Nanocomposite-based lignocellulosic fibers 2: Layer-by-layer modification of wood fibers for reinforcement in thermoplastic composites

The present study investigated the layer-by-layer (LbL) modification of wood fibers for reinforcement in thermoplastic composites. The modification process involves the manipulation of surfaces with highly controlled chemistries using polyelectrolytes and nanoparticles. Fibers were modified with layers of montmorillonite clay and poly(diallyldimethylammonium) chloride (PDDA) using sequential adsorption steps to create the LbL modified fibers. The composites were prepared by melt compounding polypropylene (PP) and unmodified/LbL modified fibers, followed by compression molding. The thermal properties of the composites were characterized by differential scanning calorimetry and dynamic mechanical analysis, while mechanical properties were tested in tensile and flexural test modes. LbL modified fiber composites have similar modulus values but lower strength values than those of unmodified fiber composites. However, composites composed of LbL modified fibers display enhanced elongation at break, increasing by more than 50%, to those of unmodified samples. DSC results indicated that crystallization behavior of PP is promoted in the presence of wood fibers. Both unmodified and LbL modified wood fibers are able to act as nucleation agents, which caused an increase of the crystallinity of PP by the presence of wood fiber. Moreover, results from tensile and flexural strength, dynamic mechanical analysis and water absorption tests reveal that the material (PDDA or clay) at the terminal (outer) layer of LbL modified fiber influences the performance of the composites.