Solid-state 13C and 1H spin diffusion NMR analyses of the microfibril structure for bacterial cellulose

To obtain further information about the cause for the rather large splitting of the C4 resonance line into the downfield (C4D) and upfield (C4U) lines in CP/MAS C-13 NMR spectra for native cellulose, C-13 and H-1 spin diffusion measurements have been conducted by using different types of bacterial cellulose samples. In C-13 spin diffusion measurements, the C4D resonance line is selectively inverted by the Dante pi pulse sequence and the C-13 spin diffusion is allowed to proceed from the C4D carbons to other carbons including the C4U carbons with use of the (13)C4-enriched bacterial cellulose sample. The analysis based on the simple spin diffusion theory for the process experimentally observed reveals that the C4U carbons may be located at distances less than about 1 nm from the C4D carbons. In H-1 spin diffusion measurements, poly(vinyl alcohol) (PVA) films in which ribbon assemblies of bacterial cellulose are dispersed are employed and the H-1 spin diffusion process is examined from the water-swollen PVA continuous phase to the dispersed ribbon assemblies by the C-13 detection through the H-1-C-13 CP technique. As a result, it is found that the C4D and C4U carbons are almost equally subjected to the H-1 spin diffusion from the PVA phase, indicating that the C4U carbons are not localized in some limited area, e.g. in the surfacial region, but are distributed in the whole area in the microfibrils. These experimental results suggest that the C4U carbons may exist as structural defects probably due to conformational irregularity associated with disordered hydrogen bonding of the CH2OH Groups in the microfibrils. (C) 2003 Elsevier Science (USA). All rights reserved.