Sol/gel transition and liquid crystal transition of HPC in ionic liquid

This work uses an ionic liquid (IL), 1-butyl-3-methylimidazolium acetate, as a solvent to study the sol/gel transition (SGT) and liquid crystal transition (LCT) of hydroxypropylcellulose (HPC) solution. The LCT concentration of HPC at room temperature is 7 wt%, which is slightly higher than its SGT concentration of 6 wt%. For HPC concentrations of over 7 wt%, three rheological approaches were utilized, and the parameters relaxation time, hysteresis ratio and loss modulus (G″) are measured to determine the LCT temperature. The relaxation study concludes that the LC-critical concentration of HPC is 7 wt%. When HPC exceeds 7 wt%, the LC transition temperatures, ranging from 45 to 51 °C, can be measured and are proportional to the HPC concentration. The rheological results are then confirmed by making observations under a polarized optical microscope. All results are highly mutually consistent. Most significantly, the rheological parameters adopted herein can be used as good indicators of the LC transition. Of those indicators, G″ changes distinctly at the LCT point, and can thus be suggested to be the most helpful indicator in determining the LCT temperature.