Pulsed Field Gradient NMR Study of the Diffusion of H2O and Polyethylene Glycol Polymers in the Supramolecular Structure of Wet Cotton

PFG NMR results are reported on H 2 O, PEG200, PEG1500, PEG8000, and PEG20000 in wet cotton fibers and H 2 O in wet cotton linters. The data are analyzed in terms of a two-site exchange model (water/cotton) and show that the probe molecules in fibers are trapped in cages. The cage size decreases from 10 to 2 μm as the probes' size increases from 0.23 to 9.2 nm, although the overall accessible volume only decreases from 50 to 20−30%. This behavior may be explained by size-exclusion effects on the connectivity and accessibility. Analysis of the diffusion coefficients at short diffusion times indicates that fiber cages are water pools held between the growth rings of the fiber. In linters these cages do not occur, and a freely diffusing signal from H 2 O in the amorphous region is observed with D = (4.5−8.9) × 10 -11 m 2 s -1, which when compared to the predicted value from the microviscosity of the amorphous regions gives a tortuosity of 2−4. Exit of all probes from linters and fibers takes 0.2 s and requires hydrogen bonds to be broken with an activation energy of 50 kJ mol -1 .