Vibrational Sum-Frequency-Generation (SFG) Spectroscopy Study of the Structural Assembly of Cellulose Microfibrils in Reaction Woods

The cellulose microfibril assemblies in secondary cell walls of tension wood and compression wood were studied with vibrational sum frequency generation (SFG) spectroscopy. The tension wood contains the gelatinous layer with highly-crystalline and highly-aligned cellulose microfibrils. The SFG spectral features of tension wood changed depending on the azimuth angle between the polarization of the incident IR beam and the preferential alignment axis of the cellulose microfibrils. The SFG spectra of the compression wood did not show any dependence on the azimuth angle, implying that the overall orientation of cellulose microfibrils in compression wood is not highly aligned. Instead, the decrease of cellulose content in compression wood brought about larger separation between cellulose microfibrils, which was manifested as changes in CH₂/OH intensity ratio in SFG spectra. These results implied that SFG spectral features are sensitive to cellulose microfibril alignments and inter-fibrillar separations.