THERMOGELATION OF METHYLCELLULOSE .2. EFFECT OF HYDROXYPROPYL SUBSTITUENTS

Thermogelation of hydroxypropylmethylcellulose (HPMC) samples (E4M, F4M and K4M from Dow) follows the two-stage mechanism observed previously for methylcellulose (A4M) and attributed to dissociation of cellulosic '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 substituents in HPMC, however, appear to inhibit intermolecular association since, in comparison with A4M, the proportion of visible high-resolution H-1-NMR signal in the solution state is higher, thermogelation does not occur until higher temperature, and the resulting gels are substantially weaker. Thermal 'demixing' of Klucel, a highly substituted hydroxypropylcellulose from Hercules, occurs at essentially the same temperature as resolubilisation on cooling, supporting the conclusion that 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 interactions.