Spruce wood (Picea asperata, about 31 growth rings) was obtained from Shanghai province, China. Wood specimens with dimensions of 35 cm × 2 cm × 20 cm in the longitudinal (L), radial (R), and tangential (T) directions were prepared. All specimens were oven-dried at 65 °C for 48 h and then at 103 °C for 24 h, and were then divided into six groups. Their average density was 0.41 g/cm3.
2.2.1 Thermal treatment processing
Thermal treatments were conducted in a vacuum vessel equipped with two metal heating plates. First, the specimens were clamped between two metal heating plates, then residual gas was removed from the vessel by pulling a vacuum of -0.9 MPa for 1 h. Then, the two metal heating plates were heated to a predetermined temperature and held for 1 h. The temperature of the metal plates was set to 160 ℃, 180 ℃, 200 ℃, and 220 ℃, respectively. Finally, the vacuum was unloaded, and the specimens were taken out.
2.2.2 Chemical composition analysis
Chemical composition analysis was performed according to a previous research method (Wang et al., 2014). The untreated and treated samples were mashed to a length of 0.18–0.25 cm. Holocellulose analysis was performed according to Wises’s sodium chlorite method, cellulose was determined by Kürschner-Hoffner’s nitric acid method, and the lignin content was determined by acid-insoluble Klason lignin. The hemicelluloses content was determined by subtracting the cellulose content from the holocellulose content. All percentages of chemical constituents were averages of three replicates.
2.2.3 X-ray diffraction measurements
The untreated and treated samples were ground using an ordinary mill (Speed was 32000 revolutions per minute) and sieved using a steel sieves (60 mesh). The samples of 0.5 mm in thickness and 7 mm in diameter were formed from 0.05 g of the sieved powder by pressing in a mold under 10kN. Three samples were formed for each treatment condition. The XRD patterns of the samples were measured using an X-ray diffractometer reflection mode (D8 Advance, BRUKER, Germany), with Ni-filtered CuKα radiation (λ= 0.154 nm) at 40 kV and 40 mA. The reflection intensity was recorded through the scanning angle (2θ) range of 5–45° at a scanning speed of 1°/min, as shown in FigS.1 (Supplementary materials Figure 1). Peaks in diffraction intensity cure were resolved using PeakFitR (Sea-Solve Software, Inc., Richmond, CA). The crystallinity index (CI) was calculated by the Segal method (Segal et al. 1959) and the following equation:
where I200 is the maximum reflection intensity of the cellulose (200) peak, and Iam is the minimum reflection intensity near the 2θ angle of 18.5°.
The crystal width is defined as the average thickness of cellulose crystallites perpendicular to the cellulose (200) plane (D200). Based on the Scherrer equation (Alexander 1969), D200 was calculated by the following equation:
where K is the Scherrer constant (K = 0.9), λ is the wavelength of the X-rays (0.1542 nm), and β1/2 is the half bandwidth (full width at half maximum, FWHM) of the (200) peak in radians, and θ200 is the Bragg angle for the (200) plane.
The microfibril angles (MFA) of thermally-treated wood samples were measured using an X-ray diffractometer transmission mode (D8 Advance, BRUKER, Germany). The samples with dimensions of 20 mm × 10 mm × 1 mm in the L, T, and R directions were fixed with double-side tape in a platform holder, with the direction of the zero scale of the platform holder parallel to the sample fiber axis. The platform rotated 360° at a rate of 0.5°/step. In the test, the incident light was perpendicular to the sample chord plane, exhibiting an angle 2θ with the receiving light. Of particular note, the relationship between (200) reflections and the azimuth angle could be measured when the setting diffraction angle 2θ was 22.1°. Subsequently, diffraction curves were fitted by GaussAmp bimodal functions (Hu et al. 2017) and the T-method average MFA values was calculated by utilizing the well-established 0.6 T method (Cave 1966), as shown in FigS.2 (Supplementary materials Figure 2).
2.2.4 Static-loading FTIR and Dynamic FTIR spectroscopies
Static tension and dynamic Fourier-transform infrared (FTIR) spectra were recorded on a VERTEX 70 spectrometer combined with a polymer stretcher kit (A555/Z, Bruker, Ettlingen, Germany). The samples used in the static tension and dynamic FTIR experiments were cut into dimensions of 25 mm (L) × 15 mm (T) × 20 μm (R), with the direction of the fiber axis parallel to the load direction. Before testing, samples were equilibrated in the sample chamber for 2 h (25 ºC, RH 65%).
Static tension FTIR spectra were recorded at different tensile strains to study the molecular responses to the loading of thermally-treated wood (Salmén and Bergstrm, 2009). A sample was mounted in the stretcher kit, with the longitudinal direction of specimens parallel to the tensile direction. The spectra were recorded at a 1 cm-1 resolution, using an average of 16 scans at each strain. Five samples were tested at each treatment intensity. The spectra were baseline corrected at 1800 cm-1 and 2300 cm-1, and the 1st derivative of each spectrum was used to determine the peak position for each specific absorption peak of wood polymers (Wang et al., 2020).
Dynamic FTIR spectroscopy can be used to observe the molecular responses of wood constituents in strained wood (Salmén et al., 2016; Wang et al., 2020). The samples were pre-stretched in the longitudinal direction by using the stretcher to apply a load of 4 N (approximately 50% of the breaking stress). A small sinusoidal strain (< 0.3% by a 4 N pre-stretched load) with a frequency of 16 Hz was applied to the sample, and the transition dipole responses were monitored as a phase lag with respect to the external perturbation. An interferometer was run in a step-scan mode with a scanning speed of 1.0 Hz. An in-phase spectrum was obtained to indicate immediate changes or elastic responses (0° phase loss angle) and an out-of-phase spectrum was used to represent the time-delayed changes or viscous response (90° phase loss angle). IR radiation was polarized by a wire grid polarizer at 0° relative to the stretching direction. An optical filter was added after the polarizer to reduce the spectral range 3000–700 cm-1. Three samples of each thermal treatment intensity were tested. The spectra were baseline corrected. All spectra were baseline corrected at 2300 cm-1, 1800 cm-1, and 700 cm-1, and were normalized to 1 at 1435 cm-1 (Salmén et al., 2008).