MEMBRANES OF SEMICRYSTALLINE ALIPHATIC POLYAMIDE NYLON-4,6 - FORMATION BY DIFFUSION-INDUCED PHASE-SEPARATION

The preparation of membranes of nylon 4,6 by diffusion-induced phase s eparation (DIPS) using formic acid as a solvent and water as a nonsolv ent was studied. Nylon 4,6 is a semicrystalline polymer; phase separat ion from a solution can occur by solid-liquid (s-1) demixing as well a s by liquid-liquid (1-1) demixing. Upon quenching films of solutions w ith low polymer concentration (< 17 wt %) in a nonsolvent bath contain ing water, the morphology of the membranes show a foam-like structure typical for 1-1 demixing. When phase separation is induced by water va por a transition in structure occurs from the cellular type to a morph ology typical for s-l phase separated films. At higher polymer concent rations 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 con centration (up to 15 wt %) resulted in s-I demixing as well. In a DIPS process s-l demixing is kinetically competitive with 1-1 demixing if nuclei are already present in the starting solutions (heterogeneous nu cleation), or if a relatively long time is available for crystal nucle i 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 d ensity during precipitation and thus large spherulites can be grown; a t higher polymer and/or water concentrations the nucleation density in creases resulting in an axialitic morphology of the membranes. (C) 199 3 John Wiley & Sons, Inc.