Permeability of N-2, Ar, He, O-2 and CO2 through biaxially oriented polyester films - dependence on free volume

Permeability, diffusion and solubility coefficients are reported for biaxia lly orientated polyester films based on poly(ethylene terephthalate) [PET], poly(ethylene naphthalate) [PEN] and copolymers containing PET and PEN moi eties. Data for cast amorphous sheets and materials produced with different biaxial draw ratios are compared. The crystallinity of the samples was ass essed using differential scanning calorimetry and density measurements. The changes in the void structure at a molecular level were investigated using positron annihilation lifetime spectroscopy (PALS). The variation of the g as diffusion behaviour with the gas used (carbon dioxide, nitrogen, argon, helium and oxygen) reflects the effects of change in morphology on the solu bility and diffusivity components of the permeability. The diffusivity of t he gas is influenced not only by both the changes in the void size and cont ent at a molecular level, but also by the effects of crystallinity on the p ercolation behaviour of the gas through the matrix. Changes in the extent o f chain alignment also have a profound affect on the solubility of the gas in the matrix. The observed behaviour for the gas permeation can be interpr eted as being the result of the complex interplay of changes in the crystal line content, the polymer chain alignment and the void structure of the amo rphous phase. (C) 2000 Elsevier Science Ltd. All rights reserved.