Rheology and turbidity measurements were performed under similar thermal histories to probe the relationship between thermoreversible gelation and phase separation for a set of three methylcellulose (MC) materials with similar degrees of substitution (DS) and contrasting molecular weights after hydration in cold water. Frequency-independent loss tangents were used to identify the gel point (T gel ) in MC solutions well over the chain overlap concentration (c ≥ 10c*). Transmittance of 633 nm laser light through the solutions revealed that all 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. Using gel and cloud points, a phase diagram was constructed which reveals that clear MC solutions transition directly into turbid gels. Frequency-independent storage moduli of fully developed MC gels scaled with φ 2.3 , consistent with theory and experiment of entangled systems. Gelation of MC has strong dependence on heating rate while the melting of the gel has little dependence on cooling rate, suggesting that thermogelation of MC proceeded by a nucleation and growth mechanism rather than spinodal decomposition.

It is well established that aqueous solutions of methylcellulose (MC) can form hydrogels on heating, with the rheological gel point closely correlated to the appearance of optical turbidity. However, the detailed gelation mechanism and the resulting gel structure remain poorly understood. Herein the fibrillar structure of aqueous MC gels was precisely quantified with a powerful combination of (real space) cryogenic transmission electron microscopy (cryo-TEM) and (reciprocal space) small-angle neutron scattering (SANS) techniques. The cryo-TEM images reveal that MC chains with a molecular weight of 300 000 g/mol associate into fibrils upon heating, with a remarkably uniform diameter of 15 ± 2 nm over a range of concentrations. Vitrified gels also exhibit heterogeneity in the fibril density on the length scale of hundreds of nanometers, consistent with the observed optical turbidity of MC hydrogels. The SANS curves of gels exhibit no characteristic peaks or plateaus over a broad range of wavevector, q, from 0.001-0.2 Å(-1). The major feature is a change in slope from I ∼ q(-1.7) in the intermediate q range (0.001 - 0.01 Å(-1)) to I ∼ q(-4) above q ≈ 0.015 Å(-1). The fibrillar nature of the gel structure was confirmed by fitting the SANS data consistently with a model based on the form factor for flexible cylinders with a polydisperse radius. This model was found to capture the scattering features quantitatively for MC gels varying in concentration from 0.09-1.3 wt %. In agreement with the microscopy results, the flexible cylinder model indicated fibril diameters of 14 ± 1 nm for samples at elevated temperatures. This combination of complementary experimental techniques provides a comprehensive nanoscale depiction of fibrillar morphology for MC gels, which correlates very well with macro-scale rheological behavior and optical turbidity previously observed for such systems.

The fibrillar structure of aqueous methylcellulose (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 (M w ) and concentration on the gel structure was explored. The fibrillar morphology was consistently observed at elevated temperatures (≥70 °C), independent of concentration and M w . 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. 14 ± 1 nm is constant for a mass fraction range of 0.01%−3.79% and for all M w investigated (49−530 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. Taking linear combinations of low temperature (solution state) and high temperature (gel state) SANS traces, the progression of fibril growth with temperature for aqueous MC materials was determined. At low temperatures (≤30 °C) no fibrils are present, whereas in the vicinity of 40−50 °C a small fraction begins to form. For temperatures ≥70 °C, virtually all of the chains are incorporated into the fibrillar structure. The persistence of the fibril structure during cooling was probed by SANS and cryo-TEM. The well-established rheological hysteresis upon cooling is directly correlated to the persistence of the fibril structures. Furthermore, cryo-TEM images taken upon heating to 50 °C showed no fibrils, whereas images for samples that were first heated to 70 °C and then cooled to 50 °C clearly display the fibrillar structure. USANS measurements revealed that heterogeneities in the gels persist beyond the largest length scale accessed in scattering experiments (∼20 μm), consistent with the observed optical turbidity.

Cryogenic transmission electron microscopy and small-angle neutron scattering recently have revealed that the well-known thermoreversible gelation of methylcellulose (MC) in water is due to the formation of fibrils, with a diameter of 15 ± 2 nm. Here we report that both the linear and nonlinear viscoelastic response of MC solutions and 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.

The weight-average molecular weight M w , z-average radius of gyration R g , and second virial coefficient A 2 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 T θ = 48 ± 2 °C, with A 2 < 0 for T > T θ , 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 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 °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 A 2 indicates that semiflexible MC dissolved in water is metastable at T < T θ , 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.