Gel point in physical gels: rheology and light scattering from thermoreversibly gelling schizophyllan

Abstract

The β-1.3/1.6 glucan schizophyllan is a fungal polysaccharide that is known to suppress tumor growth. Thermoreversible, optically transparent gels of schizophyllan were obtained on addition of sorbitol to aqueous solutions of schizophyllan. Gel temperatures were independent of the history of sample preparation. The gelation process was followed by low-amplitude oscillatory shear and by static and dynamic light scattering. Power law behavior of storage modulus G′(ω) and loss modulus G″(ω) was found at the gel temperature with the exponent n=0.5. The static LS measurements showed a strong irregularity at the gel temperature. The intensity time correlation functions (TCFs) as measured by dynamic LS during the gelation process were transformed into the electric field TCFs, taking into account the heterodyne contributions. At the gel temperature which was estimated by rheology, DSC and static LS, no power law was found for the TCF. The angular dependence of the intensity TCF was measured and converted to the field TCF by use of a non-ergodicity treatment and by the heterodyne approximation, respectively. Both methods result in mean relaxation times that agree quite well. Two relaxation modes were found in the whole temperature range (above and below the gel temperature) indicating the existence of inhomogeneities in the gel.