||Photo Tunable Liquid Crystalline Elastomers for Controlled Actuations
||Department of Materials Science and Engineering
liquid crystal elastomers
In recent years, smart materials have been widely used in the fabrication of actuators with potential applications in the fields of micro-robots, microfluidic transport, sensors and artificial muscles. Among many actuators, liquid crystal actuators have unique properties due to the combination of liquid crystal orientational order and the elasticity of polymer network structure. In this study, two types of thermal responsive liquid crystal actuators were fabricated via predesigned methods. Firstly, commercially available liquid crystal monomer RM105, difunctional liquid crystal monomer RM257, poly(ethyleneglycol) diacrylate and photo-initiator (Irg184) were mixed and photopolymerized in a glass cell with parallel and perpendicular alignment layers. Thermal properties of the synthesized liquid crystal elastomeric films (LCEs) were analyzed using DSC and POM. The liquid crystalline actuators exhibit reversible bending when close to and away from the heat source. Secondary, commercially available bifunctional liquid crystal monomer RM257, photo-initiator (Irg184), and EDDET and PETMP with thiol groups were dissolved in toluene, and then a small amount of diluted DPA was added to carry out Michael addition. After first polymerization, the polydomain liquid crystalline elastomer was uniaxial stretched and then photo-polymerized to obtain monodomain liquid crystal elastomer. The synthesized monodomain LCEs were confirmed using POM and X-ray diffractometer (XRD). To develop thermal actuators, the synthesized LCE was combined with a layer of polyurethane (PU). The fabricated LCE actuator shows reversible thermal responsive bending actuation. To enhance near infrared sensitivity, polydopamine (PDA) was coated on the synthesized LCE surface. The fabricated PDA coated LCE shows reversible NIR responsive actuations. To develop a photo-mechanical-electrical energy conversion device, the NIR responsive LCE was further combined with a triboelectric generator. Based on the results, fabrication of energy transformation device via such designed LCE films is possible. Application of the synthesized LCEs on actuators and energy transformation devices are expected.
List of Scheme IX
List of Table IX
List of Figures X
1. Introduction 1
1.1 Preface 1
1.2 Research Motivation 3
2. Literature Review 4
2.1 Introduction of Liquid Crystals 4
2.2 Classification of Liquid Crystals 6
2.2.1 Thermotropic Liquid Crystal 8
18.104.22.168 Nematic Liquid Crystal 9
22.214.171.124 Smectic Liquid Crystal 11
126.96.36.199 Cholesteric Liquid Crystal 13
2.2.2 Lyotropic Liquid Crystal 16
2.3 Anisotropic Properties of Liquid Crystals 17
2.3.1 Birefringence of Liquid Crystals 17
2.3.2 Dielectric Properties of Liquid Crystals 19
2.4 Introduction of Liquid Crystalline Polymeric Materials 20
2.5 Introduction of Liquid Crystalline Actuators 22
2.5.1 Thermally Driven LC Actuators 23
2.5.2 Solvent-Sensitive LC Actuators 27
2.5.3 Light-Induced LC Actuators 31
2.6 Versatility Surface-Assisted Alignment of Liquid Crystals 37
2.6.1 Surface Alignment of Physically Induced 37
2.6.2 Surface Alignment of Chemically Induced 39
2.6.3 Photoalignment 40
2.7 Introduction and Application of Polydopamine 42
2.7.1 Polydopamine Application in LCE Actuators 44
2.8 Introduction of the Triboelectric Nanogenerators 47
3. Experimental 58
3.1 Materials and Instruments 58
3.2 Experimental Procedure 61
3.2.1 Preparation of Polydopamine 61
3.2.2 Fabrication of Tilt Alignment LCEs 62
3.2.3 Fabrication of Bilayer LCEs 66
3.2.4 Fabrication of Triboelectric Generator (TEG) 71
3.2.5 Morphology Observation of Polydopamine and LCEs 74
3.2.6 Thermal Properties of LCEs 74
3.2.7 Mechanical Properties of LCEs 75
3.2.8 Fabrication of TEG Based on LCEs 76
4. Results and Discussion 77
4.1 Synthesis of Polydopamine Particles 77
4.2 Fabrication of Tilt-Alignment LCEs 78
4.2.1 Components of LC Mixtures 78
4.2.2 Thermal Properties of LC Mixtures 79
4.2.3 POM of LC Mixtures 80
4.2.4 Thermal Properties of LCE Films 82
4.2.5 Mechanical Properties of LCE Films 84
4.2.6 Morphology of LCE Films 85
4.2.7 Thermal Actuation of LCE Films 87
4.2.8 NIR Responsive LCE Films 92
4.3 Synthesis of LCE Films Based on Michael Addition 95
4.3.1 Components of the LC Mixtures 95
4.3.2 First Polymerized LCE Films 96
4.3.3 Thermal Properties of LCE Films 97
4.3.4 Mechanical Properties of LCE Films 99
4.3.5 Analysis of Monodomain LCE Film 100
4.3.6 Actuators Based on the Synthesized LCE 102
188.8.131.52 Shape Memory LCE Films 102
184.108.40.206 Bilayer LCE Films 105
4.3.7 NIR Responsive Bilayer LCE Films 106
4.4 Application of LCEs on Triboelectric Generator 108
4.4.1 Electrical Test of the Triboelectric Generator 108
5. Conclusions 111
List of Scheme
Scheme 3.1 Polymerization of dopamine. 61
Scheme 3.2 Schematic diagram of the fabrication of liquid crystal cells. 64
Scheme 3.3 Schematic diagram of the fabrication of liquid crystal films. 65
Scheme 3.4 Schematic diagram of the fabrication of PDA-coated LCEs. 65
Scheme 3.5 Mechanism of base-catalyzed Michael addition, EWG denotes electron withdrawing group. 67
Scheme 3.6 Schematic diagram of the fabrication of polydomain LCEs. 68
Scheme 3.7 Schematic diagrams of the fabrication process of the monodomain liquid crystalline elastomers into stripe and helical shape. 69
Scheme 3.8 Schematic diagrams of the fabrication process of the bilayer liquid crystalline elastomers. 70
Scheme 3.9 Schematic diagram of the fabrication of NIR responsible LCEs. 71
Scheme 3.10 Schematic diagrams of the fabrication process of the PAAm-LiCl hydrogels. 72
Scheme 3.11 Schematic diagram of the fabrication of TEG. 73
List of Table
Table 2.1 List the triboelectric series of commonly used materials. 52
Table 3.1 List of materials used in the study. 58
Table 3.2 List of instruments used in the study. 60
Table 4.1 Components of monomer mixtures. a 78
Table 4.2 Composition of LC mixtures. a 96
List of Figures
Figure 1.1 Molecular buildup and deformation behavior of a liquid crystal polymer film with (a) twisted alignment (b) splayed alignment (c) azimuthal alignment (d) radial alignment. 2
Figure 2.1 (a) Rod-like, and (b) disc-like liquid crystal molecules. 5
Figure 2.2 Classification of liquid crystalline molecules. 6
Figure 2.3 Typical liquid crystal molecular structure of mesogens. 7
Figure 2.4 Phase transition of (a) enantiotropic liquid crystal, and (b) monotropic liquid crystal. 8
Figure 2.5 Schematic representation of different types of rod-like liquid crystal phase (a) nematic (b) smectic, and (c) cholesteric phase. 9
Figure 2.6 Schematic diagram of the molecular arrangement of the nematic liquid crystal phase. 10
Figure 2.7 Schematic representation of different kinds of smectic liquid crystal phases. 11
Figure 2.8 Optical textures of (a) Focal conic fan-shaped texture of the smectic-A phase. (b) Schlieren texture of the smectic-C phase. 12
Figure 2.9 A schematic illustration of the molecular arrangement of cholesteric liquid crystals. 13
Figure 2.10 (a) The reflection of light by a cholesteric liquid crystal. (b) Possible changes in the reflection band of a cholesteric liquid crystal under external stimuli. 14
Figure 2.11 Schematic diagrams of the lyotropic liquid crystalline phases commonly found in neutral lipid/water systems. (a) Lamellar phase (b) reverse hexagonal phase (c) reversed micellar cubic of Fd3m (d) reversed bicontinuous cubic (Im3m) (e) reversed bicontinuous cubic (Pn3m) (f) reversed bicontinuous cubic (Ia3d). 16
Figure 2.12 The anisotropic properties of rod-like liquid crystal molecules. 17
Figure 2.13 Schematic illustration of the birefringence of (a) positive uniaxial LCs and (b) negative uniaxial LCs. 18
Figure 2.14 The molecular alignments of liquid crystals with (a) positive dielectric constant, and (b) negative dielectric constant under the electric field. 19
Figure 2.15 Schematic diagrams of molecular structure and chemical composition about (a) LCPs, (b) LCNs and (c) LCEs. 21
Figure 2.16 Schematic diagram of the reversible uniaxial deformation behavior in liquid crystal polymer under the stimulus. 22
Figure 2.17 (a) Fabrication processes of the tubular liquid crystalline actuator. (b) Operation of the tubular liquid crystalline actuator. (c) Schematic of the assembly of the robot and its video screenshots of robot walking (one cycle for 240 s). Scale bar, 2 cm. 24
Figure 2.18 Soft locomoting robot with automatic and programmable control. (a) The design and the control system of the soft locomoting robot. (b) Reversible bending actuation of the actuator with injecting hot and cold water. (c, d) Schematic diagrams and screenshots of soft locomoting robot with walking-like motion. (e) The relationship between displacement of the soft robot and time. Scale bars, 2 cm (b and d). 25
Figure 2.19 Schematic diagram of the reprogrammable methods and the reversible actuation of the SS-LCE film. 27
Figure 2.20 Schematic diagram of bending behavior of the LC actuator with porous structure. 28
Figure 2.21 Various shape changes of the solvent-sensitive LC actuator, including helical, W shape, hinge and 3D shape. 28
Figure 2.22 (a) Schematic diagram of the fabrication process of the bilayer LC film. (b) Storage modulus of bilayer LC films as a function of director angle in different condition. (c) Three kinds of the shape change with different alignment of the stiff axis of LCE layer. 29
Figure 2.23 (a) Real images of the bilayer LC films with different director angle were soaked in the water. (b) Number of twists and pitch as a function of direction angle. (c) The relationship between helical angle and direct angle. 30
Figure 2.24 Schematic diagram of the bending mechanism of the azobenzene-containing cross-linked LC polymers by irradiating two kinds of visible light. 31
Figure 2.25 Schematic diagrams of the fabrication processes of the cilia-like liquid crystalline actuator. (a) PVA release layer (1) was deposited and patterned on the substrate. (b) The polyimide alignment layer (2) was deposited, cured and buffed at the second step. (c) The monomer mixtures containing DR1A (3) and A3MA (4) were inkjet printed in designated area and cured it. (d) Dissolving the PVA release layer. 33
Figure 2.26 (a) and (b) A paramecium uses the swing motion of cilia (with different forward and backward strokes) for self-propulsion. (c) A cilia-like liquid crystalline actuator can produce asymmetric motion by controlling the light. (d) Schematic diagram of the orientation of the molecules. (e) The real images of the asymmetric motion of the cilia-like liquid crystalline actuator. Scale bar, 20 μm (a) and 5 mm (e). 33
Figure 2.27 (a) The Schematic diagrams and (b) real images of the actuation behavior of the light-driven polymer crane. 34
Figure 2.28 (a) A real image (right photo) and schematic diagram (left photo) of the device, constructing the two kinds of the azobenzene-containing liquid crystalline films (A1 and A2 expressed in yellow and red) with a tilt alignment. (b) Screenshot images and (c) schematic diagrams of the untethered cargo picks up motion. (d) Screenshot images and (e) schematic diagrams of the untethered cargo transportation and drops off motion. 36
Figure 2.29 Schematic representation of rubbing process. 38
Figure 2.30 The alignment patterns of (a) radical flower, and (b) concentric circle. 39
Figure 2.31 Schematic diagrams of LC molecules alignment and the treated substrate with (a) DMOAP and (b) MAP. 40
Figure 2.32 Schematic diagram of the photoalignment process with PI film. 41
Figure 2.33 The reaction mechanism for dopamine polymerization via two pathways (A) a pathway of covalent oxidative polymerization and (B) the other pathway of physical self-assembly trimer. 43
Figure 2.34 (a) Optical and thermal images of blank and PDA-xLCE samples under NIR laser (808 nm) irradiation. Light intensity: 1.0 W / cm2, PDA doping content: 2 wt%. (b) The relationship between temperature and power density for the blank sample and the samples doped with PDA. 44
Figure 2.35 (a) Reversible actuation of a PDA-xLCE sample. Scale bar, 0.5 cm. (b–e) Schematic diagrams of the procedure used to prepare various welded blank/PDA-xLCE sample films into deferent shapes and real images of the 3D structures. Light intensity: 1.0 W/cm2. Scale bar, 1 cm. 45
Figure 2.36 Schematic diagrams of the deformation result with various PDA-coating patterns. 46
Figure 2.37 Schematic diagram of TENG's area power density and time axis. 48
Figure 2.38 Schematic diagram of the TENG’s operating modes (a) vertical contact-separation mode, (b) lateral-sliding mode, (c) single-electrode mode and (d) freestanding triboelectric-layer mode. 49
Figure 2.39 The operating principle of the (a) open-circuit condition and (b) short-circuit condition of the TENG for dielectric-to-dielectric in contact-separation mode. 51
Figure 2.40 (a) Schematic diagrams and (b) real images of an implantable TENG. SEM images of (c) PDMS film and (d) aluminum foil. (e) Operating principle of the implantable TENG. 53
Figure 2.41 Schematic diagrams of the structure and design of a fan typed TENG. Scale bar, (b) 2 μm and 500 nm. 54
Figure 2.42 (a) Schematic diagrams of operating principle of a fan typed TENG and (b) the results of the measurement data with VOC and JSC. 55
Figure 2.43 (a) Schematic diagrams and (b) real image of a common glasses equipped with msTENG. Real images of the (c) fixers and the msTENG. (e) The operating principle and potential simulation of the TENG. Scale bar, (a) 5 mm, (b) 2 cm and (c-d) 1 cm. 56
Figure 3.1 Chemical structures of the used compounds. 62
Figure 3.2 Chemical structures of the used compounds. 67
Figure 3.3 Chemical structures of the used compounds. 72
Figure 3.4 Photography of the DMA set up. 75
Figure 3.5 Schematic diagram of the experimental set up of the electric signal measurement for the fabricated TEG based on NIR responsible LCEs. 76
Figure 4.1 Fabrication of polydopamine particles at various pH values of (a) 7.5 (b) 8.0 (c) 9.0 and (d) 10.0. Scale bars, (a-b) is 1 μm and (c-d) is 100 nm. 77
Figure 4.2 DSC curves of the LC mixtures (M0, M2, M4, M6, M8) showing TNI in first cooling cycle. 79
Figure 4.3 POM textures of the mixture M4 at (a) 49.2 ℃ (isotropic phase), (b) 44.8 ℃ (nematic phase), (c) 44.8 ℃ for 10 minutes and (d) 44.8 ℃ for 60 minutes. Scale bars are 0.5 mm. 81
Figure 4.4 POM textures of (a) mixture M6 kept at 38 ℃ for a few minutes and (b) mixture M8 kept at 30 ℃ for 1 hour. Scale bars are 0.5 mm. 81
Figure 4.5 TGA curves and thermal degradation temperature of the LCE films with different PEGDA content. 83
Figure 4.6 DSC curves of the LCE films with different PEGDA content in second heating cycle. 83
Figure 4.7 Tensile stress-strain curves of LCE films with various amounts of poly(ethylene glycol) diacrylate. 84
Figure 4.8 SEM image of the parallel alignment side of LCE film. Scale bar is 100 μm. 85
Figure 4.9 SEM image of the vertical alignment side of LCE film. Scale bar is 100 μm. 86
Figure 4.10 SEM image of the cross section of the tilt-alignment LCE film. Scale bar is 10 μm. 86
Figure 4.11 (a) Schematic diagram of the cutting direction and top view of the LCE films, and (b) real images of the repeatedly U-shape bending in heating and cooling cycles. 88
Figure 4.12 (a) Schematic diagram of the cutting direction and top view of the LCE film, and (b) real images of the repeatedly helical bending actuator in heating and cooling cycles. 88
Figure 4.13 Schematic illustration of LC molecular arrangement variation in heating and cooling cycles. 89
Figure 4.14 Real images of the bending angle measured at 60 °C of the LCE M4 films with different thickness of (a) 25 μm, (b) 50 μm, (c) 100 μm and (d) 155 μm. 89
Figure 4.15 Dependence of bending angle of LCE M4 films on film thickness. 90
Figure 4.16 Real images of the bending angle measured at 60 °C of LCE M4 films with different curing time of (a) 1 min, (b) 10 min, (c) 30 min and (d) 60 min. 91
Figure 4.17 Dependence of bending angle of LCE M4 films on curing time. 91
Figure 4.18 Real images of PDA-coated LCE films with different coating times. Scale bar is 1 cm. 92
Figure 4.19 EDS scanning results of LCE film at (a) particle and (b) blank area, atomic analysis data were shown. 93
Figure 4.20 SEM images of PDA coated LCE films with various coating days of (a) 1 day, (b) 2 days, (c) 3 days, (d) 4 days and (e) 5 days. 93
Figure 4.21 Real images of NIR responsive LCE films with different coating time. 94
Figure 4.22 Dependence of bending angle of NIR responsive LCE films on various coating days. 95
Figure 4.23 Real images of LCE film via Michael addition (a) before and (b) after vacuum. Scale bars are 1 cm. 97
Figure 4.24 Real images of LCE films with various feed molar ratios via Michael addition after vacuum at 60 ℃ for overnight. Scale bar is 1 cm. 97
Figure 4.25 TGA curves of LCE films with various feed molar ratios. 98
Figure 4.26 DSC curves of LCE films with various feed molar ratios. 99
Figure 4.27 Tensile stress-strain curves of LCE films with various molar ratios of components. 100
Figure 4.28 The real images of LCE film (a) before and (b) after 250% stretching. 101
Figure 4.29 (a) Schematic illustration of molecule arrangement and (b) POM images of monodomain LCE film before and after rotation with 45º. 101
Figure 4.30 XRD pattern of (a) 0% strain, (b) 100% strain and (c) 250% strain LCE films. 102
Figure 4.31 Schematic diagrams of the reversible deformation of the monodomain strip film and monodomain helical film under the temperature controlled. 103
Figure 4.32 Real images of actuation behavior of the predesigned (a) strip film and (b) helical curved film, respectively. 103
Figure 4.33 Thermal induced strain of LCE films prepared from mixture T1, T2 and T3. 104
Figure 4.34 The thermal actuation of LCE film T3 at various temperature. 104
Figure 4.35 (a) Schematic diagram of bilayer LCE film, and (b) dependence of bending angle on temperature of bilayer LCEs with different PU thickness. 105
Figure 4.36 SEM images of LCE films coated with PDA for (a) 1, (b) 2, (c) 3, (d) 4 and (e) 5 day(s) coating period. 106
Figure 4.37 Real actuation of NIR responsive bilayer LCE film with various PDA coating periods of (a) 1, (b) 2, (c) 3, (d) 4 and (e) 5 day(s). (Light intensity is 120 mW/cm2) 107
Figure 4.38 Measurement of output (a) voltage and (b) current of tapping triboelectric generator induced by palm. 109
Figure 4.39 Measurement of output voltage of tapping triboelectric generator induced by NIR sensitive tilt-alignment LCE film. 109
Figure 4.40 Schematic diagrams of a novel designed TEG using a NIR responsible LCE film. 110
Figure 4.41 (a) Schematic illustration of the experimental set up, (b) measurement of the output voltage and current with TEG by a synthesized NIR responsive tilt-alignment LCE film. 110
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