Mesh size analysis of cellulose nanofibril hydrogels using solute exclusion and PFG-NMR spectroscopy

The pore structure of TEMPO-mediated oxidized CNF hydrogels, chemically cross-linked with water-soluble diamines, is studied. A solute exclusion method and pulsed-field-gradient NMR are used to estimate the mesh size distribution in the gel network in its hydrated state. Dextran fractions with the nominal molecular weights in the range of 10-2000 kDa are used as probes. The results show a nonuniform network structure, consisting of a group of large openings that contain ∼50% of water, and regions with a more compact structure and smaller mesh units that restrict the diffusivity of the dextran molecules. A biexponential model is proposed for the NMR echo amplitude decay due to the probe diffusion into the gel network. A typical single exponential model does not fit the experimental data when the probe molecular size is comparable to the network mesh size. The results obtained with NMR, using the proposed biexponential model, are in very good agreement with those determined with solute exclusion. Precise mesh size estimation with solute exclusion using pore models is subject to restrictions, and vary with the assumed pore geometry. The average mesh size obtained using a spherical pore model, ∼35 nm, in the compact regions of the hydrogel, is in good agreement with the theoretical value in a network of rodlike particles. Neglecting the wall effects leads to underestimation of the mesh size with both techniques.