Surface modification of polysulfone ultrafiltration membranes by free radical graft polymerization of acrylic acid using response surface methodology

Abstract

In this research, polysulfone (PSf) ultrafiltration (UF) membranes were prepared by a phase inversion method. Surface modification of the PSf membranes was carried out via grafting of acrylic acid as a hydrophilic monomer by free radical graft polymerization initiated by redox reaction. A central composite design (CCD) of response surface methodology (RSM) was applied to design the experiments. The process variables were acrylic acid concentration (CAA), redox system contact time (T1), and acrylic acid polymerization time (T2), while the contact angle (CA), pure water flux (PWF), and flux recovery ratio (FRR) were considered as the responses. Analysis of variance (ANOVA) demonstrated that the second-order polynomial models were appropriate and significant. The optimal conditions for variables was found to be CAA of 1.6 mol/L, T1 of 41.9 min, and T2 of 101.7 min. The predicted responses were 52.9° for CA, 149 L/m2 h for PWF, and 74.5% for FRR, which were in good agreement with the results obtained from the confirmation experiments, with values of about 53.2°, 145.3 L/m2 h, and 74.3%, respectively. A comparison of the results between the optimized and unmodified membranes indicated that the CA of the modified membrane was reduced significantly, about 30.5%, and FRR was increased considerably, about 32.2%, indicating that the modified membranes possessed enhanced fouling resistance, while the PWF declined 40%. Membrane characterization also revealed that acrylic acid was grafted uniformly on the membrane surface, and the modified membranes became smoother, as observed by SEM and AFM. Moreover, the pore size of the modified membranes as measured by MWCO was smaller than that of the unmodified membrane.