Gelation Mechanism of κ- and ι-Carrageenan Investigated by Correlation between the Strain−Optical Coefficient and the Dynamic Shear Modulus

Abstract

The temperature and frequency dependence of the dynamic shear modulus, strain−optical coefficient, and the optical transmission of κ- and ι-carrageenan aqueous solutions were measured in order to clarify the gelation mechanism. The growing of the gel network and the effect of cation species (K, Cs, and Ca) and the amount of added KCl are discussed on the basis of the correlation between the shear modulus and strain−optical coefficient. For all samples, these correlations can be described by a power function in the low-temperature region in the gel state. The exponent can be interpreted as a characteristic parameter reflecting the growing process of the gel network. We found two groups in terms of the value of the exponent. In the case of 2.0 wt % (high polymer concentration) K-form and Cs-form κ-carrageenan, the exponent is much higher than unity, but in the case of 1.0 wt % K-form (low polymer concentration) κ-carrageenan with added KCl and 2.0 wt % K-form ι-carrageenan, it is close to unity. For the former group, the stress required to induce the unit orientation increases, while for the latter group it does not increase much with decreasing temperature. In other words, the structure inside cross-link region changes markedly during the growing process of gel network only for the former group. We concluded that increasing schemes of the number of aggregated helical molecules during the gel growing process strongly depends on the cation species and carrageenan type.