| Summary: | 碩士 === 淡江大學 === 化學工程與材料工程學系碩士班 === 102 === The purpose of this study is to investigate the effect of bath temperature and casting thickness on the preparation of membranes from EVOH/1,3-propanediol binary mixture system via the cold solvent induced phase separation (CIPS) process. When the temperature of the precipitation bath (45℃) was below the crystallization line and above the liquid - liquid phase separation line, the membrane exhibited a crystalline particulate structure; when the bath temperature (25℃) was just below the spinodal decomposition temperature, it presented a bi-continuous network structure and bore slight crystallization characteristics of particles; when the bath temperature (5℃&-20℃) sit deeply in the spinodal zone, it presented a pure bi-continuous network structure. With different casting thickness and immersed in 5℃bath, the formed membranes all presented bi-continuous network structure typically arising from spinodal decomposition. The water permeation flux, wettability and tensile strength of the membranes were measured and the results indicated that they were correlated with the porosity, pore size, and membrane morphology. In addition, X-ray diffraction (XRD) and differential scanning calorimetry (DSC) analyses indicated that the membranes had crystallinity of 38~42 %. The DSC data also showed that all membranes had a similar crystal melting behavior with Tm close to 184℃.
We then chose the poly (vinylidene fluoride)/triethyl phosphate/water ternary system and used a method that combined features of non-solvent induced phase separation (NIPS) and thermal induction phase separation (TIPS) to prepare porous PVDF membranes. In the phase diagram we found on a special dope composition such that the solvent/non-solvent ratio in the dope was equal to that in the precipitation. Therefore, the composition of the bath could be held constant by itself, and the bath can be used repeatedly (termed reusable bath). By varying the composition of the precipitation bath and casting thickness, a series of membranes with different porous structures were formed. The effects of the membrane structure and filtration performance have been studied subsequently. Results showed that when water, 30wt% TEP and 60wt% TEP were used as the precipitation bath, the membrane presented a skin layer at the top surface. In contrast, when precipitation in the reusable bath, it did not show a skin layer near the top, and with increasing of the TEP concentration in the bath, thinner skin layer tended to form. The thickness of the skin layer was found to affect the porosity and mechanical strength of the membranes; the thinner the skin, the higher the porosity, and thus the lower the mechanical strength. When films with different thickness were immersed in the reusable bath, we found from the FE-SEM images that thinner casting thickness tended to yield membranes that bore lacy structure. In addition, DSC measurements showed that the melting points of the membranes were ~159℃, while XRD analyses indicated that the crystallinity of membranes was about 63 to 64%. The water flux and filtration experiments showed that the membranes prepared from the reusable bath exhibited both high permeability and selectivity.
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