Investigation into the rheological properties of PES/NMP/nonsolvent membrane-forming systems

From the perspective of a polymer solution, the rheological properties of t he popular polyethersulfone (PES)/N-methyl-2-pyrrolidone (NMP)/nonsolvent ( NS) membrane-forming system were investigated thoroughly with a controlled stress rheometer (HAAKE RS75, Germany). The scope of the study included mea surements of the controlled-stress flow curve, creep recovery, and dynamic oscillation. H2O, 1-butanol, ethylene glycol, and diethylene glycol were us ed as NS additives. The effects of the polymer concentration and the qualit y of the solvent mixture, as characterized by the approaching ratio, on the rheological behavior of the dopes were studied. Up to 38 wt % PES and extr emely adjacent to the phase separation (i.e., the approaching ratio of the dope was 0.95), the viscous property dominated all the dopes, which behaved as Newtonian fluids. Moreover, all the membrane-forming dopes investigated were in the crossover regime in the semidilute region, in which the chains were overlapping but unentangled. (C) 2001 John Wiley & Sons, Inc.