Thermogelation of methylcellulose:: rheological considerations

Methylcellulose is a natural polymer which gels on heating. This gelation is thermoreversible and attributed to hydrophobic interactions. Irs mechanism is not well understood but the presence of hydrophobic zones is needed to initiate the physical crosslinking. Dynamic mechanical experiments performed in a large frequency range are presented and allow discussions for improving our understanding of the gelation mechanism. Two temperature domains are studied on both sides of the gelation temperature range. In the low temperature domain the frequency spectra are typical of an entangled solution and the activation energy is determined. From the high temperature domain the gelation temperature may be determined and may be related to the experimental temperature at which the viscosity increases and turbidity occurs. The rheological results are interpreted as the signature of heterogeneities that become more and more important with temperature and are superimposed on the chain entanglements in semi-dilute solutions. The junctions in the inhomogeneities are most likely due to hydrophobic interactions between highly substituted units, as previously demonstrated by fluorescence spectroscopy. (C) 1999 Elsevier Science Ltd. All rights reserved.