Thermodynamics of Aqueous Methylcellulose Solutions

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• 作者：John W. McAllister ; Peter W. Schmidt ; Kevin D. Dorfman ; Timothy P. Lodge ; Frank S. Bates
• 刊名：Macromolecules
• 出版年：2015
• 出版时间：October 13, 2015
• 年：2015
• 卷：48
• 期：19
• 页码：7205-7215
• 全文大小：685K
• ISSN：1520-5835

文摘

The weight-average molecular weight M_w, z-average radius of gyration R_g, and second virial coefficient A₂ have been determined between 15 and 52 掳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 掳C, reveal that the theta temperature for MC in water is T_胃 = 48 卤 2 掳C, with A₂ < 0 for T > T_胃, indicative of lower critical solution temperature (LCST) behavior. However, after annealing a solution for 2 days at 40 掳C evidence of high molecular weight aggregates appears through massive increases in the apparent M_w and R_g, 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 掳C reveal the presence of micron size fibrils with a diameter of 16 卤 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 < T_胃, even though the solvent quality is apparently good. The minimum temperature required for MC solutions to aggregate is estimated to be 30 掳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鈥揌uggins solution theory. We speculate that the underlying equilibrium phase behavior of aqueous MC solutions involves a nematic order parameter.