Thermogelation of hydroxypropylmethylcellulose (HPMC) samples (E4M, F4
M and K4M from Dow) follows the two-stage mechanism observed previousl
y for methylcellulose (A4M) and attributed to dissociation of cellulos
ic 'bundles' as a necessary precursor to hydrophobic association. All
four samples show the same unusual form of shear thinning, indicating
similar macromolecular organisation in solution. The hydroxypropyl sub
stituents in HPMC, however, appear to inhibit intermolecular associati
on since, in comparison with A4M, the proportion of visible high-resol
ution H-1-NMR signal in the solution state is higher, thermogelation d
oes not occur until higher temperature, and the resulting gels are sub
stantially weaker. Thermal 'demixing' of Klucel, a highly substituted
hydroxypropylcellulose from Hercules, occurs at essentially the same t
emperature as resolubilisation on cooling, supporting the conclusion t
hat the thermal hysteresis observed between formation and dissociation
of methylcellulose and HPMC gels arises from melting and re-formation
of the postulated 'bundle' structure, and not from hydrophobic intera
ctions.