Though nonsolvent-induced phase separation has been widely used for a long time to prepare commercial polymer membranes, it still remains a challenge to get a clear insight into the mechanism associated with membrane formation. The main focus of the presentation is on how the polymer chain entanglement in casting solution affects its viscoelasticity and the resulting phase separation mechanism and membrane morphology.
We observed that the phase-separation mechanism of a polymer solution strongly depended on the degree of chain entanglement in the solution. A solution with higher degree of chain entanglement was more stable in the metastable region between binodal and spinodal, thus having higher tendency not to phase separate in the meta-stable region but to phase separate in the unstable region. So the solution inclined to phase separate via the mechanism of spinodal decomposition, resulting in membranes with interconnected pores. On the other hand, as the solution had lower degree of chain entanglement, the solution tended to phase separate via the nucleation and growth mechanism, forming membranes with cellular pores. It was also observed that chain entanglement had influences on polymer crystallization for a casing solution of semi-crystalline polymer, such as PVDF. A semi-crystalline polymer solution with higher degree of chain entanglement had slower crystallization rate; therefore, liquid-liquid phase separation dominated during membrane formation. And polymer crystallization became the dominant mechanism with a casting solution having lower degree of chain entanglement.
The chain entanglement in polymer solution was observed to be strongly related to the polymer-solvent interaction. The results we obtained indicated that lower quality solvent made solution with higher degree of chain entanglement. And the addition of nonsolvent in polymer solution could also enhance chain entanglement. Besides, the effect of molecular weight of polymer on chain entanglement and membrane morphology has been investigated. The results we obtained indicated that polymer chain entanglement, characterized by the viscoelastic property of casting solution, plays an important role in determining its phase separation mechanism and the resulting membrane morphology. Therefore, knowledge of the rheology of polymer solution is needed to clearly understand the formation mechanism of polymeric membranes.