Oxygen-barrier properties of copolymers based on ethylene terephthalate

The oxygen-barrier properties of amorphous polyethylene terephthalate-based copolymers with various acid comonomers were examined. The incorporation o f increasing amounts of isophthalate, phthalate, or naphthalate gradually r educed the permeability P toward the low values obtained for the correspond ing homopolymers. The permeability of poly(ethylene 3,4'-bibenzoate) homopo lymer was only slightly lower than that of polyethylene terephthalate, and the copolymers correspondingly exhibited a very gradual decrease in P as th e amount of 3,4'-bibenzoate (3,4'BB) increased. In contrast, copolymerizati on with the linear isomer, 4,4'BB, produced a substantial increase in P. Ge nerally, comonomer affected the solubility S less than the diffusivity D, a nd therefore changes in P reflected primarily changes in D for the polymers studied. The diffusivity and solubility depended on the copolymer composit ion in accordance with static and dynamic free-volume concepts of gas perme ability in glassy polymers. The solubility S correlated with the amount of free volume as determined by the glass-transition temperature. Correlation of the diffusivity D with the magnitude of the subambient gamma relaxation identified dynamic free volume with thermally activated conformational chan ges and segmental motions. Correspondence in the activation energy confirme d the relationship. (C) 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Po lym. Phys 39: 1889-1899, 2001.