水分引发的木材细胞壁无定形物质对纤维素结晶结构的影响

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

【目的】聚焦于水分引发的木材细胞壁纤维素结晶结构变化,利用温和的方法不同程度脱除细胞壁无定形物质(半纤维素、木质素),探讨典型的水分状态(绝干状态、近纤维饱和点、饱水状态)对纤维素结晶结构的影响及其与无定形物质的相互作用。【方法】以杨木为实验材料,将20 mm(轴向)×3 mm(径向)×20 mm(弦向)的木片在常温下,以亚氯酸钠和冰醋酸脱除木质素,以氢氧化钠脱除半纤维素,分别制得不同程度脱除木质素、半纤维素以及二者共同脱除的试样,并将其和未处理试样的水分状态分别调至绝干、近纤维饱和点和饱水状态,再利用X射线衍射技术测定各组试样(200)、(1-10)、(110)晶面的衍射峰的位置(2θ),分析晶面间距。【结果】绝干试样(200)晶面衍射峰的位置随无定形物质脱除而改变,其(200)晶面间距随无定形物质脱除率的提高而呈减小的趋势;各试样的(200)晶面间距均随试样内水分含量的增加而减小,且对于脱除木质素或半纤维素的试样,木材在水分达到近纤维饱和点时的晶面间距减少量占至饱水状态时晶面间距减少量的百分比较大;(1-10)和(110)晶面的衍射峰随试样中水分的增加而出现分离的趋势。【结论】在绝干状态下,无定形物质收缩对纤维素结晶结构施加拉力;木材细胞壁物理化学环境的变化会影响无定形物质与水分的相互作用,水分进入无定形物质使其膨胀而释放对结晶结构的拉力,该作用主要由进入细胞壁的水分引起,(200)晶面为主要受力的晶面。

[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 CH_(3)COOH/NaClO_(2) 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.