The influence of thermo-hygro-mechanical treatment on the micro- and nanoscale architecture of wood cell walls using small- and wide-angle X-ray scattering

Tracking the changes of cellulose crystallites upon thermo-hygro-mechanical treatment is essential to understand the response of wood cell walls to steam and compression. In this paper the influence of Compression combined with Steam (CS) treatment on wood cellulose crystallites and pores structure of Chinese fir (Cunninghamia lanceolata) was studied under different steaming temperatures and compression ratios. Small-angle X-ray scattering and wide-angle X-ray scattering were used to investigate the changes of cellulose crystallites dimension, aspect ratio, fibril diameter distribution, non-crystalline fraction, the number of chains in each microfibril, as well as the fractal dimension and size of pores in response to CS treatment conditions. Results indicate that the crystallinity increased due to CS treatment, but did not show alteration with varying CS treatment conditions, i.e. seemed nearly unaffected by higher temperatures or compression ratio, both for earlywood and latewood. The cellulose crystallite diameter depended on processing parameters: it increased with increasing treatment temperature. No considerable differences were found for earlywood and latewood. We interpret our findings as a rearrangement of adjacent cellulose chains towards higher crystalline perfection attributing to the increase in crystallinity. The same effect allows a larger coherence length of crystalline order and therefore features an increasing cross-sectional dimension. In general we can state that the CS treatment leads to higher crystallinity and more perfectly arranged cellulose crystals, while it does not greatly affect the microfibril diameter but rather the amorphous regions of the microfibrils and the surrounding hemicellulose and lignin.