Porous poly(vinylidene fluoride) membranes were graft modified with acrylic acid by radiation-induced grafting utilizing electron beam. When the grafted membranes were characterized by the measurements of convective permeability, diffusion, and ion exchange properties, the membranes showed properties of variable permeability membranes with respect to the permeability of solvent and the ionic semipermeability. The permeability for the studied membranes (extent of grafting varied from 0 to 93 wt %) can be controlled by the ionic strength and pH of the permeate. The more grafted the membranes are the greater are the changes in the permeability; e.g., for the extent of grafting of 85 wt % the permeability increases ca. 4 orders of magnitude when the electrolyte concentration is increased from 10 mM to 1 M. The membranes with the extent of grafting of ca. 10 wt % or more appeared to have equal charge density on the pore walls resulting in the same cation transference number.

A simple theoretical model describing the effects of pH and salt concentration on the permeability and counterion transport number of variable permeability membranes has been presented and validated experimentally for the case of poly(vinylidene fluoride) membranes graft modified with poly(acrylic acid) chains by radiation-induced grafting. The model incorporates explicitly the statistical conformations of a polyacid chain grafted onto the pore surface. The electrostatic interactions between the bound charges in the chains are screened according to the Debye−Hückel theory. The charged capillary model for porous membranes is then used to evaluate the permeability and counterion transport number of the membrane. This theoretical approach is able to describe the experimental trends observed for a range of KCl concentrations and pH values when the grafting ratios are low. In particular, the fact that the membrane permeability changes by several orders of magnitude when the properties of the external solution are varied can be rationalized in terms of very simple physical principles.

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