| Under the irradiation of light with specific wavelength,the low-energy trans-isomer and the high-energy cis-isomer of azobenzene molecules can be switched reversibly.Based on this characteristic,azobenzene derivatives have been widely used in various molecular devices and functional materials.Nowadays,disulfide bonds have become one of the most commonly used dynamic covalent bonds due to the versatility and the relatively simple applicability for various polymers.Introducing disulfide bonds into polymer systems can realize the self-healing and recyclable properties of materials.Ring-opening polymerization of dithiolane groups in lipoic acid derivatives is an efficient method to prepare disulfide bonding polymers,and has been widely used to prepare various functionalized polydisulfide compounds,self-assembled gel materials and ultra-strong multifunctional elastomers containing dynamic crosslinked networks.In this paper,optically active azobenzene molecules were combined with dithiolane containing dynamic covalent bonds.A series of polymer films with polydisulfide backbones and different azobenzene structures in the side chains were prepared by ring-opening polymerization.The effects of the azobenzene structures on the photoresponsive properties of the materials were discussed.Utilizing the photothermal effect of azobenzene and the dynamic breaking and recombination properties of disulfide bonds,the films with suitable properties were selected as new photothermal adhesives and coatings,which can realize the photo-induced reversible adhesion-debonding process and can be utilized to fabricate the bilayer actuators with reversible photodeformation.Meanwhile,the photothermal adhesives and coatings showed excellent remolding and recyclable properties.The main work is as follows:(1)Nine types of azobenzene monomers LAzo-E1,LAzo-E2,LAzo-E3,LAzo-B1,LAzoB2,LAzo-B3,LAzo-C1,LAzo-C2 and LAzo-C3 were designed with three different types of electron-attracting/electron-donating terminal groups at 4-position and three types of dithiolane groups with different chain lengths at 4′-position.The crosslinker C1 containing dithiolane groups at both ends was prepared at the same time.The structures of the synthesized compounds were characterized by NMR spectrometer and high resoluyion mass spectrometer.The UV-Vis absorption spectra of nine azobenzene monomer solutions were measured to discuss the photoisomerization properties.(2)Nine types of polymers films with polydisulfide backbones containing different azobenzene groups in side chains(FLAzo-E1,FLAzo-E2,FLAzo-E3,FLAzo-B1,FLAzo-B2,FLAzo-B3,FLAzo-C1,FLAzo-C2,and FLAzo-C3)were prepared by ring-opening polymerization between the above-mentioned azobenzene monomers and crosslinker C1.The thermal and mechanical properties of all films were characterized by thermogravimetric analyzer and dynamical mechanical analyzer.The photoisomerization behavior of azobenzene groups in the films was analyzed by UV-Vis absorption spectroscopy and transient absorption spectroscopy.An infrared thermal imager was used to record the temperature changes of the films under UV light,and the mechanism of photothermal effect was discussed.(3)An optimal azobenzene polymer film was selected to be used as reversible photothermal adhesive/coating.Differential scanning calorimeter and rheometer were used to characterize the thermaldynamic properties during the solid-liquid transition.The photoreversible bonding/debonding properties,good adhesion ability and the remodeling properties were verified by glass plate bonding test.The material was used as a photothermal coating to fabricate double-layer actuators,which could perform light-responsive deformation and biomimetic motion.In addition,the photothermal coating exhibited the characteristics of easy separation and recyclability.These excellent properties make it have broad application prospects in the field of green smart materials. |
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