Adhesion Force Mapping of Raw and Treated Flax Fibres Using AFM Force-Volume

Abstract

Over the last few years, intensive research has been devoted to the exploitation of vegetal fibres in composite reinforcements. The mechanical properties of the natural fibre reinforced composite materials are largely dependant on the adhesion between the fibre and the polymer matrix. To improve the interfacial adhesion, raw flax fibres were chemically modified using an alkali and an eco-friendly enzymatic treatment. Atomic force microscopy (AFM) was used to probe the surface properties of the raw, alkali and enzyme treated flax fibres. First, topography images of the treated flax fibres, as investigated by AFM-Tapping mode, reveals the removal of pectin polysaccharides from the primary cell wall and allows for visualisation underneath the oriented cellulose microfibrils with a typical diameter of 25–30 nm. The removal of pectin polysccharides was also confirmed by the Infra-red spectroscopy analysis. Secondly, the lateral distribution of local adhesion properties of the fibres, as investigated using the AFM Force-Volume (FV) imaging mode, showed a heterogeneous distribution of adhesion forces on raw flax fibres in comparison with treated ones. The variation in adhesion forces was explained in terms of local changes in the hydrophilic properties of the raw flax fibre, due to the presence of some amorphous hydrophilic pectins. Furthermore, the difference in adhesion correlated well with the principal effect of the treatments which was mainly as the removal of the pectin materials on the primary cell wall of the fibre.