The effect of alkali and enzymatic treatments on flax fibre morphology, mechanical, and adhesion properties was investigated. The multilength scale analysis allows for the correlation of the fibre's morphological changes induced by the treatments with mechanical properties to better explain the adherence properties between flax and PLA. The atomic force microscopy (AFM) images revealed the removal of primary layers, upon treatments, down to cellulose microfibrils present in the secondary layers. The variation in mechanical properties was found to be dependent, apart from the crystalline content, on interaction between cellulose microfibrils and encrusting polysaccharides, pectins and hemicelluloses, in the secondary layers. Finally, microbond tests between the modified fibres and PLA emphasize the important role of the outer fibre's surface on the overall composite properties. It was observed here that gentle treatments of the fibres, down to the oriented microfibrils, are favourable to a better adherence with a PLA drop. This paper highlights the important role of amorphous polymers, hemicellulose and pectin, in the optimisation of the adhesion and mechanical properties of flax fibres in the biocomposite. 1. Introduction Research for environmental friendly alternatives has led the composite community to develop new “ecobiocomposites,” made from natural fibres and biodegradable polymer matrices, such as polylactic acid (PLA) [1–3]. It was reported that the specific Young’s modulus of PLA/Flax biocomposite ( ？GPa for 25% fibre volume fraction) can be as close to that of glass/polyester composites ( ？GPa) [1] and makes them suitable for interesting applications. Even though natural fibres have ecofriendly credentials, they present some major drawbacks, such as poor thermal stability, anisotropic resistance, high moisture absorption heterogeneity, and in some cases poor incompatibility with polymer matrices [3]. These drawbacks prevent the use of natural fibre reinforcements in high performance structural composite applications and limit, up to now, their use for nonstructural parts. The complex chemical and physical structure of natural fibres [4, 5] is certainly responsible for these limitations which may be overcome using different chemical or physical surface treatments. A detailed description of the flax fibre structure can be found in the literature [4, 5]. Briefly, a flax fibre consists of (i) a middle lamella region, principally made up of pectin, with small quantities of lignin that ensures bundle cohesion, (ii) a primary cell wall which forms
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