Hydrostatic pressure and long-chain organic molecule diffusion through a polymer matrix

Abstract

Earlier papers from our laboratory^1–3 have described a sensitive but relatively simple technique using IR spectroscopy to study the diffusion of labelled organic molecules through a polymer matrix. The method showed that the behaviour agreed well with de Gennes⁴ model of reptation which assumes that long organic molecules diffuse by the motion of segments consisting of 20 to 30 units in the backbone. We describe here an extension of this work and deal with the effect of hydrostatic pressure on the diffusion of two hydrocarbons (suitably labelled) with backbone units N = 18 and 45 through molten low density branched polyethylene at pressures ranging from atmospheric to 1,300 atmospheres. The results show that the diffusion coefficient D decreases as the pressure P is increased. To a close approximation log D decreases linearly with P implying an apparent activation volume, V. Typical values for N = 18, are V = 55 ų at 150 °C and 70 ų at 120 °C implying a thermal expansion of the free volume of order 0.5 ų K⁻¹. For N = 45, V = 80 ų at 120 °C.