Diffusion in molten polymers: the influence of hydrostatic pressure

Abstract
This paper describes an experimental study of the effect of hydrostatic pressure on the diffusion of linear hydrocarbons through molten polyethylene and polypropylene. The diffusants range in length from 20 to 260 carbons and are suitably tagged for identification by infrared spectroscopy. The results show that the diffusion coefficient D is reduced by the pressure p and the behaviour may be described by a relation of the form D = D 0 e - pV / kT , where V is the activation volume. For hydrocarbons ranging in length from 20 to 260 carbons diffusing through both branched and linear polyethylene, V has a value lying between about 40 and 90 Å 3 (1 Å 3 = 10 -30 m 3 ). Diffusion through polypropylene involves a small but significant increase in V . Similarly the incorporation of a bulky group in the middle of a linear hydrocarbon produces some increase in V . However, the main conclusion is that the activation volume bears no relation to the size of the diffusant: it corresponds to the volume of only three or four carbons in the chain. These results and the earlier diffusion studies of Klein & Briscoe (1976, 1979) suggest that diffusion of long chain molecules through a molten polymer proceeds by a process of reptation in which the fine scale mechanism involves cooperative rotations or crankshaft motions of both the diffusant and the matrix.