Rheological Properties of Hydrogels of Agar-agar. III. Stress Relaxation of Agarose Gels

Abstract A stress relaxation experiment on hydrogels of agarose isolated from agar-agar was made by means of a chainomatic balance relaxometer for the purpose of investigating the mechanism of gelation. The measurements were made on the agarose gels of the concentration range from 1.2 to 2.0 wt%, and also on agar-agar gels for the comparison, at various temperatures from 25 to 55°C. The relaxation curves obtained were represented approximately by a mechanical model consisting of three Maxwell models in parallel. The apparent activation energy, ΔH, which was calculated from the temperature dependence of the longest relaxation time, τ1 was about 5 kcal/mol for all gels treated in this study, which was nearly the same magnitude as that for agar-agar gels. It was observed in the curves obtained that the relaxation curves for agarose gels had quite similar characteristics to those for agar-agar gels of the same concentration and was observed that the total instantaneous modulus of the former gels amounted to about 80% of that of the latter gels. From these results it may be concluded that agarose is a main constituent in the network structure in agar-agar gels and that the hydrogen bonding plays a major role in the agar-agar gels as well as in the agarose gels.