Surface modification of ultrafiltration membranes by low temperature plasma. II. Graft polymerization onto polyacrylonitrile and polysulfone.
Low-temp. plasma-induced surface modifications of polyacrylonitrile (PAN) and polysulfone (PSf) ultrafiltration (UF) membranes were studied. Treatment with water plasma and with He plasma drastically and almost permanently increased the surface hydrophilicity of PSf UF membranes. However, in contrast to the behavior of PAN UF membranes, the PSf surface pore structure was also changed as indicated by altered water permeabilities and reduced protein retentions. The lower-permeability PSf membranes (nominal Mw cut-off 10 kD) showed slower but more extended conversion due to plasma excitation and stronger indications of pore-etching effects in comparison with 30 kD cut-off membranes. Polymer peroxides on PAN and PSf membranes created by plasma excitation were monitored by the 2,2-di-Ph 1-picryl hydrazyl (DPPH) assay. Graft polymn. of hydrophilic monomers such as 2-hydroxy-Et methacrylate (HEMA) and acrylic or methacrylic acid onto PAN and PSf UF membrane surfaces was initiated via thermal decompn. of peroxides. The degree of modification could be adjusted by polymn. conditions. Graft-polymer-modified surfaces were characterized with the help of Fourier transform IR attenuated total reflection (FTIR-ATR) and electron spectroscopy for chem. anal. (ESCA) spectra. The hydrophilic character of the modified surfaces was increased as compared to that of the parent membranes. With about 1-1.4 mmol/cm2 grafted HEMA, the contact angles (captive bubble technique; Qoctane/water) for PAN and PSf were reduced from 48 to 34 Deg and from 92 to 43 Deg, resp. A clear dependency of PAN UF membrane water permeability on the amt. of grafted monomer was obsd. The monomer type influenced the water permeation flux per mol of grafted acrylate via specific swelling of the graft polymer layer in water. Hydrophilic PAN membranes, modified either by plasma treatment or HEMA graft polymn., showed significantly reduced fouling due to static protein adsorption, and improved protein UF performance. In particular, for water-plasma-treated PAN membranes with high initial retention, higher fluxes (up to 150%) with the same or even improved retentions were obtained. Hydrophilized PSf-g-HEMA membranes can provide improved performance in protein ultrafiltration over unmodified PSf UF membranes because pore-etching effects are compensated for by the grafted layer, yielding both improved filtrate flux (>30%) and protein retention of bovine serum albumin. Hence, plasma-induced graft-polymer modification of UF membranes can be used to adjust membrane performance by simultaneously controlling the surface hydrophilicity and permeability.
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