Thermally induced association and dissociation of methylcellulose in aqueous solutions

Aqueous solutions of a methylcellulose, ranging from 0.30 to 2.49 wt %, were studied by means of micro differential scanning calorimetry (micro DSC) and rheology. The effects of polymer concentration on the thermodynamic properties of these solutions were examined through a heating process and a following cooling process at a fixed rate of 1 °C/min. Upon heating, an endothermic peak was observed at about 63 °C, which was independent of polymer concentration. The total energy defined by the endothermic peak area was found to be a linear function of polymer concentration. On the other hand, when samples were cooling from about 90 °C, a broad exothermic peak appeared at about 33 °C, and the peak height and its broadness increased with polymer concentration. A shoulder was observed above the peak temperature of 33 °C, and the shoulder became more prominent with increasing polymer concentration to eventually appear as a second peak at about 40 °C. The thermal analysis results clearly show that the association of methylcellulose molecules in water is thermorevesible but the dissociation occurred at much lower temperatures than the association temperatures. The viscoelastic properties of these solutions correlated excellently with the results obtained from the micro thermal analysis. Thermodynamic mechanisms involved in the association and the dissociation are proposed.