Thermally induced coupling of phase separation and gelation in an aqueous solution of hydroxypropylmethylcellulose (HPMC)

Thermally induced coupling of gelation and phase separation in polysaccharide aqueous solutions has a complex feature because of critical and tricritical phenomena, thermally induced hydrophobic interaction, and molecular-weight distribution of the polysaccharide. To elucidate the process, the criticality of a hydroxypropylmethylcellulose (HPMC) aqueous solution was assessed, and then dielectric relaxation and fluorescence intensity experiments were carried out. The diffusion coefficient of the solution with a weight fraction of HPMC being 0.06 could be extrapolated to zero at the cloud-point curve which showed the criticality of the solution. The fluorescence intensity increased at a temperature much lower than the cloud point and the gel point, especially for concentrated solutions, indicating the hydrophobic interaction as the driving force of the gelation coupled by the phase separation. Dielectric relaxation measurements by time-domain reflectometry revealed two characteristic relaxations of chain motions around 100 MHz and orientation of free water around 20 GHz, which is accompanied by a low-frequency tail reflecting hydration water.