Probing crystallinity of never-dried wood cellulose with Raman spectroscopy

The structure of wood cell wall cellulose in its native state remains poorly understood, limiting the progress of research and development in numerous areas, including plant science, biofuels, and nanocellulose based materials. It is generally believed that cellulose in cell wall microfibrils has both crystalline and amorphous regions. However, there is evidence that appears to be contrary to this assumption. Here we show, using 1064-nm FT-Raman spectroscopy, that (1) compared to the crystalline state, cellulose in the never-dried native state is laterally aggregated but in a less-than crystalline state wherein internal chains are water-accessible, (2) hydroxymethyl groups (CH₂OH) in cellulose exist not only in the tg conformation but also in the gt rotamer form, and (3) in native-state fibrils, low-frequency Raman bands due to cellulose crystal domains are absent, indicating the lack of crystallinity. Further evidence of the absence of crystallinity of the fibrils was the failure of the normal 64 % H₂SO₄ hydrolysis procedure to produce nanocellulose crystals from untreated wood. X-ray diffraction data obtained on wood, treated-wood, and wood-cellulose samples were consistent with the new finding and indicated that full-width-at-half-height of the X-ray diffractograms and lateral disorder in samples as measured by Raman were correlated (R² = 0.95).