Browsing by Subject "Cellulose ether"

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    Structure and Phase Behavior of Aqueous Methylcellulose
    (2015-09) McAllister, John
    Methylcellulose is a chemically modified polysaccharide that is partially substituted by methoxyl groups. When the average substitution per monomer is intermediate, MC is water soluble, and these materials see use in a wide variety of commercial products. Aqueous solutions of MC undergo gelation and phase separation upon heating. Using dynamic mechanical spectroscopy, frequency-independent loss tangents were used to identify the gel point (Tgel) in MC solutions well over the chain overlap concentration (c 10c). Transmittance of 633 nm laser light through the solutions revealed that MC solutions cloud upon gelling, with a relative transmittance of 86% closely associated with the gel point. The gelation temperature of MC solutions was found to decrease with increasing MC concentration and the results for all molecular weights superposed. The fibrillar structure of aqueous MC gels was probed using a combination of small-angle neutron scattering (SANS), ultra-small-angle neutron scattering (USANS), and cryogenic transmission electron microscopy (cryo-TEM). The effect of molecular weight (Mw) and concentration on the gel structure was explored. The fibrillar morphology was consistently observed at elevated temperatures ( 70 C), independent of concentration and Mw. Moreover, the fibril dimensions extracted from SANS by fitting to a scattering function for semiflexible cylinders with disperse radii revealed that the fibril diameter of ca. 141 nm is constant for a concentration range of 0.01% to 3.79% and for all Mw investigated (150-530 kg/mol). Comparison of the measured SANS curves with predicted scattering traces revealed that at 70 C the fibrils contain an average volume fraction of 40% polymer. The linear and nonlinear viscoelastic response of MC gels can be described by a filament-based mechanical model. In particular, large-amplitude oscillatory shear experiments show that aqueous MC materials transition from shear thinning to shear thickening behavior at the gelation temperature. The critical stress at which MC gels depart from the linear viscoelastic regime and begin to stiffen is well predicted from the filament model over a concentration range of 0.18-2.0 wt%. These predictions are based on fibril densities and persistence lengths obtained experimentally from neutron scattering, combined with cross-link spacings inferred from the gel modulus via the same model. 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 hour of heating the homogeneous solutions from 5 C, reveal that the theta temperature for MC in water is T = 48 2 C, with A2 < 0 for T > T, 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 three weeks at 40 C reveal the presence of micron size fibrils, which is 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, 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. It is speculated that the underlying equilibrium phase behavior of aqueous MC solutions involves a nematic order parameter.