Molecular deformation of wood and cellulose studied by near infrared spectroscopy

Wood (Eucalyptus regnans and Pinus radiata) and paper samples were stretched to different strain levels using a purpose-built tensile test device fitted into a near infrared (NIR) spectrometer while collecting transmission spectra. Consistent spectral changes caused by mechanical strain, assigned to OH stretching bands, were observed for all three sample types. Bands at 6286 ± 5 cm^-1 and 6470 ± 10 cm^-1 were tentatively assigned to the OH groups connected with the 2OH⋯6O and 3OH⋯5O intramolecular hydrogen bonds of crystalline cellulose Iβ, respectively. Both bands shifted to higher wavenumbers indicating the elongation of the hydrogen bonds. A linear relationship was found between band shifts and mechanical strain. Band shift rates for the 3OH bond were more than twice that of the 2OH bond, consistent with bending of the glycosidic bond. Bending tests showed that the band at around 6286 cm^-1 shifted in opposite direction when under tension or compression.