Water-induced effects of matrix in wood cell wall on cellulose crystalline structure

  Objective  To investigate changes in the crystalline structure of cellulose in wood cell wall induced by water, matrix (lignin and hemicellulose) removals at various degrees by mild method were conducted, and effects on cellulose crystalline structure as well as its interaction with matrix under typical water states (oven-dried, approximate fiber saturation point and water-saturated state) were discussed.

  Method  Poplar (Populus euromericans) wood was chosen as the research object. Specimens of chips (20 mm (longitudinal) × 3 mm (radial) × 20 mm (tangential)) were subjected to matrix removal at room temperature, where CH₃COOH/NaClO₂ solution was used to remove lignin, and NaOH was used to remove hemicellulose, to obtain samples at different removal ratios of lignin, hemicellulose and both matrix. Water states of treated and untreated specimens were conditioned to oven-dried, approximate fiber saturation point and water-saturated state, and X-ray diffraction technology was applied to detect the diffraction peak positions (2θ) of (200), (1-10) and (110) for analyzing the lattice spacing.

  Result  The (200) peaks of the oven-dried specimens were changed when the matrix was removed, and the (200) lattice spacing tended to decrease with the increase of matrix removal ratio; the (200) lattice spacing of each group of specimens reduced with increasing water content, and for the specimens removed lignin or hemicellulose, the reduction in lattice spacing at the approximate fiber saturation point accounted for a larger percentage of that at the saturated state; with an increase in water content of the specimens, (1-10) and (110) peaks tended to separate.

  Conclusion  At oven-dried condition, cell wall matrix of wood exerts tensile stress on the cellulose; change in physical and chemical environments of the wood cell wall affects the interaction between matrix and water. The swelling of matrix due to water entering caused tension on the cellulose crystal structure to release, which mainly induced by the water in cell wall and acts primarily on the (200) lattice planes.