Insight into the preparation of poly(vinylidene fluoride) membranes by vapor-induced phase separation

The present investigation revealed how the morphology and crystalline forms of a poly(vinylidene fluoride) (PVDF) membrane, prepared by using the vapor-induced phase separation (VIPS) method, were affected by the temperature at which PVDF was dissolved to form the casting solution. The results showed that there existed an important transition dissolution temperature, being referred to as the “critical dissolution temperature”, across which the morphology and crystalline forms of membranes drastically changed. With a dissolution temperature above the critical dissolution temperature, the prepared membranes were composed of nodules and the size of nodules decreased as the dissolution temperature decreased. And, with a dissolution temperature below the critical, membranes with lacy (bi-continuous) structure were obtained. In addition, the α-crystalline form of PVDF grew faster than the β-form when the dissolution temperature was below the critical, and the β-form became faster and dominant as the dissolution temperature increased to be above the critical. The existence of the critical dissolution temperature was observed for all the three solvents used in the present study, N-methyl-2-pyrrolidone (NMP), dimethylacetamide (DMAc), and N,M-dimethylformamide (DMF), indicating that the phenomena are general and not limited to a specific solvent. Also, it was observed that the two PVDF/NMP solutions, prepared with dissolution temperatures above and below the critical temperature, responded in different ways when water was added into the solutions. Though both solutions gelled, the solution with higher dissolution temperature started the gelation with a crystallization-initiation gelling process, while the other with a non-crystallization-initiation gelling. We propose that the competition between the two gelling processes play an important role in determining the PVDF membrane morphology and crystalline polymorphs, and the dissolution temperature can change the competition. And the critical dissolution temperature can be interpreted as a dissolution temperature across which the dominant gelling process switched.