Precipitation And Characterization Of High Hydrophobic Poly (Vinylidene Fluoride) (PVDF) Microporous Membranes Via Combining Immersion With Micromolding

Compared gas-liquid membrane contactors(such as membrane distillation,membrane absorption,membrane parsing)with conventional gas-liquid mass transfer processes(such as absorption,evaporation),the aspectes of separation,operation,or energy saving had certain advantages.So it had more attention in recent years.In the membrane contactors separation processes,ensuring no wetting was the key to membranes for a long time,which was largely dependented on the hydrophobic of microporous membranes.The method of micromolding combining with immersion precipitation was proposed for preparing high hydrophobic poly(vinylidene fluoride)(PVDF)microporous membranes.The sandblasting aluminum plates with artificial microstructure were used as the substrates,while N-methyl pyrrolidone(NMP)was respectively chosen as solvent in the process.The process of microporous membrane was researched for immersion precipitation.Those included preparation of casting solution,phase separation condition and composition of casting solution.The results showed: each condition was changed,which made the contact angle of the bottom reach 140 degree or more,indicating high hydrophobic property.Preparation of casting solution mainly studied maturation temperature and maturation time of casting solution from the perspective of thermodynamics.The maturation temperature had little impact on the membranous structure and hydrophobicity.But delayed maturation time made casting solution system produce microparticle,which had advantage to solid-liquid phase separation.Phase separation condition mainly researched coagulation bath composition and temperature from the perspective of dynamics.When adding NMP to the coagulation bath,the chemical grads between the cast solution and coagulation bath was decreased,so the gelation rate decreases and instantaneous type phase separation changed to delayed type phase separation;when temperature was low,moleculars moved slowly,so the gels and solvents decreased transfer rate mutually.Those were advantagous to the slow process solid-liquid phase separation.Composition of casting solution researched small molecule additive: when the compatibility of mixed solvent and non solvent is worse than the solvent and non solvent,transfer rate decreased between the solvent and non solvent mutually;when the mixed solvent have worse compatibility with polymer,casting solution system produce microparticle easily.Those had advantage to solid-liquid phase separation.These three aspects could make solid-liquid phase separation advantagous.The bottom surface showed micro-submicro binary structures,which were consisted of microstructure from reproducing microstructure substrate and micro-submicro structure from phase separation.The micro-submicro hierarchical roughnesses on the bottom surface brought an enhanced hydrophobicity of the membrane.The contact angle of the membrane with water was as high as 148 °,indicating high hydrophobic property.It was suggested that this binary micro-submicro structure held more gas stably,and at the same time,the membrane permeability was improved while the mechanical tensile properties reduced slightly.