Preparation and characterization of membrane adsorbers with tailored ion-exchange polymer brushes
Preparation of novel separation membranes can be done very efficiently with controlled surface functionalization. Photo-initiated surface-selective graft copolymerization was performed using a recently developed entrapping method for the photo-initiator benzophenone (BP), and weak cation-exchange polymer brush structures on polypropylene membrane pore surfaces were obtained using acrylic acid (AA) as functional monomer. Effect of entrapping time, photo-initiator concentration, monomer concentration and UV irradiation time on membrane degree of grafting was investigated for optimization. The optimized method was obtained with 1 wt% BP, 60 min entrapping time, 15 min UV irradiation time. Copolymerization of AA with “diluent” monomer acrylamide (AAm) and “cross-linker” monomer methylene bisacrylamide (MBAA) were done with optimized method for variations of the grafted layer. Membrane morphology and pore distribution was investigated using Scanning Electron Microscopy (SEM) and permporometry analyses. Graft copolymer composition analysis had been performed using FTIR-ATR spectroscopy. Performance characterizations had been done by measurements of membrane permeability at low and high pH as well as at different salt concentrations, by reversible binding of model proteins (Lysozyme (Lys), Bovine serum albumin (BSA) and Bovine immunoglobulin (IgG)), by inadvertent pH transient under membrane chromatography conditions, by breakthrough curves for system dispersion analysis and by preliminary separation of a model protein mixture (lysozyme-cytochrome c). The SEM and permporometry show modification not significantly change the membrane morphology. The FTIR-ATR spectroscopy, permeability and inadvertent pH transient show graft copolymer are successfully grafted on the pore surface. Reversible binding of model protein, breakthrough curve and protein separation measurements reveal the graft copolymer structures have dominant influence on membrane adsorber performance. The most important result of this study is that chemical cross-linking within grafted layers with about the same amount of functional groups than those from linear grafted polymer leads to a significant improvement of porous adsorber performance because the protein separation factor and resolution is higher, the dynamic protein binding capacity can be increased, the membrane permeability is significantly increased and it’s sensitivity to changes in eluent pH and salt concentration is much decreased, and consequently the solute dispersion within the membrane is reduced as indicated by significantly sharper breakthrough curves.