Structural transition of asymmetric poly(ether imide) membrane prepared by wet phase inversion

Abstract

The structure of membrane prepared by the phase inversion process shows a strong dependency on the casting thickness as well as composition of cast polymer solution. At the present work, these two factors are probed for poly(etherimide) (PEI) membranes prepared via the NIPS process. Three systematic variations in solvent composition from N-methyl-2-pyrrolidone (NMP) and dimethylformamide (DMF) were applied; including pure NMP, 50NMP:50DMF, and 25NMP:75DMF. The critical thickness was determined by means of solution casting at a broad range of thickness. The scanning electron micrograph of cross-section membranes reveals that critical thickness of PEI membrane is taken place at thicknesses of 9.7, 19.2 and 22.5 µm when the ratios of 0:100, 50:50, and 25:75 for NMP:DMF are employed, respectively. Shrinkage values which are calculated via a novel method can subscribe the effect of casting thickness and also quality of solvent on final membrane structure. The shrinkage values are altered exponentially by increasing the cast solution thickness in pure NMP solvent system. Gradual addition of DMF to the solvent system takes the later trend to linear form. It has been argued that increase in DMF content of solvent system leads to a greater demixing gap in the ternary phase diagram; it can also result in a higher solvent/polymer interaction parameter.