Rheological Properties of Hydrogels of Agar-agar. II. Stress Relaxation of Concentrated Hydrogels

A stress-relaxation experiment on hydrogels of agar-agar has been made over the range of concentration of about 3—11% by weight. A chainomatic balance relaxometer was used for the measurement of the relaxation Young’s modulus, and the temperature was varied over the range of about 20—70°C. The obtained relaxation curves have been analyzed using a mechanical model composed of three parallel Maxwell models. The relaxation times obtained for the gels of different concentrations at the same temperature have been found to be independent of the concentration. The elastic Young’s modulus, which may be attributed to the main network structure in the gels, has been found to be proportional to the 1.8 power of concentration. From the temperature dependence of the longest relaxation time, the activation energy for the relaxation process attributed to the destruction of junctions in the main network structure has been determined to be about 5 kcal/mol, irrespective of the concentration, over the whole range; this is of the order of the activation energy for the hydrogen bonds.