Studies On The Preparation And Properties Of UV-blocking Poly(vinylidene Fluoride)Films

Poly(vinylidene flouride)(PVDF)is a semi-crystalline polymer material that has excellent chemical resistance,thermal stability,mechanical behavior and forming ability.It has become a fairly important polymer and has been widely used in aeronautics and astronautics,ultrafiltration,bioreactor,gases treatments,ion battery as well as many other fields.Meanwhile,modification methods like surface grafting,mixing and surface coating have been applied to improve the performance of PVDF or introduce new functions.However,the fabrication and research about the superb ultraviolet(UV)proofing PVDF membranes were very few.When PVDF is applied to the multilayer materials,its high transmittance can cause the degradation of underlying light-sensitive material after a long time solar radiation.Hence,the improvement of UV-shielding performance of PVDF films has an important significance.Benzophenones and benzotriazoles are important organic UV absorbers.They can transform the energy of UV radiation into vibrational energy via the intermolecular hydrogen bonds and dissipate the energy.The migration of absorbers that is usually found in the material application process can be effectively prohibited through the film surface grafting and matrix modification method,as absorbers can be stably combined with PVDF.3,4-Dihydroxyphenylalanine(DOPA)can form into polydopamine(PDA)through self-polymerization to modify the surface of several materials and PDA bas been widely studied in last decade.PDA particles have fairly good UV shielding property apart from the hydrophilicity,adhesive ability and biocompatibility.Therefore,the fabrications of many UV absorbing composite films are about PDA.In the paper,excellent UV-proofing PVDF membranes were made by the surface and bulk grafting of organic UV absorbers as well as surface PDA deposition method.The main researche ideas and research conclusions are given in the following part:1.Alkali-treated PVDF bulk grafting of UV absorberPVDF has fairly good chemical stability and low reactivity,it is not easy to react with other monomers.The reactivity of PVDF could be improved as unsaturated double bonds were produced after the treatment of tetraethylammonium hydroxide(TEAH).Then benzoyl peroxide(BPO)could initiate the grafting of 2-hydroxy-4-(3-methacryloxy-2-hydroxylpropoxy)benzophenone(BPMA)onto PVDF chains.The modified PVDF films with UV shielding performance were prepared by the solution casting method.The effects of TEAH solution concentration,alkali treatment time as well as the mass ratio of BPMA and PVDF on the structure and performance of fabricated PVDF-g-PBPMA films were studied.In the end,the optimum condition was obtained.The results showed the transmittance of PVDF-g-PBPMA film made under best condition was less than 1%at the range of 200-388 nm,which means the UV light at that region could be completely shielded.Besides,the modified PVDF membrane still had fairly good thermal stability.2.Pre-irradiated PVDF grafting of UV absorberRadicals can be generated from the PVDF chains after the pre-irradiation.After that,the grafting of reactive UV absorber 2-[2-hydroxy-5-[2-(methacryloyloxy)ethyl]phenyl]-2H-benzotriazole(RUVA-93)can be initiated through the melt processing method.This way is easy to operate and there is no need to add initiators in the reaction process.Besides,this method can be used in large scale production.It could be found that there were obvious changes in the chemical structures of modified PVDF,through the comparison of 'H-NMR and FT-IR spectra of films before and after the grafting.There was a distinct decrease in the UV transmittance of fabricated PVDF-g-PRUVA-93 film.The transmittance of modified film within 200-387 nm and 280-320 nm was less than 1%and 0.4%,respectively.The test results proved that the PVDF film after the pre-irradiation grafting had superb UV shielding performance.3.The deposition of PDA on the PVDF film surfaceIt was reported that 3,4-dihydroxyphenylalanine(DOPA)could polymerized into polydopamine(PDA)on the surface of several materials and obtained PDA particles had good UV shielding performance.In this experiment,PVDF films were coated with UV-proofing PDA layer.Through the comparison,we could know that the hydrophilicity of PVDF film treated with KOH solution was greatly improved and the pretreated film was more beneficial to PDA coating compared with original PVDF film.Besides,the depositing temperature,depositing time,concentration of dopamine and tris buffer solution greatly affected the coating process,which further influenced the surface morphologies and UV transmittance of modified films.The results proved that the PVDF-OH@PDA film deposited for 21 h at temperature of 60? in the DOPA solution with the concentration of 2.0 g/L had the best UV shielding property.The film transmittance was less than 1%at region of 200-400 nm and the transmittance at 325 nm was less than 0.1%which means the absorbance was greater than 3.The adhesion ability and durability tests proved there was a strong interaction between PDA layer and the PVDF-OH film,therefore,PDA layer could be stably adhered to the film surface.4.PVDF membrane surface grafting of UV absorberActivator generated by electron transfer atom transfer radical polymerization(AGET ATRP)is a living radical polymerization method improved on the base of ATRP.In the polymerization,fewer catalysts are used and the reaction condition is not harsh.Here,the initiators were immobilized onto the film surface via the reaction between 2-bromoisobutyryl bromide(BIBB)and the hydroxylated PVDF membranes.Then the UV-absorber BPMA was grafted onto the film surface by the use of AGET ATRP.The absorbers migration could be effectively prevented because PVDF film and UV absorbers were connected with stable chemical bonds.The chemical structure,surface morphologies and UV shielding performance of modified films were tested.Besides,the effects of the reaction time were investigated.The test results proved the characteristic absorption bands of BPMA were emerged from the spectrum of functionalized PVDF film.And the intensity of bands increased with the prolongation of the reaction time.Besides,more absorbers were linked to the surface of PVDF film as the reaction time increased to 24 h.The UV transmittance of modified film between 240 nm and 350 nm was less than 0.4%.