Swelling and mechanical properties of cellulose hydrogels. III. Temperature effects on the swelling and compliance levels studied by dilatometry and 1H-NMR spectroscopy

The temperature dependence of the swelling and the creep compliance has been investigated for swollen isotropic cellulose hydrogels. The measurements were performed in a high precision type of dilatometer between 5 and 65°C. The thermal expansion of the gels in silicone oil (closed system) and the temperature dependence of the equilibrium swelling in water (open system) were studied. The influence of compressive stress in these experiments was also evaluated. The swelling level in equilibrium with water diminishes slightly with increasing temperature due to migration of water from the gel phase to the surrounding water phase. A secondary transition was found at 35°C where the temperature dependence of the swelling level is changed. When measured at constant gel composition the creep compliance of a highly swollen gel decreases with increasing temperature. The decrease is not, however, large enough for entropy elasticity to dominate over energetic elasticity. The energetic contribution f_U/f was determined to be 0.61 for a gel swollen to 3.9 g water/g dry gel (g/g) and 1.24 for a gel swollen to 1.05 g/g. The swelling and compliance data have also been analyzed in terms of a model where the gels are assumed to behave as a filler-reinforced rubbery network. The amorphous parts of the hydrogels are thus assumed to be described by the statistical theory for polymeric networks. In proton magnetic resonance studies of a gel swollen to 4.4 g/g the spin-lattice relaxation time T₁ was determined to be considerably longer than the spin-spin relaxation time T₂. T₂ has a maximum at 30°C. This maximum marks the onset (on the NMR time scale) of an exchange process between two types of proton species. These species are suggested to be specific hydration water and free gel water, respectively.