The Effect of System Parameters on the Pre‐Transition Swelling of Charged Hydrogels

Abstract

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.

Keywords:

• Hydrogel
• PMAA
• pH
• Swelling

Acknowledgments

This work was supported by the Commonwealth of Pennsylvania's Ben Franklin Technology Development Authority through the Ben Franklin Technology Partners of Southeastern Pennsylvania and the Nanotechnology Institute. K.P. and D.Q. acknowledge the Drexel University College of Engineering Summer Mentorship Program, funded by Drexel University.