Detecting the Changes on Fibers’ Surfaces after Different Chemical Treatments using Scanning Electron Microscopy

Abstract

In the last decade, jute fibers are within the scope of a wide variety of investigations thanks to their possibility for functionalization by using different chemical treatments. Among them, alkali and oxidative treatments represent the most direct, simplest, economical, and efficient methods for fibers’ functionalization and therefore for improving their quality [1]. Namely, alkali treatments using sodium hydroxide lead to selective hemicellulose removal, increased the content of cellulose exposed on the fiber surface, and decreased the crystallinity index, while oxidation using sodium chlorite is used to selectively remove lignin that is accompanied by conversion of fiber carbonyl to carboxyl groups. After both chemical treatments, scanning electron microscopy (SEM) was employed to confirm the removal of the non-cellulosic components and to evaluate the changes in jute fiber surface morphology. As can be seen from Figure 1a, the surface of multicellular raw jute fiber is uneven and rough and covered with fats, waxes, and pectin. It has to be noted that elementary fibers within the raw jute fiber cannot be seen since they are joined together into bundles by the hemicelluloses and lignin. By observing the SEM photographs of the fibers after hemicellulose or lignin removal, some changes in the fiber surface morphology can be spotted. More precisely, after the alkali treatments under different experimental conditions (sodium hydroxide concentration and/or treatment duration), the fibers’ surfaces are relatively clean and smooth (Figures 1b and c) indicating the removal of the hemicelluloses and other non-cellulosic components located on the fibers’ surfaces. As a consequence, new free spaces in the fiber structure were created, which is followed by pronounced elementary fibers’ liberation, as is evident in Figures 1b and c [2]. In the case of the most severe alkali treatment (17% NaOH for 45 min, Figure 1d), the multi-cellular fiber structure is clearly visible as well as some of the elementary fibers stick out away from the fiber bundles [3]. The changes in the fiber surface morphology are also evident after the oxidative treatments using 0.7% sodium chlorite. Significant lignin removal and elementary liberation were observed after the oxidation for 90 min, Figure 1d. The applied alkali and oxidative treatments are an appropriate starting point for obtaining multifunctional fibers. For example, to improve the jute fibers’ dielectric properties, in situ synthesis of Cu-based nanoparticles (NPs) could be performed on the alkali and oxidatively modified fibers’ surfaces [4]. Besides different characterization methods, SEM is usually used to confirm the presence of NPs on fibers’ surfaces. A few smaller agglomerates of Cu-based NPs were observed across the surface of alkali treated fiber, while smaller irregularly shaped and larger agglomerates of Cu-based NPs (along with a low amount of fiber impurities) were distributed over the oxidatively treated fibers surface, Figure 2 [5].