Study On The Mechanism Of Polylactic Acid Porous Membrane Prepared By Phase Inversion And Its Performance Regulation

Phase inversion is the mainstream technology to produce membrane materials.The theoretical research on the phase inversion technology is greatly important to reveal the mechanism of the membrane formation,as well as to control the membrane performance.The phase inversion process involves a wide range of parameters,and the change of the parameters in the phase transformation membrane process has significant effects on the thermodynamics and kinetics of the phase inversion.It has been reported in the literature that the theoretical research of improved phase inversion membrane formation technology has never stopped.In order to study the film formation mechanism of the phase inversion method under specific temperature and solvent system composition,this paper improved the traditional phase inversion method film formation technology by adjusting the temperature and solution system composition during film formation.Polylactic acid porous membranes(PLA porous membrane),with different structures and properties,were made.The film formation mechanism of polylactic acid porous membrane during the phase inversion process and the regulation of the properties of polylactic acid porous membrane were studied.The research was carried out in the following three areas:(1)Temperature,determines the evaporation rate of the solvent and the exchange rate between the solvent and the non-solvent,is a key parameter to control the morphology of the polylactic acid porous membrane prepared through phase inversion which induced by atomized microbeads.The morphology of the prepared membrane was observed using a scanning electron microscope,and the volatilization rate of the solvent and the exchange rate between the solvent and the non-solvent were tested separately.The results showed that the doublelayered PLA porous membrane was successfully prepared.The average pore diameter on the upper surface of the membrane decreased as the temperature increased from 25 °C to 75 °C.The evaporation rate of the solvent was faster than the exchange rate between the solvent and the non-solvent.The formation of macropores in the upper layer of PLA membrane was attributed to the thermal energy carried by microbeads,while the formation of homogeneous micropores in the lower layer was due to the depletion of thermal energy carried by microbeads after the formation of macropores in the upper layer.The PLA porous membrane with the target aperture could be prepared by adjusting the ambient temperature.(2)PLA porous membranes were prepared by microbead-induced phase inversion under different temperatures.The crystallization properties of the membranes were characterized by differential scanning calorimetry and X-ray diffraction,and the thermal stability of the membranes was characterized by thermogravimetric analysis.The results indicated that the ratio between imperfect and perfect crystals in the membrane was influenced by the preparation temperature,and resulting in a doublet melting peak in the DSC thermogram.The influence of the preparation temperature on the thermal stability of the membrane was negligible,which could be manifested in the small change of the initial decomposition temperature in the thermogravimetric analysis curve.By adjusting the ambient temperature,a PLA porous membrane owning a target pore size and better crystallinity could be prepared.(3)Poor solvents were introduced into the initial solution system to improve the phase inversion method to prepare polylactic acid porous membranes.The composition of the solution system thermodynamically determined the way in which phase inversion occurred.The PLA co-solvent system membranes with different mass ratios of good solvent and poor solvent were prepared by screening suitable poor solvent and preparing thermally stable solution.The structure of the PLA co-solvent system membranes was greatly different,which was observed by a scanning electron microscopy.And the polymer aggregate morphology changed under different mixing ratios.Differential scanning calorimetry and X-ray diffraction studies showed that under the high mass proportion of poor solvents,the crystallinity of PLA membrane was significantly improved.By adjusting the composition of the solution system,the membrane with high crystallinity and ordered porous structures could be prepared.