Time-Domain NMR Elucidates Fibril Formation in Methylcellulose Hydrogels
Denise Besghini, Michele Mauri, Payam Hashemi, Matthias Knarr, Roland Adden, Petra Mischnick, Roberto Simonutti
Abstract
The reversible gelation of aqueous methylcellulose (MC) solutions at high temperature is followed stepwise with low-field time-domain nuclear magnetic resonance (LF-TD-NMR). The gel transition does not influence the self-diffusion coefficient of water but is associated with a decrease of proton transverse relaxation times 1H T2 by more than 60%. The effect of molecular weight and concentration of polysaccharide chains on transition temperature and gel strength is probed on several MCs with the same degree of substitution. The measured trends connect the NMR relaxation with macroscopic observables and agree with literature results from more demanding techniques like small-angle X-ray and neutron scattering (SAXS and SANS). These findings support the idea of a network structure of disorderly arranged fibrils, with an inhomogeneous mesh size at least on the order of tens of microns, which is in accord with the opaque appearance of these hydrogels. Magic sandwich echo (MSE) NMR measurements performed at 80 °C by substituting water with D2O also reveal the appearance with thermogelation of a rigid fraction involving about 25% of MC. This is consistent with the formation of fibrils mostly constituted by an amorphous matrix strongly permeated by water and strengthened by well-dispersed MC crystallites.