Cellulose and cellulose poly(vinyl alcohol) blends .2. Water organization revealed by solid-state NMR spectroscopy

Cellulose as well as two cellulose/poly(vinyl alcohol) blends with compositions 60/40 and 80/20 w/w exposed to water are investigated by H-1-, H-2-, and C-13 solid-state NMR spectroscopy. For pure cellulose, the lower temperature, secondary dielectric relaxation process can be attributed to the onset of motion of adsorbed water molecules as revealed by H-2-NMR spectroscopy. This water is not crystalline below 270 K. Three distinct kinds of water bound to the polymer matrix are detected, as far as dynamic behavior is concerned. First there is nonfreezable, strongly bound water that is rigid but amorphous at low temperatures. The second component is highly mobile and exhibits isotropic motion even below 270 K. Interestingly, there is a third component of water molecules that undergo well-defined 180 degrees flips around their bisector axis with a rate greater than 10(5) s(-1) due to anisotropic constraints. In contrast to the first two kinds, this component cannot be removed from the polymer matrix by drying even at elevated temperatures and its motional process is observed over the whole temperature range, investigated from 190 to 370 K. All three kinds of matrix water coexist in a wide temperature range. In the blends, 2D H-1-C-13 heteronuclear wide line separation (WISE) NMR spectroscopy shows that at our low concentrations the water is predominantly associated with the cellulose backbone. No water can be detected in the immediate vicinity of the poly(vinyl alcohol). Applying spin diffusion, we detected nanoheterogeneities in the range of about 3 nm within these systems.