Study on the mechanism of membrane formation from polysulfone/polyethersulfone/N-methyl-2-pyrrolidone/ water system by means of immersion precipitation

Abstract

To construct the phase diagram of polysulfone (PSf)/polyethersulfone (PES)/ N-methyl-2-pyrrolidone (NMP)/water, cloud point measurement was carried out by a titration method. Six interaction parameters were estimated and the calculated phase diagram fitted the cloud point measurement well. In addition to the usual polymer/liquid phase separation, polymer/polymer phase separation resulting PSf-rich phase and PES-rich phase was observed with the addition of small amount of nonsolvent. This polymer/polymer separation is due to the immiscibility between the PSf and PES and the miscible region in the quaternary phase diagram was narrowed with the addition of PES in PSf. The boundary separating these two modes of phase separation could be well described and predicted from the calculated phase diagrams using the estimated interaction parameters of the components. The membranes made of PSf/PES blend show the distinctive morphology that are developed horizontally - horizontal macrovoids and horizontal ladder-like structure, which is considered to be caused by the polymer/polymer phase separation due to the immiscibility of PES and PSf.