Swelling Behaviors of pH- and Salt-Responsive Cellulose-Based Hydrogels

Ampholytic hydrogels with pH and salt responsive properties have been synthesized by cross-linking quaternized cellulose (QC) and carboxymethyl cellulose (CMC) with epichlorohydrin (ECH) in NaOH aqueous solution. The swelling behaviors of the QC/CMC hydrogels were studied as a function of the polymer composition, pH, and salt concentration. The equilibrium swelling ratio of the hydrogel in ultrapure water strongly depended on the composition, and increased dramatically from 8.6 to 498 g/g with the change of the weight ratio of QC to CMC from 3:1 to 1:3 (w/w). The hydrogel (Gel32) consisted of QC and CMC in the ratio of 3:2 (w/w) had the minimum swelling ratio, and it was electrically neutral. The results from ζ-potential experiments were in good agreement with the theoretically calculated stoichiometry for balanced charge, confirming that the weight ratio of QC and CMC was 1.5, corresponding to Gel32. Furthermore, all hydrogels exhibited excellent pH sensitivity in the range of pH from 1 to 13 and shrunk significantly at pH 12 on the whole. The hydrogels displayed smart swelling behaviors in NaCl, CaCl2, and FeCl3 aqueous solutions. The results revealed that CMC mainly contributed to increasing the swelling as a result of strong water adsorption, whereas QC played a role in the controlling of the charges in the QC/CMC system, leading to the pH sensitivity.