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. We recently discovered that MC chains associate into fibrils on heating. The fibrillar structure 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 associate into fibrils with a remarkably uniform diameter of 15 ± 2 nm over a range of concentrations and molar masses. Vitrified gels also exhibit heterogeneity in the fibril density on the length scale of hundreds of nanometers, consistent with the observed optical turbidity. The fibrillar nature of the gel structure was confirmed by fitting the SANS data with a model based on the form factor for flexible cylinders. The gels showed strain hardening behavior, in a manner anticipated by models for fibrillar networks. Light scattering measurements on very dilute solutions reveal an interesting competition between two order parameters, one reflecting polymer-solvent interactions and the other chain conformation. Finally, there is a wide region in the temperature-concentration plane where an apparently homogenous solution is actually metastable compared to the growth of fibrils.

New insights into the thermoreversible gelation of methylcellulose

250th National Meeting (2015)

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