An AFM study of the effect of chemical treatments on the surface microstructure and adhesion properties of flax fibres

The mechanical properties of fibre-reinforced polymer composites are largely dependant on the adhesion between the matrix and the fibre. In order to enhance the interaction between flax fibres and unsaturated polyester resins, raw fibres were chemically modified using sodium hydroxide, sodium hydroxide plus acetic anhydride and formic acid-based treatments. The physical properties of the modified fibres were investigated by means of the atomic force microscopy. At first, the morphological analysis of the surfaces shows that after the chemical treatments, the fibres surface appear to be less heterogeneous in topology and smoother. Nonetheless, no significant roughness difference was found between the different treatments. Secondly, adhesion forces measurements were performed between a standard AFM silicon nitride tip and the fibres. The adhesion forces were found to vary according to the chemical treatment. The sodium hydroxide-based treatment was found to increase the adhesion force between the fibre and the AFM tip whereas the lowest adhesion force was found for the formic acid-based treated fibre. These results were attributed to the different hydrophilic character of the modified fibres. Due to the importance of the water layer adsorbed on the fibres, the adhesion forces between the AFM tip and the different samples are found to be mainly dominated by capillary forces in relation with the fibre's surface hydrophilicity.