Thermal gelation of methylcellulose in water: Scaling and thermoreversibility

Thermally induced gelation was studied for aqueous solutions of methylcellulose with a substitution degree of 1.8 and molecular weight of 310 000. The dynamic viscoelastic and microcalorimetric properties were measured as functions of temperature and polymer concentration. An attempt was made to elucidate the validity of scaling laws and the mechanism involved in the thermoreversible gelation of methylcellulose. For a given high concentration of methlycellulose, the gel elasticity evolved with temperature, but it did not obey a single scaling law. At the gelation temperature of 70 degreesC, the gel elasticity increased with polymer concentration, and it could be expressed with a two-step scaling, from which the weak gels and the strong gels could be defined. For the weak gels (c less than or equal to 1 wt %), the elasticity G(e) at 70 degreesC scaled with polymer concentration c as G(e) - c(1.34), while G(e) scaled with c as G(e) similar to c(3.03) for the strong gels (c > 1 wt %). The thermoreversibility was examined through a cycle of heating to cooling, and the rheological thermoreversibility is found to correlate excellently with the microcalorimetric thermoreversibility. It is also proved that the gelation of methylcellulose is an entropy-driven process.