Tailoring of the morphology and polymorphism of poly(vinylidene) fluoride membranes by adjusting polymer dissolution temperature and adding hydrated salt
Date Issued
2010
Date
2010
Author(s)
Hsieh, Wen-Hsin
Abstract
Recent studies have shown that the poly(vinylidene) fluoride （PVDF） dissolution temperature used to prepare a casting solution can affect the nuclei density in the solution and thus plays an important role in determining the resulting membrane morphology. It has also been shown that adding hydrated salt in the PVDF casting solution dramatically affects the membrane crystalline polymorphism. In the present study, PVDF membranes were prepared by vapor-induced phase separation（VIPS）method and using triethyl phosphate （TEP） as the solvent. There are two main topics in the present work. One is about how the PVDF dissolution temperature affected the membrane morphology and polymorphism. To get insight into the effect of the dissolution temperature, falling-ball experiments were performed to study how the casting solution gelled and and Fourier-transform IR（FTIR）analysis on the solution was conducted to detect when ordered chain conformation was formed during the gelation. The other topic is abou the influence of adding hydrated salt （Ca(NO3)2‧4H2O） on the morphology and polymorphism of PVDF membranes.
The PVDF dissolution temperature dramatically influenced the morphology of the resulting membranes. A dissolution temperature of 60℃ resulted in membranes with bi-continuous structure, while 120℃ brought about nodular membranes. Both membranes mainly contained alpha crystalline form, which is characterized by TGTG’ conformation. For both cases, the PVDF/TEP solutions gelled after the addition of water in them. And the TGTG’ conformation was detected before the solution began to gel, indicating that the gelation was initiated by crystallization. We believe that the polymer-solvent interaction was the mechanism responsible for the formation of TGTG’ conformation and the bi-continuous （fibrillar） structure. Also, we propose that the dependence of the nuclei density in the solution on the dissolution temperature is a possible explanation for why the dissolution temperature can affect the membrane structure. The bi-continuous （fibrillar） gel structure could only be retained when the nuclei density was high enough, to result in enough connectivity among the crystalline domains in the fibrils that the fibrils would not break up when the gel was immersed in water.
A dramatic change in membrane morphology and polymorphism was observed with adding 2 g/L of hydrated salt in the PVDF/TEP solution. The addition of salt interfered the polymer-solvent interaction and inhibited the formation TGTG’ conformation. Therefore, we detected TTTT conformation prior to solution gelation. The resulting membrane contained nodules with beta crystalline form that is composed of all trans conformation.
The addition of hydrated salt not only increased the beta form content but also converted the membrane structure from bi-continuous to nodular. We thus can adjust the membrane morphology and the beta ratio by changing the concentration of the added salt. When the concentration was lower than 2 g/L, we observed competition between TGTG’ and TTTT conformations. We found 1 g/L was a critical salt concentration, higher than which the dominant membrane structure switched from bi-continuous to nodular and the dominant polymorph changed from alpha to beta. When the concentration was higher than 2 g/L, the TGTG’ conformation was almost fully inhibited and the ratio of beta-form reached 90%. With increasing salt concentration, TTTT conformation occurred earlier and the size of nodules became smaller.
Subjects
poly(vinylidene fluoride)
vapor-induced phase separation
nuclei density
hydrated salt
gelation
Type
thesis
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