Thermogelation of methylcellulose (A4M from Dow) shows two distinct 'waves' of increase in G', preceded by an initial reduction at lower temperature. The reduction and first wave of increase are accompanied by a sigmoidal change in optical rotation (indicating a co-operative conformational transition). Light transmission and detectable H-1-NMR reach their maximum values at the end of the first structuring process, but drop to almost zero over the temperature range of the second. Both structuring processes are reversible on cooling, but offset to lower temperature, and are accompanied by enthalpy changes in DSC (endothermic on heating; exothermic on cooling). About 40% of the high-resolution NMR signal remains undetectable in the solution state at low temperature, and the shear rate dependence of viscosity is appreciably different from that of disordered polysaccharide coils. The proposed interpretation of these findings is that methylcellulose chains exist in solution as aggregated 'bundles', held together by packing of unsubstituted or sparingly substituted regions of cellulosic structure, and by hydrophobic clustering of methyl groups in regions of denser substitution. As the temperature is raised, the ends of the bundles come apart, exposing methyl groups to the aqueous environment and causing a large increase in volume (with consequent increase in G'). At higher temperature the methyl substituents shed structured water, and form a hydrophobically crosslinked network (giving the second 'wave' of increase in G'). As in other polysaccharide systems, the thermal hysteresis is attributed to aggregation stabilising the cellulosic 'bundles' to temperatures higher than those at which they will re-form on cooling.
