Evidence for Polyelectrolyte/Ionomer Behavior in the Collapse of Polycationic Gels

The interaction of slightly cross-linked positively charged gels of poly(diallyldimethylammonium chloride) (PDADMACl) with sodium salts of chloride, bromide, iodide, and acetate in aqueous media has been investigated. At the critical concentration of sodium iodide, a phase transition of the network to the collapsed state was observed. A wide hysteresis accompanied this transition. The presence of other salts did not initiate the network collapse in the investigated concentration range. A decrease in the charge density of the cationic network by copolymerization of DADMACl with acrylamide resulted in the disappearance of the phase transition. In this case, the usual contraction of polyelectrolyte gels in the presence of the mentioned salts was observed. The rate of volume change increased sharply with increasing concentration of sodium iodide. For some cases, two well distinct regions in the kinetic behavior were observed: a region of fast polyelectrolyte contraction and a region of slow collapse which, to our knowledge, had not been observed previously. The experimental results can be explained by the formation of ion pairs and multiplets (ionomer effect) for the sufficiently charged networks in the collapsed state. As a result, the collapsed ionomeric state of the network becomes competitive with the swollen polyelectrolyte state at high enough fractions of the ion-containing segments because of the tendency of the ion pairs to associate. A potential barrier which separates the ionomeric and the polyelectrolyte states of the gel can be used to explain the two-step kinetics of the collapse