Anisotropic Thermal Conductivities And Stimuli-tunable Motion Modes Of Liquid Crystalline Elastomer Materials

Over the past few decades,the liquid crystalline(LC)materials have attracted increasing global interest.Particularly,liquid crystalline elastomers(LCEs)LCEs are materials that combine the self-organization of liquid crystals and the entropy elasticity of the polymeric elastomers,which endow these materials with wide applications in a variety of fields,like artificial muscles,actuators,electro-optic or nonlinear optic materials,micropumps.In this thesis,a variety of LCE materials are prepared by in-situ polymerization,hydrosilylation or thiol-ene chemistry.The anisotropic thermal conductivities and stimuli-tunable motion modes of these materials and the potential applications in different fields have been studied in depth.Liquid crystalline polymers(LCP)exhibit spontaneous local anisotropy,and have been recognized as ideal thermally conductive insulation materials.Two basic strategies have been developed to exploit these potentialities:one is starting from LCPs to prepare anisotropically-oriented LCP films and another one is aligning LC monomers instead of LCPs,and then appling in situ polymerization methods to synthesize anisotropically-oriented LCP films.However,the alignment effects of the former strategy were modest and the thermal conductivitivies was unsatisfactory.Most of the previous researches using the latter strategy were limited to homogeneously-aligned LCP films,which were inconsistent with the thermal conduction direction of electronic equipments.In order to overcome the problems,homeotropically-aligned LCP films are fabricated by in-situ photopolymerization of LC monomers oriented by alternating current(AC)electric fields respectively.In this thesis,LC thiol-ene monomer T6E5 and LC acrylate monomer A444 have been designed and synthesized.The homeotropically-oriented main-chain LCP film xMELCP and side-chain side-on LCP film xSSLCP are successfully prepared by in-situ photopolymerization of monomer T6E5 and monomer A444 oriented by AC electric fields respectively.Both polarized optical microscopy(POM)observations and two dimensional X-ray scattering(2D-WAXS)investigations of the xMELCP and xSSLCP films confirm the homeotropic orientation of the nematic director in the films.Evaluated by laser flash analysis(LFA)method,the room temperature thermal conductivities of these two xLCP films in the film normal direction are both dramatically higher than those along the horizontal direction.In particular,the main-chain xMELCP film shows the highest anisotropic thermal conductivity(λ//=2.439±0.025 W·m-1·K-1,λ///λ⊥=11.6).These novel xLCP materials have potential applications as anisotropic thermal insulators.LCEs can perform reversible phase transition under stimuli which can dramatically impact the microscopic orders or molecular structures of uniaxial-aligned LC mesogens and further change the macroscopic shapes of the whole LCE materials.These unique characters endow LCE materials with potential applications as mechanical actuators,artificial organs,smart surfaces,and microrobots,etc.However,most of traditional LCE materials only performed a single deformation under stimuli,and there was a rare example of multi-stimuli responsive LCE materials.In order to overcome these shortcomings,the thermal-induced LC-to-isotropic phase transition effect,azobenzene trans-cis tautomerization effect and the photo-thermal effect are combined into one LCE system to prepare a heat/UV/near-infrared(NIR)triple-stimuli-responsive LCE material.In this thesis,LC monomer V444 and azobenzene-containing LC monomer A44V6 have been designed and synthesized.A polysiloxane-based side-chain side-on azobenzene-containing LCE matrix embedded with 1 wt% single-walled carbon nanotubes are successfully prepared by Finkelmann 5s two-step crosslinking process.The multi-stimuli responsive behaviors of this LCE/CNT composite film are investigated under different external stimuli(heat,UV,NIR).The composite film can realize a reversible deformation between contraction and extension under heating/cooling cycles and a three-dimensional deformation(bending)under UV-light irradiation.On the contrary,long wavelength light(NIR laser)forces the composite film into shrinking instead of bending.The shape transformations(three-dimensional bending vs two-dimensional shrinking) of this novel shape memory material can be tuned by the light wavelength(UV vs NIR)? which might endow this LCE material with potential applications in control devices and logic gate devices,etc.Reversible photo-isomerization process of azobenzene moieties can deeply impact the order of mesophase,in /rara-conformation,the azobenzene-containing molecules are prone to form an anisotropic phase;On the contrary,in c/.y-confomiation,the azobenzene moieties bend and the ordered molecular arrangement is disrupted,leading to the formation of an isotropic phase.Most of the previously reported polysiloxane-based photo-responsive LCPs ’containing azobenzene moieties were prepared by hydrosilylation of polymethyl hydrosiloxane (PMHS) with mesogenic alkenes using Pt complex catalysts.However,classical hydrosilylation reactions are plagued by several critical problems,including uncontrollable catalyst grafting ratio,expensive Pt complex catalyst,etc.In order to overcome the disadvantages,a novel polysiloxane-based azobenzene-containing LCP material PMMS-A44V6 are designed and synthesized in this thesis via a facile thiol-ene click chemistry method by grafting a side-on azobenzene mesogenic group A44V6 onto PMMS backbone,which possesses one thiol group in every monomer unit,to take place of tradition PMHS as the starting polysiloxane backbone.lH NMR,and GPC are utilized to verify the degree of polymerization.TGA and DSC are carried out to measure the thermal properties of the materials.POM and WAXS are used to investigate the mesogenic properties and photo-responsive behavior of PMMS-A44V6.Taking advantage of the azobenzene’s trans-cis isomerization effect,PMMS-A44V6 can perform an isotropization process under UV irradiation in its nematic phase.Initially,the isotropization process starts in a linear decrease manner with a rate of ca-6.5 x 105 intensity/s,and eventually finishes in an exponential decrease regime to form cis-azobenzene moieties.The reversible UV-response behavior of PMMS-A44V6 can be performed in a relatively low temperature range of 30 °C ~75 °C,which might help this azobenzene-containing LCP material to find potential application in control devices,etc.In nature,plant tendrils can produce two fundamental motion modes,bending and chiral twisting(helical curling) distortions,under the stimuli of atmospheric conditions.Learned from these biological mechanisms,many artificial plant-like devices have been developed.However,all the reported soft actuator materials could not be able to fully mimic a plant tendril which can realize not only bending but also chiral twisting(left-handed and right-handed)in one single piece.In order to overcome the disadvantages,a real plant tendril mimic LCE material is synthesized in this thesis,which is capable of performing tunable,reversible bending and chiral twisting motions under two different external stimuli.In this thesis,LC monomer V444,azobenzene-containing LC monomer A44V6 and organic croconaine dye YHD796 are designed and synthesized A dual-layer,dual-composition polysiloxane-based azobenzene-containing LCE film is successfully prepared by Finkelmann two-step crosslinking process.The multi-stimuli responsive behaviors of this composite film are investigated under two different light stimuli(UV,NIR).The composite film can realize a reversible three-dimensional deformation(bending)under UV-light irradiation and a reversible three-dimensional deformation(chiral twisting)under NIR-light irradiation.In other words,this soft actor can produce two fundamental motion modes,bending and chiral twisting(helical curling)distortions,just like plant tendrils.This is the first example of soft actuator materials capable of performing two different reversible three-dimensional transformations(bending vs chiral twisting)under irradiations of two lightsources with different wavelength ranges(UV vs NIR),which might have potential applications in control devices and biomimetic devices,etc.Most of the super-hydrophobic surface films prepared in laboratories have complicated preparation process,unstable microstructures,and cannot perform deformations between superhydrophilic and superhydrophobic structures.In order to overcome these shortcomings,a series of LCE materials which can perform reversible roughness changes are prepared by Finkelmann’s two-step crosslinking process.The thermodynamic properties and thermal-responsive surface microstructures deformations of these LCE films are also investigated.Especially,the LCE film PMHS-MBB-A prepared by coins as the female mold can perform a reversible surface microstructures deformation(rough and smooth) under heating/cooling cycles,which might have potential applications in self-cleaning and other fields.