Polyelectrolyte gel transitions: experimental aspects of charge inhomogeneity in the swelling and segmental attractions in the shrinking

This paper aims to provide a systematic discussion based on our experimental results both previously published and unpublished, to promote better understanding of volume-phase transitions in polyelectrolyte gels. Special attention was paid to the distribution of network charges as well as to the attractive interaction among polymer segments. From looking at how these effects appear in the swelling curves, an exploration of the nature of polyelectrolyte gel transitions was attempted. Two sorts of polyelectrolyte gels, temperature-responsive ionic gels based on N-isopropylacrylamide (NIPA) and cationic poly(ethyleneimine) (PEI) gels, were mainly employed with various modifications. The charge inhomogeneity within the gel phase was created by surfactant binding, immobilized enzyme reaction and physical entrapment of polyions. The attractive interactions holding the gel in a collapsed state were studied in comparison with phase separations of the corresponding linear polyelectrolyte. The main conclusions are summarized as follows: (i) The charge inhomogeneity exhibits a large influence on the volume transition in ionic gels. (ii) Hydrogen bonding and hydrophobic association, other than electrostatic attraction, can be considered to play an important role in the segmental association. (iii) Stably associated segments via one or more of these attractive interactions causes a large hysteresis in the swelling process, in which the repulsive interaction among the fixed charges on the network is dominant as shown in the Katchalsky's model. (iv) A distribution of "neutral but hydrophilic" moieties (e.g., ion pair or salt-linkage formed between the opposite charged groups) within the gel shows a marked effect on the temperature-induced volume collapse, the aspect of which is similar to that observed in the gels with a charge inhomogeneity.