The effect of system parameters on the pre-transition swelling of charged hydrogels

Although studies of charged hydrogel swelling focus on the swelling transition, hydrogels display pre-transition swelling which may affect their performance in applications such as sensors. In this paper we investigate the effect of system parameters on the pre-transition swelling of charged poly methacrylic acid (PMAA) hydrogels. We find that the pre-transition swelling can be quite significant in magnitude (in our case ∼40% of the maximal swelling). Increasing the sub-chain molecular weight increases swelling in the pre-transition region only in the case where no salt is added; Contrary to expectation, in moderate ionic strength solutions the degree of swelling of longer chains is lower than that of shorter sub-chains. Also contrary to expectation, in the case of longer sub-chains the addition of salt decreases the gel volume in the pre-transition region, thereby indicating that in this system a significant number of charges are dissociated. These trends can be understood by accounting for the combined effect of pH and the ionic strength on the degree of charge dissociation.