Effect on the Thermo-Gelation Process of the Degree and Molar Substitution of HPMC Polymer Hydrogels

Hydroxypropyl methylcellulose (HPMC) is a polymer made of a cellulose backbone with etherified methoxy and hydroxypropyl substitutions that undergoes a reversible inverse thermo-gelation upon heating. The thermo-gelation follows a two-step mechanism: first, phase separation occurs, then fibrils, formed in the polymer-rich phase, entangle to form the three-dimensional gel network. This work aims at understanding the influence of degree (DS) and molar substitution (MS) on the thermo-gelation process. DS is the number of methyl groups, and the MS is the hydroxypropyl content per anhydroglucose unit. The thermo-gelation process is here rheologically investigated for two HPMC samples, which have similar molecular weight but different MS and DS: thermal ramps are coupled with time sweep tests at selected temperatures and with frequency sweep experiments. Results show an important effect not only on the transition temperatures (beginning of the phase separation, beginning of the gelation) but also on the kinetics of the different phases of the process.