Visualisation of dislocations in hemp fibres: A comparison between scanning electron microscopy (SEM) and polarized light microscopy (PLM)

Abstract

Dislocations are areas of the cell wall in natural fibres where the direction of the microfibrils (the microfibril angle) differs from the microfibril angle of the surrounding cell wall. Such regions affect the quality of natural fibres for industrial use. Dislocations may be studied using a number of techniques, and in this study two of them were compared: scanning electron microscopy (SEM) and polarized light microscopy (PLM). Images are presented that show exactly the same dislocations using these two techniques. The comparison showed that dislocations are more numerous and more clearly visible in images from PLM than in images from SEM, as dislocations are only seen in SEM if they are so large that they result in topographical changes of the fibre surface, i.e. a buckling of the S1 wall or a folding of the whole cell wall. However, for such large dislocations details are seen in the cell wall using SEM, while only a large bright region is seen in PLM.

Introduction

Dislocations are areas of the cell wall in natural fibres where the direction of the microfibrils (the microfibril angle) differs from the microfibril angle of the surrounding cell wall. Dislocations appear as a result of excessive compression load in the direction of the fibre axis. The resultant shear stresses dislocate one part of the cell wall relative to the other along a shear band. The origins and characteristics of dislocations in wood pulp fibres were reviewed by Nyholm et al. (2001). The increase of microfibril angle in zones of dislocations results in an increase of moisture-induced longitudinal movements and a decrease of stiffness (Hoffmeyer, 1993, Hoffmeyer and Davidson, 1989). Dislocations also function as starting points for chemical degradation (Ander et al., 2005, Thygesen, 2006), and have been found to lower fibre-matrix adhesion in fibre composites (Hughes et al., 2000). Terziev et al. (2005) induced dislocations in solid wood prior to pulping and found that the mechanical properties of the resulting paper were inferior to those of paper made from untreated wood. Whether dislocations represent zones of low tensile strength remains unclear, as results do not agree (Hudson, 1961, Davies and Bruce, 1998, Bos et al., 2002, Baley, 2004, Thygesen et al., 2006). Even though the relationship between dislocations and fibre quality does not appear to be trivial, the occurrence of dislocations nevertheless indicates that fibres have been subjected to excessive compression stress. It is therefore possible that the quality of natural fibres for industrial use to some extent may be predicted from the size and number of dislocations, even though the quality is not determined solely by the amount of dislocations. For this to be studied it is necessary to develop adequate techniques for the quantification of dislocations (Ander et al., 2005, Thygesen, 2006, Thygesen and Ander, 2005, Thygesen and Hoffmeyer, 2005).

Crystalline cellulose is birefringent and consequently polarized light microscopy (PLM) using crossed polars may be used to visualize dislocations. The colours that are seen when this technique is employed are due to interference between the fast and the slow components of the light emerging from the fibre after passing through it (see for example Preston, 1974 for a description of the technique). Some dislocations may also be studied using scanning electron microscopy (SEM), but with this technique only the surface of the object under study may be seen, i.e. for otherwise undamaged fibres studied without extensive sample preparation, dislocations may only be discerned if they result in topographical changes of the fibre surface. According to Bos and Donald (1999) dislocations only occur in the S2 layer of the cell wall, i.e. SEM detection is only possible if a dislocation in the S2 layer makes the S1 layer buckle. This means that PLM and SEM are complementary techniques: PLM gives an “X-ray type” transmission image of areas in the fibre where the microfibril angle differs from that of the bulk wall, while SEM gives a detailed image of the fibre surface. Here we present PLM and SEM images of exactly the same dislocations in hemp fibres in order to compare the information offered by the two different visualisation techniques. To the best of our knowledge no such comparison has been presented earlier for any type of natural fibres.