Gas permeation properties of polyethylene-layered silicate nanocomposites

Abstract

The inorganic cations of clays with different cation exchange capacities (CECs) have been exchanged with alkyl ammonium ions, in which 1–4 octadecyl chains are attached to the nitrogen atom. In this way, organo-montmorillonites (OMs) with different surface coverage and alkyl chain packing density were obtained. Nanocomposites of the OMs and high-density polyethylene (HDPE) were prepared and the influence of the filler surface coverage and chain packing density on the exfoliation as well as on the composite properties was studied. The morphology of the nanocomposites was investigated by wide-angle X-ray diffraction (WAX) and transmission electron microscopy (TEM). Increased packing density of the alkyl chains led to larger d-spacing (up to 4 nm) and augmenting exfoliation. Identical chemical structure of the organic monolayer and the polymer as well as full surface coverage did not lead to complete exfoliation, but to partial delamination and a mixed morphology (exfoliated layers + OM tactoids) resulted. Partial exfoliation was achieved, although no polymer intercalation was observed. The oxygen permeability of 2.8 vol% composites decreased to almost half that of the neat HDPE. Films of the nanocomposites retained the optical homogeneity of the polymer and are useful for food and drug packaging. The gas permeability was measured as a function of the filler loading and was correlated to the impermeable inorganic volume fraction of the composite. The microscopically observed orientation of the inclusions was a function of the observation scale. Comparison of the measured permeability with numerical predictions for composites of parallel oriented or misaligned disk-shaped inclusions allowed the estimation of a macroscopic average aspect ratio for the inclusions.