Thèse de doctorat en Matériaux polymères et composites
Sous la direction de Yves Grohens.
Soutenue en 2010
Interactions interfaciales dans les biocomposites à base de fibres de lin et de matrice PLA : des surfaces modèles aux surfaces réelles
The overall mechanical properties of natural fibre reinforced biocomposites are largely governed by the intrinsic strength of reinforcement fibres as well as by the level of adhesion between the fibres and the matrix polymer. The research work reported in this manuscript presents a model investigation of the adhesion properties of the major polysaccharides in flax and a polylactic acid polymer matrix, and a correlation between these interactions and the final mechanical properties of the fibre and the composite. The main objective of this multi-length scale analysis is to better understand the complex role of the different polysaccharides present in flax fibres at the biocomposite interface. We first submit a raw flax fibre to two separate treatments, an enzymatic and an alkali one, which are known to indirectly promote the adherence between the reinforcement fibre and a PLA matrix. The detailed investigation of the morphology, mechanical and adherence properties of the treated fibres allows to observe the removal of the middle and primary layers down to the cellulose microfibrils networks of the secondary layers, and underlines the importance of limiting the attack of encrusting amorphous hemicellulose matrix within the secondary layer that is essential both in order to maintain interesting material properties and to promote adhesion with PLA. In a second time, we adapt the colloidal force microscopy to directly probe interaction forces between PLA and the different polymers in flax fibre. The system was first tested on cellulose and PLA to estimate the Hamaker constant and the work of adhesion. From the Nardin and Schultz equation, the experimental data obtained by local techniques such as AFM were found to be more realistic, in contrast, with those obtained from averaging techniques that are known to be not suited for heterogeneous samples such as flax fibres. The colloidal force microscopy was thus used to selectively measure all the possible. Interaction between the different polysaccharides of the fibres and the PLA matrix to qualitatively identity the weakest interaction system in a biocomposite. The results underline the important interactions of PLA matrix with both hemicellulose and pectin materials, even at low humidity rate. We postulated that the high water content of these two polysaccharides may be responsible for several mechanisms of interaction with PLA, such as hydrogen bonds, capillary forces or inter-diffusion processes. In parallel, adhesion force mapping of real raw and treated flax fibre’s surface by AFM force-volume technique give complementary results that underline an important adhesion with these two polysaccharides
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