Thermodynamics of Aqueous Methylcellulose Solutions

The weight-average molecular weight Mw, z-average radius of gyration Rg, and second virial coefficient A2 have been determined between 15 and 52 °C for dilute aqueous solutions of methylcellulose (MC) with three different molecular weights and constant degree of substitution (DS) of 1.8 using static light scattering. These measurements, conducted within 1 h of heating the homogeneous solutions from 5 °C, reveal that the theta temperature for MC in water is Tθ = 48 ± 2 °C, with A2 < 0 for T > Tθ, indicative of lower critical solution temperature (LCST) behavior. However, after annealing a solution for 2 days at 40 °C evidence of high molecular weight aggregates appears through massive increases in the apparent Mw and Rg, a process that continues to evolve for at least 12 days. Cryogenic transmission electron microscopy images obtained from a solution aged for 3 weeks at 40 °C reveal the presence of micron size fibrils with a diameter of 16 ± 4 nm, structurally analogous to the fibrils that form upon gelation of aqueous MC solutions at higher concentrations and elevated temperatures. Growth of fibrils from a solution characterized by a positive A2 indicates that semiflexible MC dissolved in water is metastable at T < Tθ, even though the solvent quality is apparently good. The minimum temperature required for MC solutions to aggregate is estimated to be 30 °C, based on the rate-independent gel-to-solution transition determined by small-amplitude oscillatory shear measurements conducted while cooling 0.5 and 5.0 wt % solutions. These results cannot be explained based solely on separation into two isotropic phases upon heating using classical Flory–Huggins solution theory. We speculate that the underlying equilibrium phase behavior of aqueous MC solutions involves a nematic order parameter.