Probing Interfacial Interactions in Natural Fibre Reinforced Biocomposites Using Colloidal Force Microscopy
The mechanical properties of reinforced biocomposites, such as flax fibre/poly(lactic acid)(PLA), are largely governed by the level of interfacial interactions between the two constituents, apart from their intrinsic properties. The hierarchical organization of various polysaccharides present in natural fibres results in complex mechanisms at the interface which are still poorly understood and difficult to analyse through traditional approaches that rely on indirect assessments.
In this work we present an experimental set-up based on the colloidal force microscopy technique, which allows probing direct interaction forces between molecularly smooth surfaces in sphere–plane contact geometry. Interfacial interactions in flax/PLA biocomposites are described here by measuring adhesion forces between model surfaces of major polysaccharides present in the flax fibre (cellulose, pectin and hemicellulose) and a PLA microbead that represents the matrix polymer. Adhesion force measurements performed under ambient and low relative humidity (2% RH) conditions show the important role of water molecules on force measurements. The PLA/cellulose system was identified to be the weakest, in terms of interactions in a biocomposite. However, for hydrated polysaccharides, such as pectins and hemicelluloses, PLA showed strong adhesion forces even at low relative humidity. The high adhesion force on hydrated polysaccharides was presumed to be due to the contribution from complementary adhesion mechanisms, including capillary forces and interdiffusion phenomena. Finally, the Nardin–Schultz relation between interfacial shear stress and work of adhesion was tested for flax/PLA biocomposites. The work of adhesion (