Gelation of chemically cross-linked methylcellulose studied by DSC and AFM

Gelation of methylcellulose (MC) and chemically cross-linked MC via urethane linkage (MCPU) with various molecular weights was investigated in a concentration range from 0.1 to 4.0 wt %. On heating of aqueous solution of MC, three transitions, clear sol to turbid sol, sol to gel and phase separation due to water separation were observed. With increasing molecular weight the transition temperatures decrease. In contrast, no effect of molecular weight on the transition temperatures was observed for MCPU. Structural change of water restrained by MC and MCPU was investigated by DSC. Melting and crystallization of water in both series of sample showed no significant difference, however, a molecular weight dependency of the glass transition was observed for MC. The results obtained in this study indicate that hydrophobic aggregation is restricted by cross-linking. Images of atomic force microscopy (AFM) indicated that from 6 to 16 molecules piled in two layers form a bundle. By chemical cross-linking, molecular chains align in the mono layer, molecular bundles consisting of more than 10 molecules coaggregate and form a large flexible ring.