Thermoreversible hydrogel of short-chain O-(2,3-dihydroxypropyl)cellulose/borax aqueous solution. Microscopic versus macroscopic properties

The thermoreversible hydrogel of short-chain (unentangled) O-(2,3-dihydroxypropyl)cellulose/Borax aqueous solution was studied by (11)B NMR and dynamic viscoelastic measurements. The total number X(T) Of transient cross-links formed by the (diol/ion) 2/1 complexation could be quantitatively estimated by (11)B NMR. The dynamic viscoelastic data gave the number N(E) of elastically effective chains and the relaxation time of the gel. The N(E) vs X(T) 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 N(E) to the theoretical N(E) is a function only of polymer concentration C. The gel point estimated by the zero extrapolation of the experimental N(E) vs X(T) data with a fixed C reasonably agreed with the mean-field prediction only at high C. The relaxation time a (and hence the viscosity eta) of the gel strongly depended on the pH of the system but not explicitly an C or on X(T) (or N(E)). For example, the value of tau 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 tau in this system can be simply identified with the lifetime tau(0) of the transient cross-links.