Impact of Molecular Weight on Fibrillar Methylcellulose Hydrogels

Methylcellulose (MC) is a partially methoxy-substituted cellulose ether. Aqueous solutions of MC undergo thermoreversible gelation upon heating. Although MC has been a commercial material for over 80 years, the mechanism of MC gelation has been a subject of debate. Recently it has been demonstrated that MC gelation is due to the formation of a fibrillar network, with fibril diameters of ca. 15 nm as measured by small-angle neutron scattering and cryogenic transmission electron microscopy (cryo-TEM). With this new understanding we have investigated the MC molecular weight dependence of the fibrillar network properties. Small amplitude oscillatory shear reveals that the gel modulus increases monotonically with increasing molecular weight. Utilizing cryo-TEM and small angle x-ray scattering (SAXS), we characterize the fibrillar structure. Fitting SAXS patterns to a semi-flexible cylinder model reveals that molecular weight does not impact the structure or concentration of fibrils. We therefore attribute the change in modulus to a decrease in fibrillar network heterogeneity with increasing molecular weight of MC.