Characterization of novel weak amphoteric charged membranes using ζ-potential measurements:: Effect of dipolar ion structure

Weak amphoteric charged membranes were prepared by the radiation-induced graft copolymerization of the weakly charged poly(ethylene glycol) derivatives onto high-density polyethylene porous membranes. In this study, two kinds of amphoteric charged membranes, which are the amphoteric ion-pair side chain type and the mixed grafted chain type, were prepared and then characterized using the zeta -potential measurements. For the amphoteric ion-pair type membrane, the zeta -potential/pH profile showed a plateau in the range of pH 6-11 which was characteristic of the dipolar ion structure of an amphoteric ion-pair. On the other hand, the g-potential for the mixed grafted chain type membrane showed an intermediate pH dependence between the separate positively charged chain- and negatively charged chain-grafted membranes. The apparent surface charge density on these membranes was obtained from the zeta -potential. For the amphoteric ion-pair side chain type membrane, the experimental results could be explained by the theoretical model based on the dissociation of charge groups on the pore surface. For the mixed graft. chain type amphoteric charged membrane, however, the difference between the experimental result and the calculated one appears in the pH range of 4-9. This would be caused by the change in the surface states with pH, that is, a conformational change in the grafted polyelectrolyte chains with the change in the charging property. Finally, the unique charging property of the amphoteric ion-pair became apparent in this study.