Moisture-related changes in the nanostructure of woods studied with X-ray and neutron scattering

Paavo A Penttilä,Monika Österberg,Lauri Rautkari,Isabelle Morfin,Ralf Schweins,Nico Carl,Michael Altgen,Peter Van Der Linden

doi:10.1007/s10570-019-02781-7

Abstract

Wood and other cellulosic materials are highly sensitive to changes in moisture content, which affects their use in most applications. We investigated the effects of moisture changes on the nanoscale structure of wood using X-ray and neutron scattering, complemented by dynamic vapor sorption. The studied set of samples included tension wood and normal hardwood as well as representatives of two softwood species. Their nanostructure was characterized in wet state before and after the first drying as well as at relative humidities between 15 and 90%.Small-angle neutron scattering revealed changes on the microfibril level during the first drying of wood samples, and the structure was not fully recovered by immersing the samples back in liquid water. Small and wide-angle X-ray scattering measurements from wood samples at various humidity conditions showed moisture-dependent changes in the packing distance and the inner structure of the microfibrils, which were correlated with the actual moisture content of the samples at each condition. In particular, the results implied that the degree of crystalline order in the cellulose microfibrils was higher in the presence of water than in the absence of it. The moisture-related changes observed in the wood nanostructure depended on the type of wood and were discussed in relation to the current knowledge on the plant cell wall structure.Graphic abstract

Highlights

Wood is an extremely abundant, renewable material that has been used in various applications throughout the history of humankind
Opposite trends were observed in the tension wood samples of beech, where the interfibrillar distance increased by about 2% and the power-law scattering slightly weakened as a consequence of drying and rewetting. These results suggest that the drying-induced changes in the wood nanostructure, in the cross-sectional diameter and packing distance of cellulose microfibrils (CMF), were specific to the type of wood and were not fully recovered by immersing the samples back in liquid water for several days
Our results showed a reversible decrease of the interfibrillar distance a from above 6 nm at high moisture content to around 4 nm at low moisture content (Fig. 6), as well as an increased disorientation of the CMFs at low moisture contents

Summary

Introduction

Wood is an extremely abundant, renewable material that has been used in various applications throughout the history of humankind. It has been widely utilized as a construction material, raw-material for paper and, more recently, as a source of cellulose nanofibrils and other nanoscale constituents for advanced materials and applications (Jiang et al 2018). It is well known, that moisture content and history are crucial factors determining the properties of wood as a material. Understanding the interactions between water and the nanostructure of wood is of utmost importance

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