Abstract
The solidification behavior of poly(vinylidene fluoride) (PVDF) solution during membrane preparation by thermally induced phase separation (TIPS) was investigated. Apparatus newly developed in our laboratory was used to quantitatively measure membrane stiffness during phase separation. In this apparatus, a cooling polymer solution, placed on a stage, is moved upwards and the surface of the polymer solution contacts a sphere attached to the tip of a needle. The displacement of a blade spring attached to the needle is then measured by a laser displacement sensor. Different phase separation modes, such as liquid-liquid (L-L) phase separation and solid-liquid (S-L) phase separation (polymer crystallization) were investigated. In the case of S-L phase separation, the stiffness of the solution surface began to increase significantly just before termination of crystallization. In contrast, L-L phase separation delayed solidification of the solution. This was because mutual contact of the spherulites was obstructed by droplets of polymer-lean phase formed during L-L phase separation. Thus, the solidification rate was slower for the L-L phase separation system than for the S-L phase separation system.
Highlights
Polymeric porous membranes are generally prepared by phase separation of polymer solutions [1].Phase separation can be induced by cooling or by the presence of nonsolvent
We developed a new apparatus for directly measuring membrane stiffness and quantitatively analyzed the solidification behavior of polymer solutions during the nonsolvent induced phase separation (NIPS) process [24]
Similar S-L phase separation was observed for the poly(vinylidene fluoride) (PVDF) (30 wt %)/diethyl phthalate (DEP) (70 wt %) system
Summary
Polymeric porous membranes are generally prepared by phase separation of polymer solutions [1]. Phase separation can be induced by cooling or by the presence of nonsolvent. The former is thermally induced phase separation (TIPS), while the latter is nonsolvent induced phase separation (NIPS). In membrane preparation by TIPS, a polymer is dissolved in a diluent at high temperature, and the homogeneous polymer solution is cooled to induce the phase separation. After the polymer is solidified by crystallization or glass transition, the diluent is extracted by solvent exchange and the extractant is usually evaporated to yield a microporous structure. There are two main types of phase separation for the crystalline polymers usually used in the TIPS process. The solution separates into a polymer-rich continuous phase and a polymer-lean droplet phase
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