Gradient variations of cellulose supramolecular structures in moso bamboo culm: from nano- to microhorizons

The variation of cellulose microfibril orientation within the bamboo has been confirmed to be one of the critical factors exerting a strong influence on the mechanical properties of bamboo fibers, which is regarded as the response of the cell wall to the internal and external stimulus. In this paper, the radial gradient variation of the cellulose supramolecular structures of bamboo culm was systematically studied by XRD and the linear polarized confocal Raman microspectroscopy to deepen the understanding of the origin of bamboo micromechanics and its functionally graded properties. XRD analysis indicated that bamboo yellow (By) had the isotropic crystallite arrangements, while bamboo timber (Bt) and bamboo green (Bg) displayed preferred orientation of crystals. Moreover, both the crystallinity and crystallite sizes notably grew from the inner By to the Bg. At cell wall level, the variations in the distribution of microfibril orientation were visualized by Raman imaging, with the fiber secondary wall areas adjacent to compound middle lamella displaying higher Raman intensity. Furthermore, Raman band ratio (I-1095/I-2939) was used to predict the microfibril angle (MFA) in different cell wall types and morphologically distinct cell wall layers semi-quantitatively. The results showed that the ratio was the highest in parenchyma, followed by narrow layer of fiber wall, and the lowest in the broad layer, which indicated the high MFA in the parenchyma. Interestingly, the ratio decreased along the successive and alternating broad and narrow lamellae of fiber wall, in accordance with cell wall micromechanical trend.