Photoinduced Deformation Of Photoresponsive Azobenzene-Based Composites And Polymers

As a kind of smart materials,stimuli-responsive materials(SRMs)have always been a research hotspot.Upon external stimuli,the molecular configuration of SRMs will change,thus affecting their physical or chemical properties,and then SRMs show corresponding special functions.As is wellknown,light is more advantageous over other stimuli because it can be controlled remotely,quickly and precisely.Upon alternating irradiation of ultraviolet(UV)and visible(Vis)light,azobenzene and its derivatives can undergo the reversible isomerization,and thus inducing rapid,complex and reversible deformations of materials.Such photodeformable property will be broadly applicable in the fields of soft robots,micromechanical devices,microfluidic control and energy conversion,etc.Although great progresses have been made in azobenzenecontaining photodeformable materials,there are still some challenges to be solved.(1)Currently,a majority of azobenzene-containing photodeformable materials are only responsive to UV light,while UV-Vis-near infrared(NIR)light-deformable azobenzene-containing materials are rarely reported.(2)Generally,the crosslinking treatment is essential in the preparation of azobenzene-containing photodeformable materials,especially for azobenzene-containing liquid-crystal(LC)polymer network or LC elastomer,which increases the difficulty in the preparation of photodeformable materials.In addition,the above-mentioned materials are difficult to be reprocessed and reshaped due to the crosslinking.In this thesis,a series of UV-Vis-NIR light-deformable composite films have been successfully prepared by doping azobenzene and upconversion nanoparticles(UCNPs)into a shape-memory polyurethane(SMPU)matrix.In addition,when an azobenzene-containing LC mixture and graphene oxide(GO)are incorporated into the SMPU matrix,not only the UV-Vis-NIR light-deformations are achieved,but also the degree of deformation and responsive speed of the composite film are significantly improved.Furthermore,an azobenzene-containing polymer film has been successfully prepared without any treatment of crosslinking,which shows light-induced reversible deformations.Moreover,a light-switchable azobenzenecontaining polymer tough adhesive has been successfully prepared,which exhibits a good ductility while maintaining a high adhesive strength.The main research contents of this thesis are summarized as follows:(1)A series of UV-Vis-NIR light-deformable composite films have been successfully prepared by incorporation of 4-cyano-4'-pentyloxyazobenzene(5CAZ)and UCNPs into the SMPU matrix.During the film-forming process,5CAZ tends to migrate to the top surface(air interface side)of the film,resulting in a higher 5CAZ content in the upper part than that in the lower part of the film,and finally forming the composite film with a bilayer-like structure.Upon UV/Vis/NIR irradiation,the stretched composite films bend towards the side with higher 5CAZ content,independent of the incident direction of the light source.Subsequently,upon heating at 90?,the bent composite films can revert to their original flat shapes rapidly.Moreover,based on the light-induced different bending degrees of the composite films,the biomimetic actuators of simulations of finger bending are successfully fabricated.(2)A series of azobenzene-containing LC mixtures have been obtained by mixing 5CAZ with 4-cyano-4'-pentyl biphenyl(5CB)in different molar ratios,and then a series of UV-Vis-NIR light-deformable 5CAZ/5CB/GO/SMPU composite films have been successfully prepared by simply doping the azobenzene-containing LC mixture and GO into the SMPU matrix.During the film-forming process,5CAZ and 5CB tend to migrate to the top surface(air interface side)of the film,resulting in a higher content of the LC mixture in the upper part than that in the lower part of the film,and finally forming the composite film with a bilayer-like structure.Upon UV/Vis/NIR irradiation,the azobenzene-containing LC mixture in the composite film undergoes the photochemically or photothermally induced LC-to-isotropic phase transition,and then the stretched composite films show rapid and larger bending deformations towards the side with a higher content of the LC mixture,which can recover to their original flat shapes upon high-intensity NIR exposure due to the photothermal effect of GO.Moreover,the biomimetic circadian rhythms of acacia leaves and the biomimetic bending/spreading of fingers are successfully achieved on account of the excellent photodeformablity of the composite film.(3)An azobenzene-containing main-chain LC polymer has been successfully synthesized by stepwise polymerization,and then the polymer film obtained by the drop-coating method without any treatment of crosslinking exhibits lightinduced reversible deformations.Upon UV irradiation,the polymer film bends towards the light source.Subsequently,upon 530 nm green light exposure,the bent polymer film recovers to its original flat state.When exposed to alternating UV and green light,the polymer film can reversibly bend and unbend.In addition,at the same light intensity and irradiation time,the bending angles of the polymer films gradually increase with the decrease of light wavelength.Furthermore,upon UV or blue light exposure,the bending angles of the polymer films increase with the increase of light intensity or elongation ratio.Moreover,when the angle between the cutting direction and the stretching direction changes,the deformation modes of the polymer films will change accordingly.(4)A light-switchable siloxane-based azobenzene-containing main-chain polymer tough adhesive has been successfully synthesized by stepwise polymerization.Here,introduction of siloxane chains into the azobenzenecontaining polymer not only reduces the concentration of azobenzene moieties for decreasing the molar extinction coefficient of the polymer,but also enhances the flexibility of the polymer,endowing the adhesive with a good ductility while maintaining a high adhesive strength.UV exposure reduces the adhesive energies to 1.9%of the original values within 30 s,and subsequent green light irradiation raises the adhesive energies to 98.3%of the original values in 60 s.In addition,upon alternating irradiation of UV and green light,the bonding/debonding can be reversibly light-switched for various substrates.Attractively,based on the excellent light-switchable adhesion of the adhesive,a polymer-glued plastic ring is successfully fabricated,which can be reversibly opened and closed by exposure to alternating UV and green light with a pressure on the adhesive position.