Small Molecule Diffusion in Spherulitic Polyethylene : Experimental Results and Simulations

The diffusion of small-molecule penetrants in polyethylene is hindered by impenetrable crystals and by the segmental constraints imposed by the crystals on the penetrable phase. Liquid and vapour n-hexane sorption/desorption measurements were performed on metallocene catalyzed homogenous poly(ethylene-co-octene)s. It was shown that the fractional free volume of the polymer penetrable component increased with increasing amount of penetrable polymer. It also increased with the relative proportion of liquid-like component in the penetrable polymer fraction. The detour effect was found to increase with decreasing crystallinity. The experimental study of the morphology of the polymers showed that the geometrical impedance factor followed the same trend with increasing crystallinity as the data obtained from n-hexane desorption. The changes in phase composition and character upon n-hexane sorption were monitored with Raman spectroscopy, WAXS and NMR spectroscopy. Partial dissolution of the orthorhombic and the interfacial component was observed upon nhexane sorption. Changes in the character of the components were furthermore analyzed: an increase of the density in the crystalline component and a decrease of the density in the amorphous component were observed in the n-hexane-sorbed-samples. Molecular dynamics simulations were used for studying diffusion of n-hexane in fully amorphous poly(ethylene-co-octene)s. The branches in poly(ethylene-co-octene) decreased the density by affecting the packing of the chains in the rubbery state in accordance with experimental data. Diffusion of n-hexane at low penetrant concentration showed unexpectedly that the penetrant diffusivity decreased with increasing degree of branching. Spherulitic growth was mimicked with an algorithm able to generate structures comparable to those observed in polyethylene. The diffusion in the simulated structure was assessed with Monte Carlo simulations of random walks and the geometrical impedance factor of the spherulitic structures was calculated and compared with analytical values according to Fricke’s theory. The linear relationship between geometrical impedance factor and crystallinity in Fricke’s theory was confirmed. Fricke’s theory, however, underestimated the crystal blocking effect. By modelling systems having a distribution of crystal width-to-thickness ratio it was proven that wide crystals had a more pronounced effect on the geometrical impedance factor than is indicated by their number fraction weight.QC 2010090