||Study on Biomimetic Photo/Thermal Tunable Liquid Crystalline Elastomeric Actuators
||Department of Chemical Engineering
Liquid crystal elastomer
Recently, due to the rapid progress in soft actuator and robotics, novel soft materials with different and complex shape change, such as helical curling, self-oscillation, folding, and other three-dimensional shape morphing were highly investigated. In this research, a series of photo/thermal tunable liquid crystalline actuators was fabricated via thiol-acrylate Michael addition. The molar ratio between acrylate and thiol functional groups was controlled to ensure acrylate groups are excessive for the second crosslinking reaction. To synthesize elastomeric polymers showing liquid crystal phases, RM257 was used as a main chain liquid crystalline monomer. A synthesized monomer 4,4’-bis(6-(acryloyloxy)hexyloxy) biphenyl (BAHB) was used to adjust the mechanical strength. After polymerization, the monodomain aligned film was achieved by uniaxial stretching and the anisotropic properties of the polymer film were confirmed by polarized optical microscope (POM). The fabricated liquid crystal elastomeric film shows a series of reversible shape changing, such as helical curling, elongation, and contraction. This phenomenon is ascribed to the change of molecular orientation triggered by temperature. Finally, a caterpillar-like liquid crystal elastomer (LCE) was fabricated. To enhance the photo sensitivity, the film was soaked in the polydopamine (PDA) solution for overnight. With the photo-thermal effect of the polydopamine on infrared light, the prepared LCE film shows caterpillar-like moving. The results suggest that the synthesized LCE actuator can effectively transfer the photo and thermal energy into mechanical power resulting in the caterpillar-like moving. The synthesized photo and thermal sensitive films are expected to show lots of potential applications in biomimetic devices and micro robotic system.
List of Schemes IX
List of Tables X
List of Figures XI
1. Introduction 1
1-1 Preface 1
1-2 Research Motivation 2
2. Literature Review 3
2-1 Introduction of Liquid Crystals 3
2-1-1 Historical Overview 3
2-1-2 Mesogenic Molecules and Orientational Order 5
2-2 Classification of Liquid Crystals 9
2-2-1 Thermotropic Liquid Crystal 10
2-2-1-1 Nematic Liquid Crystal Phase 10
2-2-1-2 Smectic Liquid Crystals Phase 12
2-2-1-3 Cholesteric Liquid Crystal Phase 14
2-2-2 Lyotropic Liquid Crystal 16
2-3 Introduction of Michael Addition Reactions[16-19] 17
2-3-1 The Mechanism of the Carbon Michael Addition 18
2-3-2 Heteroatomic Donors in the Michael Reaction 19
2-3-2-1 The Aza-Michael Reaction 19
2-3-2-2 Thiols as Michael Donors 20
2-4 Introduction of Photopolymerization Reaction 22
2-5 Introduction of Liquid Crystalline Polymeric Materials 23
2-5-1 Liquid Crystal Elastomers (LCEs) 25
2-5-1-1 Polydomain-Monodomain Transition in Nematic Elastomers 26
2-5-1-2 Thermal Induced Actuation of LCEs 27
2-5-1-3 Photo Induced Actuation of LCEs 31
2-6 Introduction of Polydopamine Coated Application 34
2-6-1 Polydopamine Application in LC Actuators 35
3. Experimental Section 39
3-1 Materials 39
3-2 Instruments 40
3-3 Experimental Process 41
3-3-1 Synthesis of Bifunctional Monomer BAHB 41
3-3-2 Fabrication of Liquid Crystal Elastomers 43
3-3-2-1 Preparation of Liquid Crystal Mixture 43
3-3-2-2 Fabrication of Polydomain Liquid Crystal Elastomers (LCEs) 44
3-3-3 Dynamic Mechanical Analysis of LCEs 46
3-3-4 Small and Wide-Angle X-Ray Scattering of LCEs 47
3-3-5 Design and Fabrication of Monodomain LCE 48
3-3-6 Fabrication of Near-IR Tunable LCE 49
4. Results and Discussion 51
4-1 Characterization of the Synthesized Compounds 51
4-1-1 Structure Identification of BAHB 51
4-1-2 Thermal Properties of BAHB 53
4-1-3 Optical Properties of BAHB 55
4-2 Characterization of the Synthesized LCEs 56
4-2-1 Structure Identification 56
4-2-2 Thermal Properties of the Synthesized LCEs 58
4-2-3 Dynamic Mechanical Properties of the Synthesized LCEs 64
4-2-4 Optical Properties of LCE with 15 mol% of BAHB 70
4-2-5 Small and Wide-Angle X-Ray Scattering of the LCE film 71
4-2-6 Optical Properties of Uniaxial Aligned LCE Film 73
4-3 Thermal Actuation Properties of Monodomain LCE 74
4-3-1 Schematic Illustration of Thermal Tunable LCE 74
4-3-2 Thermal Shape-Switching Behaviors 75
4-3-3 Thermal Actuation Test with a Constant Stress 76
4-4 Characterization of LCE actuator 78
4-4-1 Mechanism of Thermal Response Deformation 78
4-4-2 Thermal Actuation of the Programmed LC Actuator 79
4-4-3 Biomimetic Caterpillar-like Actuator 81
5. Conclusions 83
6. References 84
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