Influence of Polymer Dissolution Temperature On the Morphology and Polymorphism of Poly(vinylidene fluoride) Membranes

We studied in the present work how the morphology and polymorphism of poly(vinylidene fluoride) (PVDF) membranes, prepared by using nonsolvent-induced phase separation, were affected by the temperature at which PVDF was dissolved to form casting solution. The results showed that there existed an important dissolution temperature (T_dis), referred to as the “critical dissolution temperature, T_cri”, across which the morphology and polymorphism of membranes drastically changed. With a T_dis higher than T_cri, the prepared membranes were composed of nodules and the size of polymer domain (nodules) decreased as T_dis decreased. With a T_dis lower than T_cri, membranes with lacy (bi-continuous) structure were obtained. As T_dis crossed T_cri, we also observed a dramatic change in the dominant polymorphism of the PVDF crystals contained in the membranes. We found that T_cri could be interpreted as a transition dissolution temperature, above which PVDF crystalline domains could grow freely while under which the domain growth was restrained. The existence of T_cri was observed for all four solvents used in the present study, N-methyl-2-pyrrolidone (NMP), dimethylacetamide (DMAc), N,M-dimethylformamide (DMF) and triethyl phosphate (TEP), indicating that the phenomenon was general and not limited to a specific solvent.