Chemically and morphologically defined ultrafiltration membrane surfaces prepared by heterogeneous photo-initiated graft polymerization

Abstract

Polyacrylonitrile (PAN) ultrafiltration (UF) membranes with varied surface hydrophilicity and charge have been prepared by heterogeneous photo-initiated graft polymerization of various acrylates or methacrylates having polyethyleneglycol (PEG), carboxyl, sulfopropyl, dimethylaminoethyl or trimethylammoniumethyl side groups. A series of photo-modified membranes with about the same water permeabilities were obtained by selection of initial membrane pore size and appropriate modification conditions (monomer concentration and UV irradiation time, λ exc>300 nm). A comprehensive characterization of the new membranes was achieved using a multi-method approach. Gravimetry, FTIR-ATR spectroscopy, SEM and UF experiments provided basic information about the degree of graft polymer modification (DG, up to 1 mg cm −2) and integrity of the macroporous membrane structure after modification. No extra layer (with a thickness >1 μm) was deposited on top of the UF membranes but, depending on DG, active layer pores were more or less covered, causing reduced water permeabilities and enhanced solute selectivities. The modified membranes were monitored with scanning force microscopy (SFM), contact angle (CA) and streaming potential (ZP) measurements. The water-swollen graft polymer layers had a monomer-specific texture with larger grain size, but in the sub-micrometer range, compared with the unmodified membranes (SFM). However, interface interactions in terms of zeta potential (ZP) and hydrophilicity (CA) were unambiguously governed by the chemistry of the graft polymer side groups. Solid surface tensions (wetting/dewetting) from advancing and receding CAs, γ sv (adv.)/ γ sv (rec.), ranged from 35 mN m −1/60 mN m −1 for PAN-grafted-poly(dimethyl-aminoethyl methacrylate) to 71.5 mN m −1/71.5 mN m −1 for PAN-grafted-poly(sulfopropyl acrylate) and PAN-grafted-poly(tri-methylammoniumethyl methacrylate), whereas unmodified PAN membranes had 49 mN m −1/67 mN m −1. Concentration potential measurements revealed that ion-exchange graft-polymer-modified PAN membranes behave as barrier for co-ions via a Donnan exclusion mechanism. All analytical data enabled a classification of the photo-modified UF membranes into permanently anionic (sulfonic acid) or cationic (trimethylammonium) as well as ionizable (carboxyl, dimethylamino) or non-ionic hydrophilic, flexible (PEG) types. The results provide guide-lines for the development of new low-fouling UF and nanofiltration membranes.