THERMOGELATION OF METHYLCELLULOSE .2. EFFECT OF HYDROXYPROPYL SUBSTITUENTS

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.