Orientation of the wood polymers in the cell wall of spruce wood fibres

Abstract

The mechanical and physical properties of wood fibres depend to a large extent on the orientation of the polymers, mainly the cellulose microfibrils, within the supramolecular structure of the cell wall. Under moist conditions, the arrangement within the polymer matrix may play a dominant role for mechanical properties in general and, especially, in the transverse direction. In this context, it is of special interest to determine the orientation of glucomannan and xylan, being the essential components of softwood hemicelluloses, and of lignin in wood fibres. Fourier transform infrared (FTIR) microscopy was used to examine the orientation of the main wood polymers in transversal and longitudinal direction of spruce fibres. We investigated fibres made from a thermomechanical pulp, in which the outer fibre wall layers were removed by mechanical action, and chemically delignified fibres. The polarised FTIR measurements indicated that glucomannan and xylan appear to have a parallel orientation with regard to the orientation of cellulose and, in all probability, an almost parallel orientation with regard to the fibre axis. Lignin was found to be less oriented in the fibre wall, although its arrangement is not fully isotropic. In the longitudinal direction of the fibres, there were no significant changes in the molecular orientation of the studied polymers.

Raman Imaging of Lignocellulosic Feedstock

“…Furthermore the orientation of cell wall polymers can be elucidated by polarised FT-IR measurements. In spruce glucomannan and xylan appear to have a parallel orientation with regard to the orientation of cellulose and, in all probability, an almost parallel orientation with regard to the fibre axis [74]. The first evidence for lignin orientation within native wood cell walls was revealed by Raman microprobe studies [75] and later confirmed in the secondary wall of tracheids fibres of thermomechanical pulp by FT-IR [70].…”

Fourier Transform Infrared Spectroscopy for Natural Fibres

“…FTIR imaging has been found to be a remarkable tool for biological and materials analysis. It can be used extensively to investigate the chemical composition of stem [83][84][85] and cell wall structure [86] of natural fibres, and natural fibre composites [87].…”

Microspectroscopy as applied to the study of wood molecular structure

“…In the analysis of spectra, this has to be taken into account and one can even extract orientational information (Gierlinger et al 2010). By inserting a polarizer in the beam path of the IR microscope, this method can also be used to extract information on the orientation of the wood polymers (Stevanic and Salmén 2009). …”

“…Therefore, care has to be taken to proper sample and laser alignment and not to mix up changes in content with changes in orientation (Gierlinger and Schwanninger 2006). Lignin, which is less oriented than cellulose within the wood cell wall (Stevanic and Salmén 2009;Gierlinger et al 2010), can be mapped by having marker bands not overlapping with contributions from cellulose and hemicelluloses. Even small amounts in a very thin layer towards the lumen of G-layer of poplar tension wood could be visualised (Agarwal 2006).…”

“…[45°) to the polarisation direction. Stevanic and Salmén (2009) investigated the orientation of all major wood polymers (cellulose, glucomannan, xylan and lignin) within spruce wood fibres using polarisation IR microscopy and found dichroic effects for all of them (Fig. 2a).…”