Research On The Preparation Of Polyvinylidene Fluoride Microporous Membrane Via Thermally Induced Phase Separation And Hydrophilic Modification

Membrane technology has been one of the research hot spots due to its advantages such as high efficiency,low energy consumption,easy controlling and simple process.PVDF(Plyvinylidene fluoride)is a kind of semi-crystalline polymer.PVDF has become one of the most attractive polymer materials for membrane preparation because it exhibits great thermal stability,excellent chemical resistance,extraordinary mechanical property and strong radiation resistance.Compared with NIPS(Nonsolvent-induced phase separation)method,PVDF membrane prepared via TIPS(Thermally induced phase separation)method possesses high porosity and excellent mechanical strength due to its reliable and controllable TIPS technology.In the dissertation here,nine substances including diphenyl ketone(DPK),diphenyl carbonate(DPC),N-methylpyrrolidone(NMP),etc had been selected as the diluent to prepare PVDF membranes.The structure of the prepared PVDF membrane and the phase separation process of polymer/diluent system were analyzed by Hansen solubility parameters and RED(Relative energy distance)value.Smaller RED value means the stronger interaction between PVDF and the diluent and the polymer/diluent system is prone to follow the mechanism of solid-liquid(L-L)phase separation during the cooling process.The prepared membrane's cross-sectional morphology is compact spherulitic structure.When the RED value is slightly elevated,the compatibility of the system become poorer and the phase separation process changes from the S-L phase separation to the L-L phase separation,led to a bicontinuous structure in the membrane.The addition of a second diluent AA(A kind of alkyl alcohol)which is incompatible with PVDF and strongly interacts with DPK in the PVDF/DPK system made the system undergo L-L phase separation during the cooling process due to the separation of the second diluent from the casting solution.It could lower the compatibility of system and extend the L-L phase separation region to higher polymer concentration.Under the condition of a certain PVDF concentration(25 wt%),the membrane structure could be switched from spherulitic structure to bicontinuous and cellular structure by adjusting the proportion of the two-component diluent.Membrane fouling is easy to form due to the low surface energy and hydrophobic nature of PVDF so an amphiphilic copolymer(Named A)was adopted to blending modification of PVDF membrane.The optimum blending ratio was determined through the analysis of thermal properties,morphology,mechanical properties and water contact angle of the samples.Afterwards,the hollow fiber membranes with optimum blending ratio were prepared and characterized.The modified membrane's pure water flux raised to 90.6%and the amount of absorbed protein reduced 40.2%and the membrane after modification could maintain its hydrophilicity in long-term test.