Thermoreversible Hydrogel of Short-Chain <i>O</i>-(2,3-Dihydroxypropyl)cellulose/Borax Aqueous Solution. Microscopic versus Macroscopic Properties

Abstract

The thermoreversible hydrogel of short-chain (unentangled) O-(2,3-dihydroxypropyl)cellulose/Borax aqueous solution was studied by 11B NMR and dynamic viscoelastic measurements. The total number XT of transient cross-links formed by the (diol/ion) 2/1 complexation could be quantitatively estimated by 11B NMR. The dynamic viscoelastic data gave the number NE of elastically effective chains and the relaxation time of the gel. The NE vs XT data were compared to the mean-field theory of transient gels. Agreement between the theory and experiment was very poor at low C but became better as C increased. It was suggested that the ratio of the experimental NE to the theoretical NE is a function only of polymer concentration C. The gel point estimated by the zero extrapolation of the experimental NE vs XT data with a fixed C reasonably agreed with the mean-field prediction only at high C. The relaxation time τ (and hence the viscosity η) of the gel strongly depended on the pH of the system but not explicitly on C or on XT (or NE). For example, the value of τ observed for a solution at pH = 13.5 was more than 100 times as large as that observed for a similar solution at pH = 8.4, whereas the plateau moduli of the two solutions were similar in order of magnitude. It was suggested that τ in this system can be simply identified with the lifetime τ0 of the transient cross-links.