Rheological investigations of linear and hyperbranched polyethersulfone towards their as-spun phase inversion membranes' differences

We have investigated the macromolecular structure and rheological behavior of both linear and hyperbranched polyethersulfone (PES) materials. It was found that the hyperbranched PES material has a higher molecular weight and a wider molecular weight distribution than its linear analogue. Rheological studies disclose that polymer solutions made from the HPES/polyvinylpyrrolidone (PVP)/N-methyl-2-pyrrolidone (NMP) ternary system have a longer relaxation time than their linear counterparts. The less relaxation characteristics of the HPES dope not only result in a more pronounced die swelling during hollow fiber spinning, but also produce hollow fiber membranes with smaller pore sizes, narrower pore size distribution, and a smaller molecular weight cut-off (MWCO). In addition, elongational viscosity characterizations indicate that HPES possesses a more strain hardening effect than LPES. As a result, films made from the former tends to break easier and quicker under high extensional strains than those made from the latter.