DLS-Derived Apparent Mobility as a Formulation-Relevant Descriptor of Thermoresponsive Methylcellulose Gelation and Hysteresis.

Methylcellulose is a thermoresponsive polymer that undergoes thermally induced association and gelation upon heating, with behavior strongly influenced by thermal history, concentration, and ionic environment. In this work, dynamic light scattering (DLS) was used beyond conventional particle size analysis to monitor the temperature-dependent mobility of methylcellulose solutions through the apparent diffusion coefficient, complemented by transmittance and oscillatory rheology. For a 0.1 wt.% methylcellulose solution, rheology showed a sol-gel transition during heating between approximately 50 and 60 °C, but no complete gel-sol transition during cooling, indicating pronounced thermal hysteresis. Transmittance and DLS confirmed this path dependence while revealing different recovery behavior: optical turbidity recovered near 38-40 °C, whereas apparent mobility recovered at slightly lower temperatures, around 30-35 °C. A simple Arrhenius-type model described sol-state mobility but not the full heating cycle. Therefore, a two-state phenomenological model was introduced, representing DLS-derived mobility as weighted sol-like and gel-like contributions. The model captured mobility loss, recovery, and a hysteresis window of about 22 °C. Overall, DLS-derived apparent mobility provides a useful descriptor of methylcellulose association, dissociation, and hysteresis, complementing rheology and turbidity measurements.