Responsive Liquid Crystal Elastomers Based On Bisazobenzenes With High Photothermal Effect

Light-responsive liquid crystal elastomers(LCEs)as a class of stimuliresponsive materials have attracted widespread attention,because the light could realize remote and precise manipulation of LCEs and control their deformation in various ways.Intelligent responsive actuators constructed by light-responsive LCEs have broad application prospects in the fields of biomimetic devices,biomedical equipment,and micro-manipulation devices.Azobenzene is currently the most widely studied photoactive group because it could undergo reversible trans-cis isomerization under ultraviolet light.However,the photothermal effect of azobenzene under ultraviolet light has always been not appreciated.This dissertation focuses on the photothermal effect of azobenzene as following:1.Firstly,a bisazobenzene molecule with long conjugated structure and strong π-π stacking interaction is designed and synthesized,which has two acrylate groups.Differential scanning calorimetry(DSC),polarized optical microscopy(POM)and ultraviolet-visible spectroscopy(UV-Vis)are applied to characterize the phase behavior and light absorption behavior of bisazobenzene.The test results indicate that the bisazobenzene molecule has high rigidity with a certain degree of crystallinity,so that the photo-induced trans-cis isomerization in the solid state is limited,while the photothermal effect would be enhanced.Then,we use the bisazobenzene molecule as a crosslinker to construct a series of LCEs with different crosslinking degrees,whose light-responsive behavior and response mechanism are studied by irradiation with different intensity of ultraviolet light.The results show that after the in-situ photopolymerization,a crosslinked polymer network is formed and the mobility of bisazobenzene molecule in LCE film is dramatically restricted.As a result,the transcis isomerization of traditional azobenzene cannot occur under ultraviolet light,the absorbed light energy is dissipated in the form of heat,causing locally heat up and expansion of LCE and perform bending deformation.In addition,it can be observed that with the increase of crosslinking degree within a proper range,the interaction between the polymer chains gets stronger,and the overall synergy of the elastomer network is improved.Therefore,the movement restriction of bisazobenzene molecule is greater,higher photothermal effect can be produced under the same light intensity,which causes LCEs perform larger and faster deformation.2.LCE films with different bisazobenzene contents are further prepared on the basis of previous experiments.The dependence of the photothermal effect and deformation of LCE on the content of bisazobenzene is explored.Firstly,LCE without bisazobenzene was prepared.The results show that the bisazobenzene molecule is prerequisite for the photothermal effect and bending deformation of LCEs.Comparing the LCEs with different contents of bisazobenzene,we find that increasing the content of bisazobenzene have a significant promotion on the photothermal effect of LCEs,but not necessarily improve the deformation performance of LCEs.This is due to the proportion increase of rigid structure in the system would weaken the mechanical properties of LCEs.In addition,we applied splay orientation for the alignment of the mesogens.The obtained LCEs exhibit faster and larger deformation than the LCEs with parallel orientation.Furthermore,a photoresponsive actuator is constructed with curling deformation through scissoring the LCE at an angle of 45° to the orientation direction.