Photodeformation Properties Of Liquid-crystalline Elastomers And Composites Containing Long Conjugated Mesogens

There is a growing interest in the development of photodeformable materials. One of the most interesting research fields is liquid-crystalline elastomers(LCEs), which combine the elasticity of polymer networks and the intrinsic anisotropic aspects of LC phases.Various kinds of deformation of LCEs containing azobenzene chromophores have been achieved by alternating the wavelength of actinic light.UV and visible light initiate the photochemical reaction of the azobenzene moieties in LCEs,changing the alignment of mesogens and leading to a macroscopic contraction, expansion and even bending through the cooperative movement of mesogens and polymer segments.In our previous study,most of the photosensitive LCEs incorporating azobenzene moieties underwent a photoinduced deformation in response to UV light,but the UV light is harmful to human body.Therefore,we developed a new kind of LCEs,which can undergo a photoinduced deformation in response to visible light.Furthermore,this kind of LCEs exhibit reversible photoinduced bending and unbending in sunlight.Herein a novel LC monomer and cross-linker both incorporating azotolane long conjugated mesogens were synthesized.It was found that the mixture of monomer and cross-linker exhibited the nematic phase at 120℃.Compared with other LC monomers and cross-linkers containing azobenzene moieties,azotolane-containing LC compounds show LC phases in a wider range of temperature,because the long conjugated system with three phenyls is prone to the LC phases.It was also found that the maximum absorption of the molecules appeared around 384 nm,due to the absorption of theπ-π~* transition of the azotolane moieties.Additionally,not only the 366 nm UV light irradiation,but also the 436 nm short-wavelength visible light irradiation can induce the trans-cis photoisomerization of the azotolane moieties. With the increase of irradiation time,the absorbance around 384 nm decreased until a photostationary state reached.The azotolane LCE films with three kinds of cross-linking density were prepared by thermal polymerization.The DSC curves of the obtained azotolane LCE films show that the glass transition temperatures of the films are around room temperature. Periodic changes of the dark and bright images of POM reveal that the mesogens in the films are preferentially aligned along the rubbing direction of the cell.Moreover, the order parameters of the films were calculated from the polarized UV-Vis absorbance,and it was found that the order parameter increased with increasing the cross-linking density.All of the free-standing films exhibit photoinduced bending and unbending behaviour at room temperature.The films bent toward the irradiation direction of the incident light upon exposure to short-wavelength visible light at 436 nm,while the bent films completely reverted to the initial state upon exposure to visible light at 577 nm.Moreover,the intensity of light and the temperature have great influence on the bending time of the films.The bending rate is properly accelerated by improvement of light intensity and temperature.It was found that the maximum force generated upon the irradiation with 436 nm light increased with the increase of cross-linking density or light intensity,but decreased with the increase of temperature.From the stress-strain curves of the films,we found that the elastic moduli parallel to mesogen alignment were larger than the perpendicular one because of the fibre reinforcement of the aligned mesogens.Besides,the elastic moduli of films increase with the increase of cross-linking density.We also prepared LCE composite films by thermal compression bonding of a LCE film and a polyethylene(PE) film. We found that the mechanical strength of the composite film is much lager than that of LCE.After the stretch of 10%strain,the composite film still exhibited excellent photoinduced bending and unbending behaviour without deterioration or breakages of the materials.When the light irradiated two different sides,the composite film exhibited different bending behavior.If the light irradiated the LCE side,the composite film bent toward the irradiation direction of the incident light.On the contrary,if the light irradiated the PE side,the composite film bent away from the light source.Finally,we successfully demonstrate that the azotolane LCE film can exhibit photoinduced bending and unbending in sunlight by manipulating the wavelength with lens and different glass filters.Besides,the ring of LCE film rolled intermittently forward upon simultaneously irradiation from the upside left and upside right with 470 nm light and 577 nm light,respectively.