Experimental evidence for a mechanical function of the cellulose microfibril angle in wood cell walls

Wood is a natural fibre composite with a hierarchical cellular structure of a specific strength and a specific modulus of elasticity that can be compa red with those of other common construction materials. Each wood cell is ty pically built of cellulose fibrils spiralling around the macroscopic fibre direction. While it is natural to assume a relation between the microfibril angle (MFA) and the mechanical properties, a good correlation has up to no w only been established for single fibres, where a larger extensibility was found for fibres with larger MFA. In the present paper, we show for the fi rst time that this relation even exists for thin (200 mu m) sections of woo d: which provides strong evidence for the fact that the MFA optimizes the e xtensibility of wood. In a combination of tensile tests with structural inv estigations by small angle X-ray scattering on the same sample of Picea abi es, we found a remarkable increase in maximum strain with increasing MFA, a nd also a change in the elastic moduli.