Study On The Structure And Properties Of Photochromic Hydrogels

As we all know,intelligent photo-responsive materials play an important role in both industry and human daily life.Amongst,photochromic materials,as an intelligent naked eye visual photo-responsive material,have been applied in clothing industry,printing industry and daily chemical industry.However,the development of photochromic hydrogel materials is still challenging.Based on this,in this paper,photochromic materials are applied to construct photochromic hydrogels and their properties and applications have been studied.The details are as following:Firstly,a photochromic hydrogel system was designed and fabricated based on silver bromide and cuprous bromide in poly（hydroxyethyl acrylate-co-polyacrylamide）hydrogel matrix.Here,polyvinyl pyrrolidone（PVP）and glycerol were employed as a surfactant and sustainable co-solvent,respectively,whose synergistic effect make AgBr endows the hydrogel with photochromic stability and repeatability.The photochromic hydrogel can be changed reversibly between light-blue and brown color for least 15 times in air under a simulated solar light irradiation.The atoms side reaction and migration of halides in opening hydrogel system were overcome to realize photochromism.Both UV-vis absorption（approximately 408 nm）and the valence states of the metal ions were studied and verified the conversion between Ag⁺and Ag⁰.The process of color changing was studied and the fabricated hydrogel with a half-life fading period(t_1/2)was 6.78 min under a simulated solar light irradiation.Reasonably,the as-fabricated photochromic hydrogel displayed remarkable antibacterial ability to both Gr AM-positive-and Gr AM-negative bacterium with the Ag ions in the system.Meanwhile,it also demonstrated excellent non-cytotoxicity property to L929 Mouse Fibroblast cells.Since its excellent photochromic and biological properties,the hydrogel has potential applications in the field of smart materials such as naked eye visual sensors,artificial intelligent devices and bionic skin.Although photochromic hydrogel based on silver bromide nanomaterials demonstrates photofatigue resistance,the fading time of it is relatively long.In the second part,double bond functionalized spirooxazine（DSPO）was applied to endow a hydrogel photochromic performance.Moreover,the hydroxyethyl cellulose（OHEC）molecular chain was used as template molecule to obtain the high compression resisitant hydrogel based on the dynamic bond.The developed hydrogel could recover to itself without deformation once the hydrogel is released in the process of repeated loading and unloading compression.Furthermore,integrating DSPO endowed the hydrogel photo-and pH sensitivity in the transparent hydrogel（93%,T%）substrate.Under UV light irradiation,the prepared hydrogel changed from transparent colorless to blue due to the transformation of DSPO from its ring-closed form（SP）to the ring-opened merocyanine（MC）form.The protonation of MC in an acidic environment could convert the photochromic hydrogel into a fluorescent one with yellow luminescence under UV illumination.Moreover,the hydrogel possessed non cytotoxicity property Hence,the designed dual-sensitive hydrogel has application prospects in the field of wearable transparent device or display sensor,which provide a new strategy for the development of intelligent devices.Thirdly,with the development of green chemistry,materials with self-healing properties can extend the service life of materials,which makes them attract more and more attention.In this part,carboxyl functionalized organic photochromic molecule was applied to traditional polyvinyl alcohol（PVA）macromoleculas via hydrogen bonds to construct photochromic self-healing hydrogels.First,a kind of carboxyl group functionalized photochromic spirooxazine molecula（SPO-COOH）was prepared.Then SPO-COOH was dissolved in aqueous-organic mixed solution with glycerol and PVA to form a large number of hydrogen bonds.Borax was added to the system as the crosslinking point.Finally,the photochromic self-healing hydrogel was prepared by repeated freeze-thaw approach.The hydrogel exhibited blue color through the ring opening of SPO-COOH under ultraviolet light irradiation and faded under visible light irradiation.The photochromic process can be repeated many times.Moreover,the effects of different freeze-thaw time on rheological properties were studied,and the self-repairing properties were tested by rheometer.Based on the fast photochromism and self-healing,the as-prepared hydrogel can be applied in the field of fast self-erasing paper.Finally,we prepared reverse photochromic self-repairing hydrogels,namely,visible light controls photochromic hydrogels.A kind of functionalized spiropyran molecule（SP-COOH）,which exhibiting reverse photochromic behavior in the presence of rich hydrogen bonds,was prepared.Then,the spiropyran derivative was mixed with glycerol and PVA molecules via water-orgnic cosolution.The reverse photochromic self-healing hydrogel was prepared by repeated freeze-thaw approach.Compared to the hydrogel system without borax,the result indicate that borax enhanced the color of the hydrogel.Hence,the hydrogel with reverse photochromism under visible light and self-healing at room temperature without stimulation was designed and fabricated.Based on abovementioned properties,the reverse photochromic hydrogel has potential applications in wearable devices,sensors,intelligent machines and self-erasing paper.