The preparation of membranes of nylon 4,6 by diffusion-induced phase separation (DIPS) using formic acid as a solvent and water as a nonsolvent was studied. Nylon 4,6 is a semi-crystalline polymer; phase separation from a solution can occur by solid-liquid (s-l) de-mixing as well as by liquid-liquid (l-l) demixing. Upon quenching films of solutions with low polymer concentration (< 17 wt %) in a nonsolvent bath containing water, the morphology of the membranes show a foam-like structure typical for l-l demixing. When phase separation is induced by water vapor a transition in structure occurs from the cellular type to a morphology typical for s-l phase separated films. At higher polymer concentrations membranes exhibit structures consisting of spheres or smaller crystal-like units resulting from an s-l phase separation process. The addition of 2 wt % or more of water to polymer solutions with low concentration (up to 15 wt %) resulted in s-l demixing as well. In a DIPS process s-l demixing is kinetically competitive with l-l demixing if nuclei are already present in the starting solutions (heterogeneous nucleation), or if a relatively long time is available for crystal nuclei to be formed. The morphology resulting from s-l demixing is a result of spherulitic crystallization. A certain concentration of nuclei or of precursor particles already present results in a small nucleation density during precipitation and thus large spherulites can be grown; at higher polymer and/or water concentrations the nucleation density increases resulting in an axialitic morphology of the membranes.
Bulte, A. M. W., Bulte, A. M. W., Folkers, A., Mulder, M. H. V., Smolders, C. A., & Smolders, C. A. (1993). Membranes of semicrystalline aliphatic polyamide nylon 4,6: formation by diffusion-induced phase separation. Journal of applied polymer science, 50(1), 13-26. https://doi.org/10.1002/app.1993.070500103