Role of surface modification and mechanical orientation on property enhancement of cellulose nanocrystals/polymer nanocomposites

Cellulose nanocrystals (CNCs), surface-modified by grafting long alkyl chain amine onto carboxylated CNCs, have been applied as novel nanofillers to prepare nanocomposite films of poly(ethylene-co-vinylacetate) (EVA) through solution casting technique. The robust amide linkages imparted hydrophobicity to the nanocrystal surface and allowed for a better dispersion of CNCs within the EVA matrix. Atomic force microscopy (AFM) surface images and polarized optical micrographs showed that the nanocrystals were randomly oriented in the plane of the EVA film. To achieve the alignment of anisotropic nanofillers, we carried out mechanical shear experiments at an elevated temperature well above the glass-transition temperature of matrix polymer. Hot-pressing and hot-stretching resulted in preferential reorientation of nanocrystals, but not along the shear direction, which is related to the combined actions between the external tension and intermolecular interaction of amide-modified CNC during shear processing. As a result, external mechanical stress transferred from the matrix to the particles, leading to a preferential orientation of the latter. Mechanical and water vapor barrier properties were found to be drastically enhanced. The amide-based CNC/EVA showed high level of elongation at break (up to 85% retention of the initial values). These enhancements were tentatively attributed to the strong interaction occurring at the interface between CNC amide and the EVA polymer.