X-ray scattering and microtomography study on the structural changes of never-dried silver birch, European aspen and hybrid aspen during drying

The impact of drying on the structure of the never-dried hardwood cell wall was studied at nanometer level by means of wide- and small-angle X-ray scattering (WAXS, SAXS), and at micrometer level by X-ray microtomography (mu CT). Never-dried silver birch, European aspen and hybrid aspen samples were measured by WAXS in situ during drying in air. The samples included juvenile and mature wood, as well as normal and tension wood to allow comparison of the effects of different matrix compositions and microfibril angles. The deformations of cellulose crystallites and amorphous components of the cell wall were detected as changes in the cellulose reflections 200 and 004 and amorphous halo in the WAXS patterns. Especially, the width of the reflection 004, corresponding to the cellulose chain direction, increased due to drying in all the samples, indicating an increase of strain and disorder of the chains. Also, the cellulose unit cell shrank 0.2-0.3% during drying in this direction in all the samples except in hybrid aspen tension wood. According to the SAXS results of silver birch, the distance between microfibrils decreased during drying. It was detected by mu CT that the mean cross-sectional maximum width of the parenchymatous rays decreased from that of never-dried to air-dried birch by roughly 16%.