Abstract
The diffusion process of several molecules (D<sub>2</sub>O, n-butanol (OD) and t-butanol (OD)) in softwood (Sitka spruce) was investigated by means of a deuterium exchange method and Fourier transform near-infrared (FT-NIR) polarization spectroscopy. The location of OH groups in different states of order of cellulose in wood was clarified by analyzing the FT-NIR transmission spectra ranging from 7200 to 6000 cm<sup>-1</sup>. Four absorption bands were assigned to 2 × ν(OH) absorptions of the amorphous regions, OH groups in semi-crystalline regions, and two types of intramolecular hydrogen-bonded OH groups in the crystalline regions, respectively. The saturation level of accessibility was very different for these absorption bands (i.e., 70–80, 60, and 40–50% for the amorphous, semi-crystalline, and crystalline regions, respectively). However, the saturation accessibilities for each absorption band varied little with molecular structure and geometry of the diffusants. The diffusion rate of D<sub>2</sub>O was much faster than that of n-butanol (OD) and t-butanol (OD) for all states of orders. The size effect of the butanols led to slight differences in the diffusive transport in the crystalline regions.
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