Long-term water migration in hydrated corn stalk pith and hemp stalk by 250 MHz NMR relaxometry and diffusometry

Plant fibers are of current interest in biocomposites and known hydrophilicity of plant fibers, e.g. corn stalk pith and hemp stalk, require enhanced understanding of water impacts on fiber structure. Water distribution during hydration in these fibers is compared to fibrous cellulose media using nuclear magnetic resonance (NMR) $T_2$ relaxometry and diffusometry. Measurements were made every 6 h from $3\text{–}237$ h, or until equilibrium. NMR $T_2$ measured bound, intermediate bound and macropore water populations at two water mass hydrations. NMR diffusometry measured diffusion coefficients $D$ of mobile water populations in each material. The diffusion coefficient $D$ was acquired as a function of molecular migration time $\Delta$ to detect restricted diffusion. $D\left(\Delta \right)$ behavior indicated increased interpore connectivity in highly saturated hemp, versus swollen pores saturated pith. $D\left(\Delta \right)$ values were fit for surface to volume ratio $S/V$ to determine average pore radius $r$ as a function of hydration. $S/V$ values were paired to $T_2$ distributions to provide an effective average transverse magnetic surface relaxivity ${\bar{\rho }}_2$ throughout hydration, which is then used to rescale $T_2$ distributions as pore size distributions. Bound water was found in pores $<1\mu$m, semi-bound water in pores from $1\text{–}30\mu$m and macropore water in pores from $30\text{–}300\mu$m, in agreement with prior literature.