Surface characterization of poly(acrylonitrile) membranes graft-polymerized with ionic monomers as revealed by ζ potential measurement

Anionic and cationic monomers were graft-polymerized onto poly(acrylonitrile) (PAN) membrane surfaces. The graft polymerization was initiated by an oxidation-reduction system at 25 degrees C. Grafted PAN membrane surfaces yielded FT-IR/ATR spectra clearly different from that of an unmodified PAN membrane, because of the presence of the graft chains. zeta potentials of the pore surfaces of the grafted PAN membranes were obtained from streaming potential measurements. The surface properties are discussed with respect to a single site dissociation model, which is based on the protonation and deprotonation of charged surface groups. In this study, we obtained some characteristic parameters of the pore surfaces from the theoretical fits using the site dissociation model. This approach gives direct insight into the pore surface properties of the membrane. The results could be well explained by an ion-pairing effect in terms of different charge characteristics between membranes having hydrophobic or hydrophilic graft chains: charge groups of hydrophobic graft chains are likely to form ion pairs with counterions from the external solution. The local dielectric constant surrounding ion pairs plays an important role in the surface effective charge concentration.