Sol-Gel transition behavior of pure iota-carrageenan in both salt-free and added salt states

This paper describes how strongly the gelation process of iota-carrageenan is affected by addition of metallic ions from the creep and creep recovery, dynamic viscoelasticity (DVE) and DSC measurements. Creep results at T = 25 degrees C indicate that below a polymer concentration C of 3.0 wt % the salt-free system behaves as a viscous solution, and it starts to exhibit viscoelasticity as C exceeds 3.0 wt %. In the range C = 5.0-7.0 wt %, the salt-free system shows gellike behavior whereas the added salt system, measured in the low C range 1.0-2.5 wt %, showed gellike behavior at the same temperature. The sol-gel transition temperature T(c) was determined using Winter's criterion as the temperature at which both G'(omega) and G' '(omega) follow power law behavior with the same exponent n. DSC measurements reveal that salt-free and added salt systems take different types of thermal behavior within the same temperature range. The temperature T(c) is quite close to the gelation temperature T(m) determined from DSC measurement. The Eldrige-Ferry plot was performed to estimate activaton enthalpy, which shows that physical cross-links in the salt-free iota-carrageenan is not strong in comparison with those of samples which contains metal ions. We conclude from the data analysis of C dependence of the plateau modulus using the theory developed by Jones and Marques for rigid networks based on the fractal theories that addition of metallic ions gives rise to a rigid fiber like structure even at low C of iota-carrageenan in contrast to the salt-free system for which a flexible structure has been maintained at higher C.