Nanostructural deformation of high-stiffness spruce wood under tension

Lynne H. Thomas,Clemens M. Altaner,Estelle Mossou,Michael C. Jarvis,Craig J. Kennedy,V. Trevor Forsyth,Anne Martel

doi:10.1038/s41598-020-79676-2

Abstract

Conifer wood is an exceptionally stiff and strong material when its cellulose microfibrils are well aligned. However, it is not well understood how the polymer components cellulose, hemicelluloses and lignin co-operate to resist tensile stress in wood. From X-ray scattering, neutron scattering and spectroscopic data, collected under tension and processed by novel methods, the ordered, disordered and hemicellulose-coated cellulose components comprising each microfibril were shown to stretch together and demonstrated concerted, viscous stress relaxation facilitated by water. Different cellulose microfibrils did not all stretch to the same degree. Attempts were made to distinguish between microfibrils showing large and small elongation but these domains were shown to be similar with respect to orientation, crystalline disorder, hydration and the presence of bound xylan. These observations are consistent with a major stress transfer process between microfibrils being shear at interfaces in direct, hydrogen-bonded contact, as demonstrated by small-angle neutron scattering. If stress were transmitted between microfibrils by bridging hemicelluloses these might have been expected to show divergent stretching and relaxation behaviour, which was not observed. However lignin and hemicellulosic glucomannans may contribute to stress transfer on a larger length scale between microfibril bundles (macrofibrils).

Highlights

Conifer wood is an exceptionally stiff and strong material when its cellulose microfibrils are well aligned
Abbreviations FTIR Fourier-transform infrared SANS Small-angle neutron scattering WANS Wide-angle neutron scattering WAXS Wide-angle X-ray scattering NMR Nuclear magnetic resonance FWHM Full width at half maximum SD Standard deviation
The 20 μm thick samples used for FTIR were less stiff than the 0.5 mm samples used for the scattering experiments and showed larger irreversible and time-dependent elongation fractions (Table S1), but had quite similar stress-relaxation kinetics (Fig. S1)

Summary

Introduction

Conifer wood is an exceptionally stiff and strong material when its cellulose microfibrils are well aligned It is not well understood how the polymer components cellulose, hemicelluloses and lignin co-operate to resist tensile stress in wood. Attempts were made to distinguish between microfibrils showing large and small elongation but these domains were shown to be similar with respect to orientation, crystalline disorder, hydration and the presence of bound xylan. These observations are consistent with a major stress transfer process between microfibrils being shear at interfaces in direct, hydrogen-bonded contact, as demonstrated by small-angle neutron scattering. The tensile properties depend predominantly on the microfibril angle, the winding angle of the cellulose in the dominant S2 layer of the softwood cell wall[15]

Methods

Results

Conclusion