Mechanical and dielectric studies of carrageenan sols and gels

The temperature dependence of the dynamic shear modulus, strain optical coefficient, DC conductivity, and complex dielectric spectrum of κ- and ι-carrageenan aqueous solutions with K, Ca, Cs, and Na were measured in order to clarify the formation process of the cross-linking region and the gel network structure. From the correlation analysis between the shear modulus and the strain optical coefficient, we found that the stress inducing unit orientation increases with decreasing temperature, which strongly suggests that the branching number in a cross-linking region increases with decreasing temperature, which depends on counterion species. In terms of the correlation parameters, an increasing scheme of the branching number depends on counterion species. Just below the coil-helix transition temperature, dielectric relaxation arises, with relaxation time ～100μs and relaxation strength ～10³. Dielectric relaxation can be assigned to the counterion fluctuation in the parallel direction to the helical axis. The fluctuation distance of the counterion estimated from the relaxation time increases sharply in the initial stage of gelation and gradually reaches a constant value. We concluded that the longitudinal length of the aggregated region increases sharply at the initial state of gelation while the number of helical molecules bundled in a cross-linking region increases successively with decreasing temperature.