We derived typical phase diagrams for aqueous solutions of methyl cellulose
(MC) of different molecular weights via micro-differential scanning calori
metry, small-angle X-ray scattering, and visual inspection. The phase diagr
ams showed the cooccurrence of gelation and phase separation and qualitativ
ely agreed with the theoretically calculated diagrams. The sol-gel transiti
on line and phase separation line of a lower critical solution point type s
hifted toward lower temperatures and lower concentrations with an increase
in the MC molecular weight. The sol-gel transition line intersected at a te
mperature higher than the critical point of the phase separation; therefore
, both sol-gel phase separation and gel-gel phase separation were possible,
depending on the temperature. Specifically, through visual inspection of a
high molecular weight MC sample in the critical temperature region, we obs
erved phase separation into two coexisting gels with different polymer conc
entrations. (C) 2000 John Wiley & Sons, Inc.