SURFACE-MODIFICATION OF ULTRAFILTRATION MEMBRANES BY LOW-TEMPERATURE PLASMA .2. GRAFT-POLYMERIZATION ONTO POLYACRYLONITRILE AND POLYSULFONE

Low temperature plasma-induced surface modifications of polyacrylonitr ile (PAN) and polysulfone (PSf) ultrafiltration (UF) membranes were st udied. Treatment with water plasma and with He plasma drastically and almost permanently increased the surface hydrophilicity of PSf UF memb ranes. However, in contrast to the behavior of PAN UF membranes [23], the PSf surface pore structure was also changed as indicated by altere d water permeabilities and reduced protein retentions. The lower perme ability PSf membranes (nominal M(w), cut-off 10 kD) showed slower but more extended conversion due to plasma excitation and stronger indicat ions of pore etching effects in comparison with 30 kD cut-off membrane s. Polymer peroxides on PAN and PSf membranes created by plasma excita tion were monitored by the 2,2-diphenyl 1-picryl hydrazyl (DPPH) assay . Graft polymerization of hydrophilic monomers such as 2-hydroxy-ethyl methacrylate (HEMA) and acrylic or methacrylic acid onto PAN and PSf UF membrane surfaces was initiated via thermal decomposition of peroxi des. The degree of modification could be adjusted by polymerization co nditions. Graft polymer modified surfaces were characterized with the help of Fourier transform infrared attenuated total reflection (FTIR-A TR) and electron spectroscopy for chemical analysis (ESCA) spectra. Th e hydrophilic character of the modified surfaces was increased as comp ared to that of the parent membranes. With about 1-1.4 mmol/cm(2) graf ted HEMA, the contact angles (captive bubble technique; Theta(octane/w ater)) for PAN and PSf were reduced from 48 to 34 degrees and from 92 to 43 degrees, respectively. A clear dependency of PAN UF membrane wat er permeability on the amount of grafted monomer was observed. The mon omer type influenced the water permeation flux per mole of grafted acr ylate via specific swelling of the graft polymer layer in water. Hydro philic PAN membranes, modified either by plasma treatment [23] or HEMA graft polymerization, showed significantly reduced fouling due to sta tic protein adsorption, and improved protein UF performance. In partic ular, for water plasma treated PAN membranes with high initial retenti on, higher fluxes (up to 150%) with the same or even improved retentio ns were obtained. Hydrophilized PSf-g-HEMA membranes can provide impro ved performance in protein ultrafiltration over unmodified PSf UF memb ranes because pore etching effects are compensated for by the grafted layer yielding both improved filtrate flux (> 30%) and protein retenti on of bovine serum albumin. Hence, plasma induced graft polymer modifi cation of UF membranes can be used to adjust membrane performance by s imultaneously controlling the surface hydrophilicity and permeability.