Anisotropic nanoparticles as templates for the crystalline structure of an injection-molded isotactic polypropylene/TiO 2 nanocomposite

Abstract

We study the molecular origins of anisotropy in a semicrystalline polymer nanocomposite that is caused by aligned, elongated filler nanoparticles. Our study is based on spatially resolved 2D WAXS/SAXS data that indicates the arrangement of molecules, lamellae, and filler particles in the composite. Isotactic polypropylene (IPP) samples filled with anisotropic TiO2 nanoparticles were prepared by injection molding. The nanocomposite contained IPP crystals with preferential alignment, while neat IPP formed crystals with random orientation under the same preparation conditions. We studied the mechanism through which anisotropic TiO2 nanoparticles change the molecular assembly in the polymer melt and cause preferential alignment. Our hypothesis is that shear forces during injection molding align the long axis of the nanoparticles parallel to the melt flow direction, and the particles align the adjacent IPP molecules. The aligned IPP molecules in the melt then serve as nuclei in crystal growth during solidification. This templating increases the elastic modulus compared to that of neat IPP.