Study On Preparation And Photochromic Behavior Of Naphthopyran Doped Polymer Films

Organic photochromic materials have a wide range of development potential application in optical information storage,optical display and holographic imaging,photochromic glasses and apparel,smart response materials,which due to reversible photochromic reactions caused by changes in molecular structure,color,electromagnetic distributions.Photochromic materials are usually dispersed in polymer matrix.When photochromic molecules were doped in polymer matrix to satisfy the microscopic environment of photoisomerization,however,it is necessary to sacrifice the mechanical properties of matrix to achieving a higher photoresponse rate.In order to solve this contradiction,physical doping and chemical modification are usually used to achieve a balance between the two,but still face challenges.In this dissertation,on the one hand,the polymer was plasticized to increase the free volume of the polymer,so as to achieve the balance between the mechanical strength and fast photoresponse rate;on the other hand,by polymer dispersion gel method,the amphiphilic surfactant（Gemini）was used to adjust the polymer/gel interface interaction to provide the photochromic molecules with a liquid microscopic environment in the rigid polymer.Firstly,polymethyl methacrylate（PMMA）was used as a matrix,naphthopyran（NA）was used as a photochromic molecule,and dioctyl phthalate（DOP）was used as a plasticizer.The polymer dispersed NA photochromic films was prepared by solution casting method.The effects of plasticizers on the photochromic properties of NA,fading rate,fatigue resistance,and mechanical properties of the materials were systematically studied.The results showed that the addition of DOP increases the polarity of the materials,resulting in a blue shift in the maximum absorption spectrum of the photochromic molecule.At the same time,the free volume of the polymer increases,and the fade rate of NA in the film can be increased up to 3 times before the addition.The addition of NA and DOP both have a plasticizing effect on PMMA,resulting in a decrease in the elastic modulus and tensile strength of the film,reduced toughness of PMMA film.When 40 wt%of DOP plasticizer was added,the elastic modulus of NA-PMMA film was 451 MPa,the elastic modulus was reduced by 50%compared to the sample without DOP.The tensile strength was reduced to 8.4 MPa,which was a drop of 60%.The elongation was 26%and the toughness increased by 5times.Secondly,the PVA dispersed photochromic gel films were prepared by cosolvent dissolution-emulsification-solvent evaporation method,using polypropylene glycol（PPG）as the organic solvent and polyvinyl alcohol（PVA）as the polymer matrix.The use of Gemini during the preparation of the thin film improves the interfacial structure at the interface between the gel and the PVA.The results showed that PVA dispersed spiropyran（SP）gel films exhibited solution-like good photochromic behavior and fading behavior.The fade rate of SP in films was similar to that of polypropylene glycol（PPG）solution,which was 0.0418 s^-1.Interestingly,SP-PDPG 1 also has excellent mechanical property with elastic modulus and tensile strength as high as 547 MPa and23.5 MPa respectively.After 10 repeated coloration-discoloration cycles,the intensity at the maximum absorption wavelength decreased by about 35%,which was decreased dramatically comparing with that using PVA1 as a surfactant（53%）.Based on the separation force by head-end gathering process of Gemini,the anchor effect of PVA/organogel interface on SP is suppressed.Meanwhile,more SP molecules were protected within micro-solvent environment,which was homogeneously dispersed in PVA matrix.The anchor effect increases the number of SP molecules that could conduct reversible free isomerization,accelerating the fading rate and improving the fatigue resistance of the PDPG-SP thin film concurrently.In order to study the universality of this method,different emulsified films were prepared using NA,and the prepared films still had good solution-like photochromic properties.The fade rate of Gemini emulsion film（NA-PDPG 1）was 0.167 s^-1,and the fade rate of PVA1 emulsion film（NA-PDPG 2）was 0.2 s^-1,which was faster than that of NA-PPG solution(0.153 s^-1).Although the presence of gels in the film reduces the mechanical properties of the material,the tensile strength is in the range of 23.5²8.5MPa,and the elastic modulus is in the range of 500⁶50 MPa,which is 50%lower than that of the pure PVA film.The elongation at break is about 200%,which is about25%higher than the 163%of the PVA film.