Thermal decomposition of cellulose crystallites in wood

Decomposition of cellulose crystallites in wood during pyrolysis was studied by X-ray diffraction using a tension wood of Populus maximowiczii (cottonwood), which contains highly crystalline cellulose. X-ray diffraction profiles were recorded at varied temperature up to 360 degreesC. By one-hour isothermal treatments, the cellulose crystallites did not decompose at 300 degreesC, but completely decomposed at 340 degreesC. The change in equatorial diffraction profile was studied by temperature scan up to 360 degreesC and by isothermal treatment at the critical temperature of 320 degreesC. Along with the changes by thermal expansion, the changes in diffraction diagram revealed a characteristic discrepancy between the diminishment of crystalline order and the reduction in crystallite size; i.e., the intensity of crystalline reflections diminished steadily while the crystallite size decreased much more slowly. A model of highly heterogeneous decomposition is proposed to explain this behavior.