Relations entre la structure et les proprietes des membranes de polytetrafluoroethylene greffe—I. Membranes monogreffees carboxyliques ou basiques

We have determined the equilibrium properties (neutralization and swelling and the transport properties: conductivity and dialysis) of hydrophilic membranes obtained by radiation grafting of acrylic acid or 4-vinylpyridine onto thin PTFE films. The presence of strong counter-ion-membrane interaction (PTFE-PAA-K⁺ and PTFE-P4VP-ClO^?₄) has been confirmed in the beginning of neutralization. The grafting ratios of the samples ranged between a few percent and several hundreds percent. The properties have been studied in connection with the average degrees of ionization and the average molalities of the reactive groups throughout the whole thickness of the membrane. The parameters of the synthesis which determine the structure of acid or basic grafted chains also have an influence on the properties of the resulting membranes. For a high dose-rate (> 100 rad min^?1), the properties of carboxylic membranes are related to the degree of cross-linking: for smaller dose rates, the properties are related to the length of the grafted branches and/or to the state of the skeleton of PTFE. For the basic membranes, the properties are controlled by the length of the grafted branches and the importance of the micro-phase-separation between PTFE and the grafted chains; for low dose rates, when the grafted branches are long, separation of hydrophobic and ionizable zones is noticed for grafting ratios higher than 5%. The carboxylic membranes with lower degrees of grafting, prepared with a high dose rate, exhibit very good permselectivity. The pyridinic membranes with a low degree of grafting could be of practical interest, viz. the manufacture of selective electrodes for perchlorate ions.