Modification And Structural Control Of PVDF Films Fabricated By Thermally Induced Phase Separation

Environmental pollution is caused by the continuous development of industrialization.At present,the problem of more and more serious environmental pollution has emerged.There is a social focus on solving the serious problem of water pollution so as to ensure people’s healthy life.Therefore,membrane separation technology with high separation efficiency,simple process operation,no phase transition in the separation process,low energy consumption and no secondary pollution has become a research focus.In this work,thermally induced phase separation（TIPS）was used to prepare polyvinylidene fluoride（PVDF）films,and the effects of polymer concentration,quenching temperature and doping modification of silicon dioxide nanoparticles（SiO₂）coated with polydopamine（PDA）on the microstructure and properties of PVDF films were investigated.The main content of this paper is as follows:Firstly,the PVDF film was prepared by TIPS using DPC as a diluent.The effects of polymer concentration and quenching temperature on the structure and properties of the films were studied.The study showed that the monotectic point of the system was 52.6 wt%,and the microstructure of PVDF film changed from multi-layer honeycomb structure to spherulite structure with heterogeneous pore size as the polymer concentration increased.And the cooling rate in the quenching process of PVDF film has the characteristics of position dependence,which affects the crystal growth and droplet aggregation during the phase separation process,causing the formation of multi-layer structure in the films.A lower cooling rate results in a larger pore size,and the pore size of the film with high polymer concentration is much smaller than that of the film with low polymer concentration at the same cooling rate.Therefore,the change of quenching temperature can regulate the pore size of the film,which has an impact on the film properties.PVDF film with low polymer concentration generally follows the trend of water vapor transmission value（WVTR）increasing with the increase of quenching temperature,but the WVTR of the film quenched at 20℃ is higher than that at 40℃.However,the WVTR values of the film with high polymer concentration hardly changed with the change of quenching temperature.Higher quenching temperature can bring better thermal stability for PVDF films.The dynamic mechanical thermal properties of PVDF films with low polymer concentration are relatively good at a quenching temperature of 20℃;the dynamic mechanical thermal properties of PVDF films with high polymer concentration are less dependent on quenching temperature.Secondly,spherical SiO₂ nanoparticles prepared by sol-gel method were coated by oxidative self-polymerization reaction of dopamine on the surface to obtain SiO₂@PDA nanoparticles.Then,TIPS method was used to prepare SiO₂@PDA/PVDF composite films after blending modification.Polydopamine（PDA）is a perfect adhesive substance,and also has abundant hydrophilic groups such as hydroxyl groups and amino functional groups,which enhances the hydrophilicity of SiO₂@PDA nanoparticles.The packing distribution,thermal stability,dynamic mechanical thermal properties,mechanical properties,water contact angle and anti-pollution properties of PVDF composite films were studied.The SEM observation showed that the coating of PDA can improve the dispersibility of SiO₂ nanoparticles in organic solvents,and SiO₂@PDA nanoparticles are evenly distributed in PVDF composite films.Through TGA testing,it was found that the addition of SiO₂@PDA nanoparticles improved the thermal stability of PVDF films.DMTA results showed that the addition of SiO₂and SiO₂@PDA particles improved the dynamic energy storage modulus of PVDF films.As compared with SiO₂ inorganic particles,the addition of SiO₂@PDA had better effects on improvement of both rigidity and deformation resistance of PVDF films,and the toughness of SiO₂@PDA/PVDF composite films was also enhanced.The tensile properties showed that as compared with neat PVDF films,the mechanical properties of SiO₂/PVDF and SiO₂@PDA/PVDF composite films have increased to a certain extent,the tensile strength of SiO₂/PVDF films with 1.0 wt%SiO₂ nanoparticles was lower than that of SiO₂@PDA nanoparticles with the same addition amount.As the loading of SiO₂@PDA nanoparticles increased,the water contact angle of SiO₂@PDA/PVDF films showed a gradually decreasing trend.SiO₂@PDA/PVDF films were prepared by adding 0.5 wt%,1 wt%and 2 wt%of SiO₂@PDA inorganic nanoparticles,and the WVTR values of SiO₂@PDA/PVDF films decreased by 3.8%,13.6%and 19.1%,respectively,in comparison to neat PVDF film.The addition of SiO₂@PDA nanoparticles can significantly improve the anti-pollution performance of the PVDF films,and the anti-pollution performance becomes better as the content of SiO₂@PDA particles increases.