DIFFUSIONAL CHARACTERISTICS OF COEXTRUDED LINEAR LOW-DENSITY POLYETHYLENES PREPARED FROM DIFFERENT CONDITIONS OF PROCESSING

The permeability and diffusivity of oxygen, carbon dioxide, nitrogen, and helium have been obtained for a range of linear low density polyet hylene (LLDPE) films prepared from the same raw materials but with dif ferent processing conditions. The measurements were carried out by mea ns of a permeation technique over the temperature interval where the a lpha-relaxation processes were observed in earlier studies. The temper ature dependence of the permeability and diffusion coefficients of gas es shows 2 well-differentiated regions in all films. The break tempera ture of these regions is approximately located at the same temperature as the alpha-relaxation takes place. Both the permeability and their temperature dependence do not show a noticeable influence on the proce ssing conditions. The effect of processing conditions on the diffusivi ty seems to be more complex. Differences are observed for different fi lms in the diffusion coefficients, in the case of oxygen, and in their change with the temperature, which is particularly marked in the case of carbon dioxide. Fujita's free volume model has been applied to dif fusivity data in order to study the influence of films microstructure in gas permeation properties through them. (C) 1998 John Wiley & Sons, Inc.