Rheology and turbidity measurements were performed under similar thermal histories to probe the relationship between thermoreversible gelation and phase separation for a set of three methylcellulose (MC) materials with similar degrees of substitution (DS) and contrasting molecular weights after hydration in cold water. Frequency-independent loss tangents were used to identify the gel point (T)_(gel)) in MC solutions well over the chain overlap concentration (c ≥ 10c). Transmittance of 633 nm laser light through the solutions revealed that all MC solutions cloud upon gelling, with a relative transmittance of 86% closely associated with the gel point. The gelation temperature of MC solutions was found to decrease with increasing MC concentration and the results for all molecular weights superposed. Using gel and cloud points, a phase diagram was constructed which reveals that clear MC solutions transition directly into turbid gels. Frequency-independent storage moduli of fully developed MC gels scaled with φ~(2.3), consistent with theory and experiment of entangled systems. Gelation of MC has strong dependence on heating rate while the melting of the gel has little dependence on cooling rate, suggesting that thermogelation of MC proceeded by a nucleation and growth mechanism rather than spinodal decomposition.
展开▼
