4TPU6 with (a, d) PEG0; (b, e) PEG5; (c, f) PEG10 ............. 92

Figure 4-4. SEM images of PLA4TPU6 with (a, g) CNT2; (b, h) CNT2PEG5; (c, i)

CNT2PEG10; (d, f) CNT4; (e, k) CNT4PEG5; (f, l) CNT4PEG10 ........................................... 92

Figure 4-5. TEM images of (a, b) PLA/TPU/CNT4 and (c, d) PLA/TPU/CNT4/PEG5 ............ 93

Figure 4-6. FTIR spectra of different PLA/TPU/PEG/CNT composites ................................... 94

Figure 4-7. (a, b, & c) DSC curves; (d, e, f) TGA curves of PLA/TPU/PEG/CNT composites . 96

Figure 4-8. Viscosity of PLA/TPU/PEG/CNT composites ......................................................100

Figure 4-9. (a, b, c) Stress-strain curves; (d, e, f) storage and (g, h, i) loss modulus of different

PLA/TPU/PEG/CNT composites.............................................................................................103

Figure 4-10. Shape memory cycles of different PLA/TPU/PEG/CNT composites ...................106

Figure 4-11. (a, b) Room temperature stretched shape memory cycle of different

PLA/TPU/PEG/CNT composites; (c) Rr & Rf of under 80°C stretching; (d) Rr & Rf under room

temperature stretching .............................................................................................................108

Figure 4-12. (a) Electrical conductivity, (b) Joule heating performance and (c) Joule heating

time of different PLA/TPU/PEG/CNT composites (d) Demonstration of electrical shape recovery

ability of CNT2PEG10 sample ................................................................................................110

xiii

Figure 5-1. (a) SWCNT film fabricated from solvent casting, and (b) flexing of SWCNT film

...............................................................................................................................................121

Figure 5-2. Dimension of U-shape actuator ............................................................................123

Figure 5-3. Cross sectional SEM images of fabricated SWCNT/PDMS ETA ..........................126

Figure 5-4. Cross sectional SEM images of fabricated SWCNT/PDMS ETA ..........................128

Figure 5-5. Actuation, tip displacement, and bending angle of PDMS ETA ............................129

Figure 5-6. Shape programming demonstration of (a) SWCNT/PDMS ETA (b) pure PDMS and

(c, d) after programming .........................................................................................................131

Figure 5-7. Stress relaxation (left) and the following strain recovery (right) of pure PDMS and

PDMS + CNT composites under different condition................................................................134

Figure 5-8. a) Isothermal of PDMS curing behaviour, b) DSC temperature ramp of different

PDMS samples, c) 2 & 4 hours 80oC curing behaviour with respect to time, and d) 2 & 4 hours

80oC curing behaviour with respect to temperature ..................................................................136

Figure 5-9. a) Storage and loss modulus of PDMS & PDMS+CNT ETA under temperature

ramping, b) derivative of the storage modulus, c) heat-and-hold experiment with respect to

temperature, and d) heat-and-hold experiment with respect to time .........................................138

Figure 5-10. NMR spectrum of PDMS ...................................................................................139

Figure 5-11. ETA actuation from a programmed curled shape and temperature changes

compared to non-programmed (flat) configuration ..................................................................141

Figure 5-12. ETA crawling soft robot and the actuation behaviour .........................................142

xiv

Figure 6-1. Basic building blocks for 4D printing development ..............................................150

Figure 6-2. Geometry of 3D printed arc-shape component ......................................................153

Figure 6-3. Fracture surface of (a, d) PLA, (b, e) 10PEG, and (c, f) 30PEG ............................155

Figure 6-4. 3D printed fracture surface of (a, d) PLA, (b, e) 10PEG, and (c, f) 30PEG ...........156

Figure 6-5. FTIR absorption spectra of PLA/PEG composites ................................................157

Figure 6-6. (a) DSC curves of PLA/PEG composites, (b) thermal stability of PEG at 180oC, (c)

thermal degradation and (d) the derivative of thermal degradation curves ................................159

Figure 6-7. (a) Dynamic frequency sweep, (b) temperature sweep from dynamic mechanical

testing; (c) storage/loss moduli and (d) viscosity measurement ................................................162

Figure 6-8. Stress strain curves of PLA/PEG composites at (a) room temperature and (b) 70oC

...............................................................................................................................................163

Figure 6-9. 3D shape memory cycle for different PLA/PEG composites .................................165

Figure 6-10. (a) 4D printed PLA arc and (b) flattened temporary shape ...........................166

Figure 6-11. Thermal recovery of 4D printed (a) PLA, (b) 10PEG, and (c) 30PEG component

...............................................................................................................................................167

Figure 6-12. 4D printed functionally graded model.................................................................169

Figure 6-13. Thermal recovery of 4D functionally graded model ............................................169

xv

List of Symbols, Abbreviations and Nomenclature

CNF Carbon nanofiber

CNT Carbon nanotube

CTE Coefficient of thermal expansion

DEA Dielectric elastomer actuator

DMA Dynamic mechanical analyzer

DSC Differential scanning calorimetry

ΔHcc Cold crystallziation enthalphy

ΔHm Mleting enthalphy

EAP Electroactive polymer

EDS Energy-dispersive X-ray spectroscopy

ETA Electrothermal actuator

εm Applied strain

εp Recovry strain

εu Stress-free strain

FDM Fused deposition modeling

FGM Functionally graded material

FTIR Fourier transform infrared spectroscopy

HA Hydroxyapatite

IR Infrared radiation

IPMC Ionic polymer metal composite

MAG-PLA Maleic anhydrided-grafted poly(lactic acid)

MEMS Microelectromechanical systems

MMT Montmorillonite

xvi

MWCNT Multi-walled carbon nanotubes

NMR Nuclear magnetic resonance spectroscopy

OCL Oligo(ɛ-caprolactone)diol

ODX Oligo(p-dioxanone)diol

PAC Printed active composite

PAE Polyamide elastomer

PE Polyethylene

PEB Poly(ethylene-co-butylene)

PCL Poly(ϵ-caprolactone)

PDLLA Poly(D,L-lactide)

PDMS Polydimethylsiloxane

PEG Poly(ethylene glycol)

PLA Poly(lactic acid)

PLLA Poly(L-lactide)

PMMA Poly(methyl methacrylate)

POA Poly(octylene adipate)

PPDL Poly(ω-pentadecalactone)

PU Polyurethane

PVAc Poly(vinyl acetate)

PVDF Poly(vinylidene fluoride)

Rf Shape fixing ratio

Rr Shape recovery ratio

SBS Poly(styrene-butadiene-styrene)

SEM Scanning electron microscopy

xvii

SLA Stereolithography

SMA Shape memory alloy

SME Shape memory effect

SMP Shape memory polymer

SWCNT Single-walled carbon nanotube

TEM Transmission electron microscopy

TGA Thermogravimetric analyzer

TPU Thermoplastic polyurethane

Tcc Cold-crystallization tempearture

Tg Glass transition temperature

Ti Initial degradation temperature

Ti, max Maximum weight loss slope

Ti, max, 1 First maximum weight loss slope

Ti, max, 2 Second maximum weight loss slope

Tm Melting temperature

Tpermanent Permanent temperature

Ttrans Transitional temperature

t1/2 crystallization half time

UV Ultraviolet

WPU Waterborne polyurethane

wt% Weight percent

XRD X-ray diffraction

Xc% Percent crystallinity

1

Chapter 1

Introduction

1.1 Preamble

The development of actuators can be traced back to the early industrial revolution when it was

discovered that steam power could be harvested to power a variety of mechanical structures and

devices. In comparison to human or animal muscles, steam-powered machines do not experience

fatigue and could achieve much higher efficiency. Inventors and engineers all over the world have

since designed and built machines that can perform various motions to do work like lifting,

rotating, and re-positioning. Parallel to the evolution of machining, the use of electrical energy has

played a significant role in the progression of engineering development. Electricity can be easily

transmitted through a conductive material or stored as an electrical potential by electrochemical

reactions. Electricity has replaced most of the massive steam engines and has become the primary

energy source powering our society today.

The use of more complex and efficient actuators has allowed for machines to become faster and

more powerful, with precisions beyond what biological muscles can achieve. However, these

design-specific actuators often perform poorly when removed from their typical operation modes

to carry out other discontinuous or non-repetitive tasks. Despite the great force output and response

speed, traditional motors and actuators face a series of challenges in the progress of mimicking

human motions. Complex transmission systems and designs are also required for the construction

of muscle-like actuators. These unavoidable secondary systems increase the overall

weight/bulkiness and add to the manufacturing cost. Researchers have expressed interest in

overcoming these challenges by replacing the bulky electrical and mechanical systems with bio-

inspired designs. It was discovered that certain groups of material could change their physical

dimensions when external signals, such as electrical or thermal stimuli, are applied to them. The

2

behavior and response of these materials are extremely similar to biological tissues, and these so-

called “artificial muscle” materials would be suitable for biomedical applications where muscle-

like motion is required.

There were several challenges posed during the development of material-based actuators. These

range from the understanding of fundamental material properties to manufacturing and

implementing the smart materials. The purpose of this research is to provide insight into the

essential microscale material parameters and their implication on the macroscopic actuator

behaviour. Chapter 1 starts by introducing different categories of smart active material. Their

history and corresponding actuation mechanisms are discussed in detail. The motivations and

objectives of the research are also addressed. Finally, the chapter concludes with a brief layout of

the following chapters.

1.2 Definition of soft robotics smart active materials

Soft robotics is an emerging research field and can be defined as the practices of replacing

traditional motors/actuators with low compliance materials to generate motions [1-3]. These

materials are able to undergo large amount of dimensional or geometrical changes when subjected

to a proper stimulus, which can vary from electricity, light, heat, or even solvent induced chemical

reactions [4-6]. Compared to conventional robotics systems which are usually mechanically

driven, these biomimicking or bio-inspired technologies, or artificial muscle materials, are highly

flexible and can be easily manipulated to suit the desirable functionalities that range from wearable

textiles to industrial applications. The greatest advantages offered by these materials are flexibility

and adaptability, which make them favorable in applications related to biomimicking, and

complex/non-repeatable tasks. One of the currently research focuses of soft robotic materials is

the development of pneumatic based actuator [7-9]. These actuators have the capability to generate

motions by changing the amount of air or liquid pressure within a flexible tentacle-like polymeric

based tube that is designed to either to expand or contract in order to respond to the pressure

difference. It has been demonstrated that such materials have the ability to grip and manipulate

complex features and tools [4, 10, 11].

3

The fundamental definition of smart materials is the ability to respond to environmental changes

[12]. By using this special characteristic, the behaviour of the material can be controlled by

manipulating the material properties. The smart materials can then be implemented into a wide

range of applications in healthcare, biomedical, structure, and aerospace sectors [13-17]. One of

the most popular applications is to employ smart active material as artificial muscles [18-20].

Compared to traditional motors, material-based actuators offer advantages such as superior

flexibility, better biomimicry, lower cost, and higher power-to-weight ratio. These properties make

them more suitable for many applications involving lightweight prosthetics and soft robotics [20-

23]. Furthermore, some of these actuators have even demonstrated performances that surpass

biological muscles [9, 18].

Figure 1-1 shows the categories that the active material can be placed based on their actuation

mechanisms. Their microstructure and properties can respond to various stimuli, such as light

(optically active), heat (thermally active), and electric fields (electroactive) [24-27]. Deformation

or geometrical changes can be observed once the material has been subjected to the appropriate

stimuli; hence, their suitability as an actuator. The variation in actuation mechanisms can also

affect their actuation performance. For instance, the deformation produced by ceramic based

ferroelectric or piezoelectric actuators are highly accurate and controllable but occur on the

microscale level [28]. In comparison, dielectric elastomer actuators (DEA) can produce record-

breaking strain but require an extremely high electric field for actuation [29, 30]. Conductive

polymers or ionic polymer metal composites (IPMC) only require a minimum amount of electrical

power, but the actuation can only be observed when the material is present in an ionic-rich liquid

environment [31, 32]. Consequently, the choices of material-based actuation are usually

application dependent and must consider the possible and available stimuli.

4

Figure 1-1. Smart active materials and their corresponding mechanics.

Smart Active Materials

Optically Active Chemically Active Thermally Active Magnetoactive Electroactive

MagnetostrictiveShape Memory EffectIonic Electronic

Dielectric Elastomer

Shape Memory

Polymer (SMP)

Shape Memory Alloy

(SMA)

FerrorelectricIonic Polymer Metal

Composites (IPMCs)Conductive Polymer

Polymeric Gels Chiral Materials

5

1.3 Design of electrically and thermally active materials

Over the past few decades, the development of smart materials had focused on the enhancement

of electrical and thermal activation. The input voltage/current can be easily controlled by

laboratory-scale instruments, while the variation in environmental temperature can be adjusted in

an air oven or water bath. Furthermore, the electrical energy that is imparted can be converted into

thermal energy through the Joule heating phenomenon. Among the thermally activated

mechanisms, the shape memory effect and thermal expansion dominate as they can produce the

best actuation performance. The following sections focus on shape memory polymers and

electrothermal actuators, as well as possible manufacturing procedures for these materials.

1.3.1 Shape Memory Polymer

The shape memory effect (SME) is defined as the material’s ability to change back to a predefined

shape after deformation. For the recovery to take place, an external stimulus is necessary. This

stimulus can be heat, light, electrical, or magnetic fields [33-39]. Two of the most common shape

memory materials are shape memory alloys (SMAs) [40-42] and shape memory polymers (SMPs)

[35, 43, 44]. The actuation mechanism of SMA is driven by the phase transformation between

three crystalline structures: twinned martensite, detwinned martensite, and austenite. These

transformations can be induced by temperature differences or applied stress. For example,

austenite can be transformed into detwinned martensite by stretching, and the strain recovery can

take place after unloading at high temperature. The advantages of SMA are their high stress (>200

MPa), high strain rate (300%/s), and large strain (>5%) [45]. However, the major drawback of

SMAs is the significant hysteresis during phase transformation and their limited lifetime;

activation strain can drop from 5% to 0.5% after millions of cycles [46, 47].

In contrast to the SMA, SMPs are constructed from two distinct soft and hard polymer phases. The

status of the SMP is determined by the transitional temperature (Ttrans). Below Ttrans, the SMP is

rigid. When SMPs are heated above Ttrans, the material is in the soft/rubbery state. In such a state,

the SMP can transform into a new configuration by applied stress or deformation on the material.

The decrease in temperature to below Ttrans “fixes” the SMP into a new/temporary shape because

of the hard/glassy state. If heat is applied to the “fixed” SMP, the material state returns to the

6

rubbery state, recovering the overall geometry [35, 44, 48]. As a result, crosslinked polymers or

semi-crystalline are good candidates for SMP application. The crosslinked regions or “nodes”

serve as the hard phase for memorizing the shape while the amorphous regions are responsible for

the shape recovery process once the temperature is above Ttrans [44, 49]. To characterize the shape

memory performance, two parameters, shape recovery ratio (Rr) and shape fixing ratio (Rf), are

often used. Rr is associated with the ability to maintain its temporary shape while the latter is used

to determine the recovery ability. These parameters can be calculated by using Equation (1.1) and

Equation (1.2), as shown below:

𝑅𝑟 =𝜀𝑚 − 𝜀𝑝

𝜀𝑚 (1.1)

𝑅𝑓 =𝜀𝑢𝜀𝑚

(1.2)

where εm is the amount of strain applied, εp is the strain after shape recovery, and εu is the strain

after removing the deformation stress. For high-performance SMPs, both Rr and Rf approach

100%, which indicate that the material can maintain its temporary shape (Rf = 100%) and fully

recover to its permanent shape (Rr = 100%)

Compared to crosslinked thermoset polymers, thermoplastics have the advantage of remoldability

and reusability. This can be achieved because their primary polymer chains are not chemically

crosslinked [50]. Also, their shape recovery behaviours can be easily tailored by adjusting the two

different polymeric matrices [51]. Because of its biocompatibility, polylactic acid (PLA) is one of

the most studied semi-crystalline thermoplastic SMPs. The shape memory of polylactic acid was

first reported by Zhang et al. in 2009, during the touching process [52]. The group reported that

the ductility of PLA can be significantly enhanced by blending with an elastomer (5.1% to 161.5%

and 194.6% with 5 and 10 wt% of elastomer). Along with the increase in stretchability, the SME

can also be observed as PLA no longer exhibits the brittle fracture behaviour. The phrase

“polymeric blend SMP” can be used to describe this multi-phase system since the fabricated SMP

is constructed from two immiscible polymers [44, 51]. PLA-based SMPs can be created from

blending PLA with polyvinyl acetate (PVAc) [44], polyethylene glycol (PEG) [53, 54], or

thermoplastic polyurethane (TPU) [55-59].

7

1.3.2 Electrothermal actuator

The operating principle of Electrothermal actuator (ETA) is governed by the thermal expansion of

the material. The kinetic energy of individual molecules increases and vibrate more at higher

temperatures. These vibrations impose larger separations between molecules leading to an

observable volumetric expansion. One of the earliest applications to utilizing such an effect was a

micro-sized actuator made from silicon for microelectromechanical systems (MEMS) [60, 61] and

in situ microscopy testing for nanomaterials [62, 63]. To induce actuation, resistive Joule heating

is commonly employed. As the degree of thermal expansion of silicon can be predicted with high

accuracy, micro or even nanoscale movement can be precisely controlled by these actuators.

Several analytical models and studies have demonstrated that the output stress and strain for

silicon-based thermal actuator can be accurately described [60, 62, 64]. According to these models,

both the actuator geometry and material properties play important roles in determining actuation

behaviour. For example, the electrical resistivity and thermal conductivity describe the electro-

thermal relationship and energy conversion efficiency while the material’s Young’s modulus and

coefficient of thermal expansion (CTE) is related to actuation behaviour. Although the MEMS

thermal actuator can be considered as a well-established field, the micro/nanoscale displacements

are too small for other applications.

Researchers have also demonstrated that certain polymeric composites systems possess the same

actuation properties as silicon-based thermal actuators. Compared to silicon-based ETA, these

polymeric based materials have a much greater CTE and can produce a significant amount of

deformation. Since most of the polymers are electrical insulators, the ETA composites are

constructed with reinforcement fillers that are highly conductive. An applied voltage to the

composite material provides Joule heating to the polymer matrix through the conductive fillers,

thereby activating the matrix. The very first polymeric based ETA was proposed by Chen et al. in

2008; the group utilized Joule heating to drive the thermal expansion of an elastomer based

composite containing polydimethylsiloxane (PDMS) and 5 wt% of carbon nanotubes (CNTs) [65].

Under 52V or 1.5V/mm electric field, the composite was able to reach a maximum strain of 4.4%

after a temperature increase to 180°C. In comparison to other electroactive polymers reported in

the literature, ETAs demonstrate large deformation, fast response time, and low electrical field

8

requirement. It has been identified that CTE controls the overall ETA deformation performance;

nevertheless, there are very limited studies that focus on enhancing the CTE of the bulk polymer.

An alternative CTE enhancement approach was proposed by Samel et al. that uses expandable

microspheres, which can undergo a large amount of volumetric increase after heating [66, 67]. The

initial designs proposed by the group can be implemented in microfluidic systems as micro-valves

or pumps; however, the major disadvantage of the composite is that it can only provide one-time

actuation due to the irreversible expansion. Despite this, an astonishing volumetric expansion of

over 270% was reported from this design. One-time expansion actuators should not be overlooked

as they might be useful for specific applications, such as single-use smart bandages or fasteners.

Compared to the single layer CNT/PDMS film proposed in [65], Zeng et al. coated pure PDMS

onto a PDMS/CNT film and created a double layer structure. This structure enables bending

motion due to the different CTE of the two layers [68]. To further increase the actuation

performance, proposed strategies such as fiber alignment can be implemented. Chen et al.

demonstrated that by replacing randomly distributed CNT with ultra-aligned CNT sheets or “bulky

paper”, the voltage required for actuation was significantly reduced [69]. The same research group

also showed that the actuation behaviour has a strong relationship with the orientation of the bulky

paper: if the electrical current is applied in the fiber direction, less heat and actuation motion is

generated because of the lower resistance. These examples demonstrate the potential of using

polymeric ETA materials for artificial muscle applications due to their low voltage requirement

and high displacement/force response.

9

1.3.3 4D printing and 4D active materials

4D printing is a relatively novel fabrication concept which combines smart active materials and

3D printing technologies [70-73]. The 3D printing techniques enable the creation of complex

geometries, while the printed active materials allow the fabricated components to be capable of

responding to external stimuli. The 4th dimension is described as “time” as the shape or geometry

components can be designed to shift or move in the desired fashion. Similar to non-printed smart

materials, a proper environmental change, such as changes in temperature [74-76] or water

presence [70, 77, 78], is required to trigger the motion.

One of the earliest 4D printing reports was provided by Ge et al., who used the phrase “printed

active composites” (PACs) to describe the materials [79]. The group combined polymer fibers with

shape memory properties and an ultraviolet (UV) curable elastomer matrix. As the fibers and

elastomer matrix have different properties, mechanical deformation can be applied to the printed

composites, and the thermal difference can be used to recover the PAC to its original shape.

4D printing technologies consists of four fundamental elements: a 3D printing platform, printable

active materials or composites, the desired activation mechanism, and lastly, an external stimulus

for initiate the response of the material [73]. These elements have a strong correlation and are

interdependent. It is important to identify a proper 3D printer platform that can be used to print the

active material. However, not all active materials are 3D printable, and their activation

mechanisms can also vary. Lastly, the fabrication or 3D process can also influence the material

properties, and these changes might also impact the actuation behaviours of the printed 4D

material. As a result, all four elements need to be carefully considered.

10

1.4 Motivation and Objectives

Both SMP and ETA are well-established research topics from literature and have the potential to

be implemented in a variety of fields, such as artificial muscles or soft robotic applications.

Nevertheless, these material-based actuators are still in the research and development phase. To

prepare the material-based actuator for real-world applications, several factors, such as material

performances, fabrication processes, and implementation abilities, need to be carefully considered.

Before the material-based actuators can be used to replace traditional electronic-actuators, they

need to demonstrate a vast increase in actuation performances. These performances are often

characterized into possible deformation, maximum force output, and response time. For material-

based actuators, the actuation behaviour strongly depends on the material properties. By

understanding these properties, it is possible to achieve a significant amount of deformation. This

research investigated two distinct material-based actuators, the shape memory polymer (SMP)

composites and polymeric based electrothermal actuator (ETA). Each study presented in this

research started from presenting a desired property for enhancement and how such improvement

can be achieved by inducing changes in material formulation. Due to these changes, the actuation

behaviour can be controlled, and a more effective material-based actuator can be created.

Conventionally, shape memory polymer needs to be heated to a temperature higher than its Ttrans

before deformation can be applied to change its geometry into its temporary shape. Below Ttrans,

the SMP cannot be re-shaped easily due to its high stiffness. Because of this property, the usability

of SMP is limited. It is proposed that a more efficient SMP can be created if the material can be

directly deformed under room temperature condition, thus bypassing the first heating cycle. The

stiffness and mechanical properties highly depend on the formulation of the composite. As a result,

it is essential to understand the basic material properties such as crystallinity, thermal behaviour,

and the mechanisms within the soft/hard phase interaction region to fabricate highly controllable

SMP composites with room temperature deformability. The soft/hard phase interaction can be

modified by using nanoparticles such as nanoclay, while the crystalline regions can be further

strengthened with an annealing effect.

11

One of the biggest challenges for polymeric blend SMP is its high activation temperature. For

example, PLA/TPU blend SMP has an activation temperature close to 70°C as the Ttrans is

controlled by the glass transition temperature (Tg) of the PLA. The high activation temperature is

not suitable for applications related to the biomedical field. One of the desirable criteria for SMP

to be utilized in medical application is a recovery or activation temperature close to human body

temperature. In related work, crosslinked and network-based SMPs show domination over

polymeric based SMP as their Ttrans can be controlled by varying the degree of crosslinking. As

the initial recovery temperature is determined by Tg, the activation behaviour can be controlled by

using the plasticizing effect. The polymeric chain movement can be enhanced with plasticizer, and

a reduction in Tg can be expected. Because of this effect, an SMP with low activation temperature

can be created.

A rise in environmental temperature is a necessity to initiate the recovery behaviour for thermally

activated SMP. Such an effect is difficult to induce in real-world applications. Instead, the increase

in temperature can be induced by Joule heating, which converts electrical energy into thermal

energy. A common method to facilitate this is to incorporate conductive nanoparticles, such as

carbon nanotubes (CNTs), into the SMP system. However, CNT reinforced SMPs usually suffer

low tensile strain due to the high stiffness of CNT. The low tensile strain also limits the recovery

performance of SMP. It is proposed that the plasticizing effect can be used to enhance the recovery

performance as it promotes the PLA/TPU interaction. The novel SMP would possess the same

advantages as the previous studies (room temperature deformability and low actuation

temperature) while still being able to be activated by an electrically induced Joule heating effect.

The polymeric based ETA reported in the literature is often constructed by coating a thick layer of

thermally expandable thermoset material (such as polydimethylsiloxane, PDMS) onto a

conductive buckypaper made of conductive nanoparticles. ETA can only be activated from a flat

configuration because of this fabrication process. To further widen the possible applications, the

initial actuation configuration needs to be programmed into different shapes. By utilizing post-

curing and thermally induced stress relaxation behaviour, the ETA can be trained to remember a

certain geometry and actuated from such a state. This is suitable for the ETA to be directly

fabricated into a usable shape for soft robotic applications.

12

4D printed materials are often fabricated from UV-curable composites as such materials can be

easily formulated. In compassion, thermoplastic materials need to be fabricated using fused

deposition modeling (FDM). Despite the low market cost and convenience, little studies focus on

4D printing development of FDM 4D printing technologies. A huge advantage of FDM is its ability

to create functional graded material (FGM). Due to the different material composition in each

layer, the 4D printed FDM can be designed with specific actuation behaviour such as localized

deformation under different temperature condition.

1.4.1 Objectives

The primary objective for this research is to investigate novel composites materials that can be

implemented as a material-based actuator with the potential of being used in soft robotic

applications. To achieve this goal, two different thermal active materials, ETA and SMP, are

investigated. In this context, the objectives and sub-objectives are defined as the following:

➢ Shape memory polymers with room temperature deformability

➢ Shape memory composites with a low activation temperature

➢ Electroactive room temperature deformable SMP with a low activation temperature

➢ Electrothermal actuator and its shape programming ability

➢ 4D printing of a localized actuation surface utilizing a functional graded layer

13

1.5 Organization of the Thesis

The research was divided into three main parts: the development of novel SMP composites

(Chapter 2, 3, and 4), ETA performance enhancement (Chapter 5), and futuristic 4D fabrication

method (Chapter 6). In each chapter, a specific active material was studied with a focus on targeted

performance enhancement. The enhancement was achieved by establishing hypotheses from

literature reported behaviour, such as nanoparticle-induced crystallinity changes and plasticizing

effect. A parametric study was conducted to verify the hypotheses and actuation motions were

demonstrated.

Room temperature deformable SMP composites were fabricated by fine-tuning the crystallinity of

PLA/TPU shape memory composites using montmorillonite (MMT) nanoclay. The results indicate

that the incorporation of MMT can improve the compatibility of the two immiscible polymers.

Also, the presence of MMT affects crystallinity and improves mechanical properties. The room

temperature deformability was demonstrated by applying uniaxial stretching deformation to the

SMP composites. With 1 wt% MMT nanoclay, a 95% recovery ratio was achieved. The SMP

composites also exhibit good fixing ability higher than 95% as the plastic deformation was applied

at room temperature condition.

Low activation temperature polymeric blend SMP was created by utilizing the plasticizing effect.

Small amounts of poly(ethylene glycol) (PEG) were used as a plasticizer, which can lower the Tg

of the SMP composites. Due to this incorporation, significant changes in morphologies and thermal

behaviour were reported. The SMP performances (activation temperature, shape fixing ability,

shape recovery ratio, and recovery time) were improved due to the induced plasticizing effect.

With 5 wt% PEG, the transitional temperature of PLA/TPU was decreased from the typical 75°C

to 50°C while a high 85% recovery ratio was maintained. An improvement of 37.5% shorter

recovery time can also be observed with the SMP containing 10 wt% PEG at 50°C environment.

These results indicated that plasticizing is an effective method for altering the polymer chain

structures and can be used to develop fast response SMPs with low activation temperature.

14

An electroactive polymeric based hybrid SMP was developed by incorporating single-walled

carbon nanotube (SWCNT) into PEG plasticized PLA/TPU composites. Conventionally, the CNT

conductive fillers have a negative impact on SMPs due to their high tensile strength. Thus, a

reduction in the elongation is often observed with CNT reinforced SMP, which worsen shape

recovery performance. Given that plasticizer can be included to alter the microstructure of the

SMPs, a positive effect on SMP composites with conductive fillers can be expected. The PEG can

lower the activation temperature while enhancing dispersion of SWCNT. Also, it was observed

that the presence of SWCNT can stabilize the SMP system by providing additional mechanical

support. By carefully controlling the formulation of the composite, an electroactive hybrid SMP

can be created by optimizing the amount of SWCNT and PEG plasticizer.

As polymeric ETA has high potential to be used in futuristic soft robotic applications, and it is

important that ETA with different geometries can be fabricated. Previous ETA studies strongly

focused on increasing electrical conductivity and Joule heating ability by super-aligning carbon

nanotube (CNT) sheets. The resulting ETA was often formed into a flat configuration. This study

presents the programmability of ETA. The composites material can be programmed into the

desired configuration by utilizing an induced stress relaxation mechanism and post-secondary

curing. It was demonstrated that an ETA can be programmed into a curled resting state. In such a

state, the actuator can achieve a bending angle greater than 540°. The shape programming feature

also enabled tailoring of the actuator configuration to a specific application, such as a bioinspired

crawling soft robot that mimics the motion of an inchworm.

4D printing is the combination of smart active materials and 3D printing techniques. Give that an

external stimulus is required, shape memory effect was commonly employed as the activation

mechanism. Current studies primarily focus on SMPs constructed from photopolymerizable

composites. In comparison, fused deposition modeling (FDM) can produce 3D printed structures

from thermoplastic material and can be used to create functionally graded layers. This study

demonstrates that functionally graded PLA 4D structure with different degrees of plasticization

can be fabricated from the FDM technique. In each layer, a different amount of PEG plasticizer

was incorporated into the PLA, which modified the polymer chain motions. Due to the difference

in plasticization, each layer can be actuated under different temperatures, and a 4D material with

localized actuation ability was created.

15

1.6 Contribution

The major contribution of this thesis is the development of novel material-based actuation

composites that can be implemented in futuristic soft robotics or artificial muscle applications.

New composite hybrids were developed using the knowledge gained from the literature and

combined with novel polymeric mechanisms. These mechanisms range from microscopic

crystallization behaviour to novel 3D fabrication techniques. To summarize, the main

contributions are:

➢ Development of room temperature deformable SMP via tailoring crystalline phases of the

SMP with MMT nanoclay

➢ Development of low Tg and close-to-body actuation temperature SMP via induced PEG

plasticizing effect

➢ Development of electroactive SMP with both room temperature deformability and low

activation temperature

➢ Investigation of shape programming ability of ETA via the induced stress relaxation effect

and soft robotics implementation

➢ Demonstration of 4D printed functionally graded hybrid with localized actuation ability

16

1.7 References

[1] S. Kim, C. Laschi, and B. Trimmer, "Soft robotics: a bioinspired evolution in robotics,"

Trends in biotechnology, vol. 31, no. 5, pp. 287-294, 2013.

[2] C. Majidi, "Soft robotics: a perspective—current trends and prospects for the future," Soft

Robotics, vol. 1, no. 1, pp. 5-11, 2014.

[3] M. Wehner et al., "An integrated design and fabrication strategy for entirely soft,

autonomous robots," Nature, vol. 536, no. 7617, p. 451, 2016.

[4] S. I. Rich, R. J. Wood, and C. Majidi, "Untethered soft robotics," Nature Electronics, vol.

1, no. 2, p. 102, 2018.

[5] O. M. Wani, H. Zeng, and A. Priimagi, "A light-driven artificial flytrap," Nature

communications, vol. 8, p. 15546, 2017.

[6] F. Ilievski, A. D. Mazzeo, R. F. Shepherd, X. Chen, and G. M. Whitesides, "Soft robotics

for chemists," Angewandte Chemie International Edition, vol. 50, no. 8, pp. 1890-1895,

2011.

[7] R. V. Martinez, C. R. Fish, X. Chen, and G. M. Whitesides, "Elastomeric origami:

programmable paper‐elastomer composites as pneumatic actuators," Advanced functional

materials, vol. 22, no. 7, pp. 1376-1384, 2012.

[8] R. V. Martinez et al., "Robotic tentacles with three‐dimensional mobility based on

flexible elastomers," Advanced Materials, vol. 25, no. 2, pp. 205-212, 2013.

[9] R. V. Martinez, A. C. Glavan, C. Keplinger, A. I. Oyetibo, and G. M. Whitesides, "Soft

actuators and robots that are resistant to mechanical damage," Advanced Functional

Materials, vol. 24, no. 20, pp. 3003-3010, 2014.

[10] K. C. Galloway et al., "Soft robotic grippers for biological sampling on deep reefs," Soft

robotics, vol. 3, no. 1, pp. 23-33, 2016.

[11] L. A. Al Abeach, S. Nefti-Meziani, and S. Davis, "Design of a variable stiffness soft

dexterous gripper," Soft robotics, vol. 4, no. 3, pp. 274-284, 2017.

[12] M. Addington and D. Schodek, Smart Materials and Technologies in Architecture: For

the Architecture and Design Professions. Routledge, 2012.

[13] M. Shahinpoor and H.-J. Schneider, Intelligent materials. Royal Society of Chemistry,

2008.

[14] L. Van Langenhove, Smart textiles for medicine and healthcare: materials, systems and

applications. Elsevier, 2007.

17

[15] C. De las Heras Alarcón, S. Pennadam, and C. Alexander, "Stimuli responsive polymers

for biomedical applications," Chemical Society Reviews, vol. 34, no. 3, pp. 276-285,

2005.

[16] S. Hurlebaus and L. Gaul, "Smart structure dynamics," Mechanical systems and signal

processing, vol. 20, no. 2, pp. 255-281, 2006.

[17] W. Staszewski, G. Tomlinson, C. Boller, and G. Tomlinson, Health monitoring of

aerospace structures. Wiley Online Library, 2004.

[18] J. D. Madden et al., "Artificial muscle technology: physical principles and naval

prospects," IEEE Journal of oceanic engineering, vol. 29, no. 3, pp. 706-728, 2004.

[19] R. Kornbluh, R. Pelrine, J. Eckerle, and J. Joseph, "Electrostrictive polymer artificial

muscle actuators," in Proceedings. 1998 IEEE International Conference on Robotics and

Automation (Cat. No. 98CH36146), 1998, vol. 3, pp. 2147-2154: IEEE.

[20] Y. Bar-Cohen, Electroactive polymer (EAP) actuators as artificial muscles: reality,

potential, and challenges. SPIE press, 2004.

[21] E. Biddiss and T. Chau, "Dielectric elastomers as actuators for upper limb prosthetics:

Challenges and opportunities," Medical engineering & physics, vol. 30, no. 4, pp. 403-

418, 2008.

[22] J. Rossiter, P. Walters, and B. Stoimenov, "Printing 3D dielectric elastomer actuators for

soft robotics," in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference

Series, 2009, vol. 7287.

[23] K. J. Kim and S. Tadokoro, "Electroactive polymers for robotic applications," Artificial

Muscles and Sensors, 2007.

[24] H. Jiang, S. Kelch, and A. Lendlein, "Polymers move in response to light," Advanced

Materials, vol. 18, no. 11, pp. 1471-1475, 2006.

[25] X. Zhang et al., "Optically-and thermally-responsive programmable materials based on

carbon nanotube-hydrogel polymer composites," Nano letters, vol. 11, no. 8, pp. 3239-

3244, 2011.

[26] H. Koerner, G. Price, N. A. Pearce, M. Alexander, and R. A. Vaia, "Remotely actuated

polymer nanocomposites--stress-recovery of carbon-nanotube-filled thermoplastic

elastomers," Nature materials, vol. 3, no. 2, p. 115, 2004.

[27] M. Ma, L. Guo, D. G. Anderson, and R. Langer, "Bio-inspired polymer composite

actuator and generator driven by water gradients," Science, vol. 339, no. 6116, pp. 186-

189, 2013.

[28] P. Muralt, "Ferroelectric thin films for micro-sensors and actuators: a review," Journal of

micromechanics and microengineering, vol. 10, no. 2, p. 136, 2000.

18

[29] G. Kofod, P. Sommer-Larsen, R. Kornbluh, and R. Pelrine, "Actuation response of

polyacrylate dielectric elastomers," Journal of intelligent material systems and structures,

vol. 14, no. 12, pp. 787-793, 2003.

[30] R. Pelrine, R. Kornbluh, Q. Pei, and J. Joseph, "High-speed electrically actuated

elastomers with strain greater than 100%," Science, vol. 287, no. 5454, pp. 836-839,

2000.

[31] K. Jung, J. Nam, and H. Choi, "Investigations on actuation characteristics of IPMC

artificial muscle actuator," Sensors and Actuators A: Physical, vol. 107, no. 2, pp. 183-

192, 2003.

[32] M. Shahinpoor, Y. Bar-Cohen, J. Simpson, and J. Smith, "Ionic polymer-metal

composites (IPMCs) as biomimetic sensors, actuators and artificial muscles-a review,"

Smart materials and structures, vol. 7, no. 6, p. R15, 1998.

[33] D. Hartl and D. C. Lagoudas, "Thermomechanical characterization of shape memory

alloy materials," in Shape memory alloys: Springer, 2008, pp. 53-119.

[34] M. Behl and A. Lendlein, "Shape-memory polymers," Materials today, vol. 10, no. 4, pp.

20-28, 2007.

[35] P. T. Mather, X. Luo, and I. A. Rousseau, "Shape memory polymer research," Annual

Review of Materials Research, vol. 39, pp. 445-471, 2009.

[36] A. Lendlein, H. Jiang, O. Jünger, and R. Langer, "Light-induced shape-memory

polymers," Nature, vol. 434, no. 7035, pp. 879-882, 2005.

[37] R. C. O’Handley, "Model for strain and magnetization in magnetic shape-memory

alloys," Journal of Applied Physics, vol. 83, no. 6, pp. 3263-3270, 1998.

[38] Y. Liu, H. Lv, X. Lan, J. Leng, and S. Du, "Review of electro-active shape-memory

polymer composite," Composites Science and Technology, vol. 69, no. 13, pp. 2064-

2068, 2009.

[39] T. Manouras and M. Vamvakaki, "Field responsive materials: photo-, electro-, magnetic-

and ultrasound-sensitive polymers," Polymer Chemistry, vol. 8, no. 1, pp. 74-96, 2017.

[40] K. Otsuka and C. M. Wayman, Shape memory materials. Cambridge university press,

1999.

[41] T. W. Duerig, K. Melton, and D. Stöckel, Engineering aspects of shape memory alloys.

Butterworth-Heinemann, 2013.

[42] K. Bhattacharya, Microstructure of martensite: why it forms and how it gives rise to the

shape-memory effect. Oxford University Press, 2003.

[43] T. Xie, "Tunable polymer multi-shape memory effect," Nature, vol. 464, no. 7286, pp.

267-270, 2010.

19

[44] C. Liu, H. Qin, and P. Mather, "Review of progress in shape-memory polymers," Journal

of Materials Chemistry, vol. 17, no. 16, pp. 1543-1558, 2007.

[45] J. D. Madden et al., "Artificial muscle technology: physical principles and naval

prospects," Oceanic Engineering, IEEE Journal of, vol. 29, no. 3, pp. 706-728, 2004.

[46] J. Cui et al., "Combinatorial search of thermoelastic shape-memory alloys with extremely

small hysteresis width," Nature materials, vol. 5, no. 4, p. 286, 2006.

[47] H. Funakubo, "Shape Memory Alloys (New York: Gordon and Breach Science)," 1987.

[48] W. Wang, Y. Liu, and J. Leng, "Recent developments in shape memory polymer

nanocomposites: Actuation methods and mechanisms," Coordination Chemistry Reviews,

vol. 320, pp. 38-52, 2016.

[49] D. Ratna and J. Karger-Kocsis, "Recent advances in shape memory polymers and

composites: a review," Journal of Materials Science, vol. 43, no. 1, pp. 254-269, 2008.

[50] Y. Wang, Y. Li, Y. Luo, M. Huang, and Z. Liang, "Synthesis and characterization of a

novel biodegradable thermoplastic shape memory polymer," Materials Letters, vol. 63,

no. 3-4, pp. 347-349, 2009.

[51] H. Zhang, H. Wang, W. Zhong, and Q. Du, "A novel type of shape memory polymer

blend and the shape memory mechanism," Polymer, vol. 50, no. 6, pp. 1596-1601, 2009.

[52] W. Zhang, L. Chen, and Y. Zhang, "Surprising shape-memory effect of polylactide

resulted from toughening by polyamide elastomer," Polymer, vol. 50, no. 5, pp. 1311-

1315, 2009.

[53] S. Zhang, Z. Yu, T. Govender, H. Luo, and B. Li, "A novel supramolecular shape

memory material based on partial α-CD–PEG inclusion complex," Polymer, vol. 49, no.

15, pp. 3205-3210, 2008.

[54] G. Liu, X. Ding, Y. Cao, Z. Zheng, and Y. Peng, "Shape memory of hydrogen-bonded

polymer network/poly (ethylene glycol) complexes," Macromolecules, vol. 37, no. 6, pp.

2228-2232, 2004.

[55] H. M. Jeong, S. Y. Lee, and B. K. Kim, "Shape memory polyurethane containing

amorphous reversible phase," Journal of materials science, vol. 35, no. 7, pp. 1579-1583,

2000.

[56] S.-M. Lai and Y.-C. Lan, "Shape memory properties of melt-blended polylactic acid

(PLA)/thermoplastic polyurethane (TPU) bio-based blends," Journal of Polymer

Research, vol. 20, no. 5, pp. 1-8, 2013.

[57] M. Raja, S. H. Ryu, and A. Shanmugharaj, "Thermal, mechanical and electroactive shape

memory properties of polyurethane (PU)/poly (lactic acid)(PLA)/CNT nanocomposites,"

European Polymer Journal, vol. 49, no. 11, pp. 3492-3500, 2013.

20

[58] J. J. Song, H. H. Chang, and H. E. Naguib, "Biocompatible shape memory polymer

actuators with high force capabilities," European Polymer Journal, vol. 67, pp. 186-198,

2015.

[59] J. J. Song, H. H. Chang, and H. E. Naguib, "Design and characterization of biocompatible

shape memory polymer (SMP) blend foams with a dynamic porous structure," Polymer,

vol. 56, pp. 82-92, 2015.

[60] Q.-A. Huang and N. K. S. Lee, "Analysis and design of polysilicon thermal flexure

actuator," Journal of Micromechanics and Microengineering, vol. 9, no. 1, p. 64, 1999.

[61] M. J. Sinclair, "A high force low area MEMS thermal actuator," in Thermal and

Thermomechanical Phenomena in Electronic Systems, 2000. ITHERM 2000. The Seventh

Intersociety Conference on, 2000, vol. 1: IEEE.

[62] Y. Zhu, A. Corigliano, and H. D. Espinosa, "A thermal actuator for nanoscale in situ

microscopy testing: design and characterization," Journal of micromechanics and

microengineering, vol. 16, no. 2, p. 242, 2006.

[63] P. Shivhare, G. Uma, and M. Umapathy, "Design enhancement of a chevron

electrothermally actuated microgripper for improved gripping performance,"

Microsystem Technologies, pp. 1-9, 2015.

[64] D. Yan, A. Khajepour, and R. Mansour, "Modeling of two-hot-arm horizontal thermal

actuator," Journal of Micromechanics and Microengineering, vol. 13, no. 2, p. 312, 2003.

[65] L.-Z. Chen, C. Liu, C. Hu, and S. Fan, "Electrothermal actuation based on carbon

nanotube network in silicone elastomer," Applied Physics Letters, vol. 92, no. 26, p.

263104, 2008.

[66] B. Samel, J. Melin, P. Griss, and G. Stemme, "Single-use microfluidic pumps and valves

based on a thermally responsive PDMS composite," in Micro Electro Mechanical

Systems, 2005. MEMS 2005. 18th IEEE International Conference on, 2005, pp. 690-693:

IEEE.

[67] B. Samel, P. Griss, and G. Stemme, "A thermally responsive PDMS composite and its

microfluidic applications," Journal of microelectromechanical systems, vol. 16, no. 1, pp.

50-57, 2007.

[68] Z. Zeng et al., "Low-voltage and high-performance electrothermal actuator based on

multi-walled carbon nanotube/polymer composites," Carbon, vol. 84, pp. 327-334, 2015.

[69] L. Chen et al., "High-performance, low-voltage, and easy-operable bending actuator

based on aligned carbon nanotube/polymer composites," ACS nano, vol. 5, no. 3, pp.

1588-1593, 2011.

[70] S. Tibbits, "4D printing: multi‐material shape change," Architectural Design, vol. 84, no.

1, pp. 116-121, 2014.

21

[71] Q. Ge, C. K. Dunn, H. J. Qi, and M. L. Dunn, "Active origami by 4D printing," Smart

Materials and Structures, vol. 23, no. 9, p. 094007, 2014.

[72] Z. X. Khoo et al., "3D printing of smart materials: A review on recent progresses in 4D

printing," Virtual and Physical Prototyping, vol. 10, no. 3, pp. 103-122, 2015.

[73] F. Momeni, X. Liu, and J. Ni, "A review of 4D printing," Materials & Design, vol. 122,

pp. 42-79, 2017.

[74] K. Yu, A. Ritchie, Y. Mao, M. L. Dunn, and H. J. Qi, "Controlled sequential shape

changing components by 3D printing of shape memory polymer multimaterials,"

Procedia Iutam, vol. 12, pp. 193-203, 2015.

[75] C. P. Ambulo, J. J. Burroughs, J. M. Boothby, H. Kim, M. R. Shankar, and T. H. Ware,

"Four-dimensional Printing of Liquid Crystal Elastomers," ACS applied materials &

interfaces, vol. 9, no. 42, pp. 37332-37339, 2017.

[76] H. Wei, Q. Zhang, Y. Yao, L. Liu, Y. Liu, and J. Leng, "Direct-Write Fabrication of 4D

Active Shape-Changing Structures Based on a Shape Memory Polymer and Its

Nanocomposite," ACS applied materials & interfaces, vol. 9, no. 1, pp. 876-883, 2016.

[77] A. S. Gladman, E. A. Matsumoto, R. G. Nuzzo, L. Mahadevan, and J. A. Lewis,

"Biomimetic 4D printing," Nature materials, vol. 15, no. 4, pp. 413-418, 2016.

[78] D. Raviv et al., "Active printed materials for complex self-evolving deformations,"

Scientific reports, vol. 4, p. 7422, 2014.

[79] Q. Ge, H. J. Qi, and M. L. Dunn, "Active materials by four-dimension printing," Applied

Physics Letters, vol. 103, no. 13, p. 131901, 2013.

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Chapter 2

Room Temperature Deformable Shape Memory Composite with Fine-

tuned Crystallization Induced via Nanoclay Particles 1

It is known that particular types of semi-crystalline/elastomer polymer blends exhibit shape

memory effects (SME) due to the dispersion of two immiscible phases. In this study, the crystal

structure of polylactic acid (PLA)/ thermoplastic polyurethane (TPU) based shape memory

polymer (SMP) is altered by incorporating small amounts of montmorillonite (MMT) nanoclay.

The results indicate the incorporation of MMT can improve the compatibility of the two different

polymers. Moreover, the presence of MMT affects the total crystallinity of the SMP and improves

mechanical properties. Lastly, uniaxial stretching deformation can be applied to the SMP at room

temperature conditions while maintaining its shape memory properties. With 1 wt% MMT

particles, the recovery ratio (Rr) was nearly 95%, which indicated a strong recovery effect. The

shape fixing ratio (Rf) remained above 95% for all composites due to plastic deformation applied

at room temperature.

1 The content of this chapter has been published in the Journal of Polymer Science Part B: Polymer

Physics;

Sun, Y., Cai, S., Ren, J. and E. Naguib, H. (2017), Room temperature deformable shape memory composite with fine‐

tuned crystallization induced via nanoclay particles. J. Polym. Sci. Part B: Polym. Phys., 55: 1197-1206.

doi:10.1002/polb.24370. Reproduced by permission from John Wiley and Sons (License # 4617680192905).

23

2.1 Introduction

Shape memory polymers (SMPs) are a special class of polymers that have the ability to maintain

one or more temporary shapes and revert back to their initial shape once exposed to an external

stimulus [1]. Such stimuli can be light, heat, or solvent-induced chemical reactions [2-7]. The

shape memory effect (SME) of different types of SMPs have been well documented over the past

few decades, with thermally activated SMPs being studied the most [4]. Thermally activated SMPs

usually consists of two different phases: a soft and thermally reversible phase responsible for

maintaining a temporary shape, and a hard phase that has the ability to “memorize” a permanent

shape. Furthermore, the shape recovery proccess of thermaly activated SMPs can be characterized

by two critical temperatures: the permanent temperature (Tperm) for programming the permanent

shape of the SMP, and the transition temperature (Ttrans) for triggering the recovery from a

temporary shape back to its permanent shape. For physically cross-linked thermoplastic polymers

or cross-linked block copolymers, Tperm is determined by the degree of cross-linking at the

molecular level. As a result, the melting temperature (Tm) of the polymers can be utilized for

programing its permanent shape [4].

When a SMP is heated above its Ttrans, a large drop in the elastic modulus of the soft phase can be

observed. Under such condition, deformation can be applied to the SMP to form its temporary

shape. This temporary shape can then be fixed by reducing the temperature of the SMP to below

its Ttrans so that the micro-Brownian movement within the polymer chain can be frozen. By re-

heating the SMP in its temporary shape to a temperature above Ttrans, the SMP would then be able

to recover back to its permanent shape. As a result, it is common to select the soft phase based on

a polymer with a glass transition temperature (Tg) equivalent to the desired Ttrans [8]. Due to the

special properties of SMPs, it is proposed that they can be used in a wide variety of applications

such as actuator textiles [9, 10], aerospace [11], and biomedical devices [12, 13]. Lan et al.

demonstrated a prototype solar panel hinge that can be activated by SMP composites [14]. Bye et

al. developed a morphing SMP skin that has to potential to be used in shape shifting unmanned air

vehicles [15]. Xue et al. fabricated a fast self-expandable SMP stents that can be used as biomedical

insert [16].

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SMPs can be further classified into two distinct group: covalently/physically cross-linked polymer

network structures [17-19] and polymer blends [4, 20]. For network based SMPs, the hard phases

are usually in droplet form and act as the cross-linking nodes while the soft phases act as the

switching phase. In contrast, polymer blend SMPs consists of two different polymers that are

immiscible: a crystalline polymer that acts as the hard phase, and an amorphous polymer that acts

as the switching phase. Examples includes poly(ω-pentadecalactone) (PPDL)/poly(ϵ-

caprolactone) (PCL) copolymer [21], polyurethanes (PU)/PCL [22], poly(styrene–butadiene–

styrene) tri-block copolymer (SBS)/PCL [20], polyvinylidene fluoride (PVDF)/polyvinylacetate

(PVAc) [4], and polylactic acid (PLA)/PVAc [4]. Compared to network SMPs, polymer blend

based SMPs are easier to fabricate. Furthermore, their activation and shape recovery mechanism

can be tailored by carefully designing the ratio of amorphous/crystalline polymer [20]. Recently,

PLA has become one of the most popular polymer choices for the crystalline polymer in SMP

composites due to its mechanical properties and biocompatibility[23-25]. One of the earliest

studies on PLA based SMPs was reported by Zhang et al. in regards to the toughening process of

a PLA matrix [26]. The purpose was to modify the brittleness of PLA by blending it with

polyamide elastomer (PAE), which is a polymer that has both polyether and polyamide groups.

Previous research have demonstrated that the polyether groups in PAE have good compatibility

with PLA, while the polyamide groups form good interfacial polymer-polymer interactions with

PLA [27]. By incorporating 5 and 10 wt% of PAE into the PLA matrix, the elongation at break

increased from 5.1% to 161.5% and 194.6% respectively. Moreover, the PLA/PAE blend also

exhibited SME properties: 92% recover ratio (Rr) was reported for the 10 wt% PAE sample after

100% stretching. Ever since this discovery, intensive studies on PLA based SMP composites have

been conducted. For instance, Yuan et al. fabricated a fully bio-based SMP by combining PLA,

natural rubber, and thermoplastic vulcanizates together [28]. Huang et al. verfied that SMP

behaviour can be observed from polylactide-b-poly(ethylene-co-butylene)-b-polylactide (PLA–

PEB–PLA) triblock copolymers and that its perfomance can be controlled by adjusting the

crystallizability of the PLA segments [29]. Samuel et al. reported a novel SMP composite that has

multi-shape memory capability by integrating poly(L-lactide) (PLLA) with poly(methyl

methacrylate) (PMMA) [30].

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In order to induce SME in PLA based materials, the incorporation of soft segments is required,

which can done by blending in thermoplastic polyurethane (TPU) [23-25, 31, 32] and polyethylene

glycol (PEG) [33, 34]. TPU is widely selected due to its biocompatibility and the fact that it can

be easily blended within the PLA matrix. Since SMP performance and behavior highly depends

on its crystalline structure and hard/soft phase interface, therefore it is essential to understand basic

polymer properties including crystallinity and the mechanisms within the soft/hard phase

interaction region. It is long known that the polymer’s crystalline structure can be altered

dramatically through the incorporation of compatibilizers or coupling agents. Recently, Zare

evaluated the SME of several different nanoparticle enhanced SMP composite systems and the

work indicated that polymer-particle interfacial regions play an important role in SMP activation

performance [35]. Montmorillonite (MMT) is a commonly used filler due to its large surface area

and high aspect ratio of its laminar structure [36, 37]. Zou et al. showed that octadecylammonium-

treated MMT filler has good compatibility when blend with TPU; an increase in mechanical

property was reported with only 3 wt% filler content [38]. Kim et al. intercalated sodium-modified

MMT particles with PEG polymer segments and found that the segments behave as physical cross-

linkers in the poly(ethyl methacrylate) matrix [39]. More recently, Kelnar et al. investigated the

influence of organophilized MMT when blended with PLA/TPU composites [40]. The group

reported that the TPU dispersion in the PLA matrix is dramatically improved in addition to

enhancements in crystallinity and polymer-polymer compatibility.

Despite the advances in research related to different SMP composite systems and filler-controlled

polymer integration, there are only a few studies that focus on adapting nanoparticle induced

crystalline/amorphous changes and applying such alteration for fine-tuning SMP behaviour. For

instance, Tan et al. reported that MMT can be used to improve the mechanical properties of

polyurethane-epoxy composites, but critical SME parameters such as glass transition temperature,

storage modulus, and shape recovery ratio remained the same [41]. In this study, we report the