Time-Domain NMR Elucidates Fibril Formation in Methylcellulose Hydrogels

The reversible gelation of aqueous methylcellulose (MC)solutionsat high temperature is followed stepwise with low-field time-domainnuclear magnetic resonance (LF-TD-NMR). The gel transition does notinfluence the self-diffusion coefficient of water but is associatedwith a decrease of proton transverse relaxation times H-1 T (2) by more than 60%. The effect ofmolecular weight and concentration of polysaccharide chains on transitiontemperature and gel strength is probed on several MCs with the samedegree of substitution. The measured trends connect the NMR relaxationwith macroscopic observables and agree with literature results frommore demanding techniques like small-angle X-ray and neutron scattering(SAXS and SANS). These findings support the idea of a network structureof disorderly arranged fibrils, with an inhomogeneous mesh size atleast on the order of tens of microns, which is in accord with theopaque appearance of these hydrogels. Magic sandwich echo (MSE) NMRmeasurements performed at 80 degrees C by substituting water with D2O also reveal the appearance with thermogelation of a rigidfraction involving about 25% of MC. This is consistent with the formationof fibrils mostly constituted by an amorphous matrix strongly permeatedby water and strengthened by well-dispersed MC crystallites.