Surface Nanostructuring of Polysulfone Membranes by Atmospheric Pressure Plasma-Induced Graft Polymerization (APPIGP)
Surface nano-structuring of polysulfone (PSf) membrane surrogate surfaces was accomplished by grafting hydrophilic polymers, poly(methacrylic acid) (PMAA) and poly(acrylic acid) (PAA), using the atmospheric pressure plasma-induced graft polymerization (APPIGP) approach. Atmospheric pressure (AP) plasma was used to activate the polysulfone substrate surface, and subsequent graft polymerization of hydrophilic monomers was carried out with an aqueous monomer solution. Optimization of generating the surface initiation sites was guided by water contact angle (WCA) measurements of the PSf surface after plasma treatment at different gas compositions, radio frequency (RF) power, and the treatment time. Optimal surface activation of the PSf substrate surface was achieved for AP helium plasma of RF power of 50 W and treatment time of 15 s. Graft polymerization was carried out at initial monomer concentration in the range of 5 - 20 vol% and reaction time of 0.5 - 2 h at 60°C and 70°C for grafting PMAA and PAA, respectively. Compared to the PSf surrogate surface (WCA: approx. 97 - 100º), the surface hydrophilicity of the grafted surfaces was improved for all grafting conditions, with the water contact angles of the PMAA- and PAA-grafted polysulfone surfaces decreasing by 28.8% and 34.0%, respectively, both at the lowest initial monomer concentration of 5 vol% and the longest graft polymerization time of 2 hours. Surface topography of the grafted surfaces was evaluated by atomic force microscopy (AFM). Both the surface feature number density and root-mean-square surface roughness (Rrms) increased with initial monomer concentration, from 5 - 20 vol%, suggesting that a higher density of surface grafted chains with a wider chain size distribution was achieved with increased initial monomer concentration. All polymer grafted polysulfone surfaces were significantly more hydrophilic than the original polysulfone surface, and high grafting densities were observed. However, relative to lower initial monomer concentration (5 vol%), increased roughness of the surface at high initial monomer concentration (20 vol%) resulted in somewhat reduced surface hydrophilicities quantified by 15.9% and 18.2% contact angle decrease for PMAA- and PAA-grafted PSf, respectively, after 2 h of graft polymerization. Results of the present study are encouraging in indicating that the APPIGP methodology could be beneficial for surface modification of commercial polysulfone membranes for improving the surface hydrophilicity and hence fouling mitigation.