Porous polypropylene membranes with different carboxyl polymer brush layers for reversible protein binding via surface-initiated graft copolymerization

Photoinitiated surface-selective graft copolymerization onto polypropylene (PP) microfiltration membranes was performed using two different methods for coating the photoinitiator, benzophenone (BP), on the membrane surface. An already established adsorption method and a novel method based on preswelling of the PP in heptane, subsequent solvent exchange, and thus entrapping of the BP in the surface layer of the PP had been evaluated. With acrylic acid (AA) as the monomer, functional polymer brush structures on the entire membrane pore surface were obtained. Further variations of the grafted layer had been achieved by copolymerization of AA with acryl amide (AAm) and methylene bisacrylamide (MBAA). Characterization had been done mainly by detailed measurements of membrane permeability including pH dependency as well as the reversible binding of a protein (lysozyme, Lys) under membrane chromatography conditions. Compared with BP adsorption, the BP entrapping method yielded a less dense grafted layer with longer PAA chains at the same degree of functionalization (DG). This was due to somewhat lower immobilized BP amounts, but also less side reactions via nonselective photoinitiated cross-linking by dissolved BP. Unexpected properties of the PAA-co-AAm brush layers were their even larger swelling/deswelling as a function of a pH change (above and below the pK(a) of PAA) as compared with the PAA brushes. Both PAA-co-AAm and cross-linked PAA-co-MBAA layers showed Lys binding capacities-more than 10 times higher than monolayer adsorption onto the unmodified PP membrane surface-and quantitative recoveries similar to PAA of the same DG; and the highest efficiencies of protein binding (Lys amount relative to amount graft copolymer) were achieved with the membranes prepared by the entrapping method. In general, the more controlled BP entrapping method had distinct advantages in terms of the control of grafted layer structure leading to an improved membrane adsorber performance.