Cellulosic derivatives have been used as promising natural gelling agents in diverse biomedical, environmental, and pharmaceutical settings. Their natural origin together with their thermo-responsive behavior has made them strong candidates in the production of smart gels, reducing our reliance on petroleum resources and thus our carbon footprint. However, the dynamics of thermally-induced phase transition at the lower critical solution temperature (LCST) is poorly understood for these polymers. Here, with experiments and theoretical considerations, we address how molecular architecture dictates the phase transition dynamics and precipitated particle morphologies for two naturally-derived polymers: methyl cellulose (MC) and hydroxypropyl cellulose (HPC). We find that the LCST changes significantly with the number of hydroxyl groups (n) within the molecular structure—the greater n is, the smaller the LCST becomes—causing HPC to have an LCST ~20 °C smaller than that for MC. When n is sufficiently large, HPC polymer chains collapse into globules at their LCST, making spherical particles. These soft particles are comprised of a physically entangled polymer network, and they disassociate into free polymer chains at temperatures just below their LCST. In contrast, MC polymers with fewer n first dehydrate at their LCST then diffuse in bulk to self-assemble into fibrillar structures. We show that this kinetically controlled process is the consequence of competition between the entropic free energy of polymer chains and the polymer diffusion coefficient. Unlike for the HPC globules, the disruption of these MC fibrils occurs with a significant hysteresis at temperatures much lower than their LCST. Our findings provide the required fundamental and practical knowledge to design thermo-responsive gels with desired rheological and mechanical properties.

Dynamics of reversible phase transition of thermo-responsive natural polymers

ACS Fall 2021

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