Shrinking of Chemically Cross-Linked Polymer Networks in the Postgel Region

In this paper, the syneresis behaviors in the post-gel region of hydrogels of hydroxyethylcellulose (HEC) and its hydrophobically modified analogue (HM-HEC) were investigated by means of a new high precision swell-ratio-tester. The gels were prepared by cross-linking cellulose ether derivatives with divinyl sulfone (DVS) in alkaline solution at various cross-linker concentrations and temperatures. Increasing the cross-linker density promotes a faster shrinkage of the gel, and a more compressed gel. The compression of the 1 wt % HEC gel starts at an earlier time at 40 °C than at 25 °C, because the increased mobility of the chains is more favorable to a faster formation of interpolymer cross-links. The results from the deswelling measurements show that the hydrophobic modification of the polymer yields less contracted gels. This novel finding is ascribed to the fact that some of the hydroxyl groups for the formation of intermolecular cross-links are deactivated through the incorporated hydrophobic groups at these sites.