How Gold Nanoparticles Influence Crystallization of Polyethylene in Rigid Cylindrical Nanopores

Even high amounts of gold nanoparticles (AuNPs) only moderately influence crystallization of bulk polyethylene (PE). However, under the rigid two-dimensional confinement of aligned cylindrical nanopores in anodic aluminum oxide (AAO) the presence of Au turns nucleation-dominated crystallization of PE at high supercooling into growth-dominated crystallization at lower supercooling. Transmission electron microscopy investigations revealed formation of larger Au crystals from AuNPs by Ostwald ripening. These larger Au crystals apparently acted as heterogeneous nucleation sites initiating PE crystallization in AAO nanopores. Thus, PE/Au composites in AAO exhibited significantly higher crystallization and melting onset temperatures as well as significantly weaker dependence of crystallization half-times on crystallization temperatures. X-ray texture analysis revealed for pure PE in AAO the existence of two copopulations of crystals with different orientations (indicative of nucleation-dominated crystal growth); PE/Au composites showed uniform alignment of the fastest growing PE crystal direction with the AAO nanopore axes (indicative of growth-dominated crystallization). The prevailing alignment of the [020] direction of orthorhombic PE with the AAO nanopore axes suggests that properly oriented crystals may form on pre-existing crystal surfaces by secondary nucleation. These secondary crystals grow along the AAO nanopores if, under the conditions of growth-dominated crystallization, competing crystals clogging the growth path are absent while the confinement of the AAO nanopore walls stabilizes the (020) growth faces.