Oxygen-barrier properties of polyethylene terephthalate modified with a small amount of aromatic comonomer

The oxygen-barrier properties of amorphous copolyesters based on ethylene t erephthalate with 10%, or less, of an acid comonomer were examined. Comonom er units were isophthalate, phthalate, 1,5-naphthalate, 1,8-naphthalate, 2, 6-naphthalate, 1,8-anthracenate, 2,6-anthracenate, and 2,7-pyrenate. Even 2 .5 mol % comonomer significantly affected the permeability. Linear comonome rs decreased the permeability. In contrast, small amounts of a kinked comon omer increased the permeability. However, increasing the amount of kinked c omonomer further (gradually) decreased permeability P below that of polyeth ylene terephthalate. Generally, comonomer affected solubility S less than d iffusivity D; therefore, changes in P reflected primarily changes in D. The solubility and diffusivity depended on copolymer composition in accordance with static and dynamic free-volume concepts of gas permeability in glassy polymers. The solubility correlated with the amount of free volume, as det ermined by the glass-transition temperature. This study also explored the r elationship of dynamic free volume, which determines D, to thermally access ible segmental motions of the polymer chain. The effect of comonomer on D c orrelated with the intensity of the gauche component of the subambient gamm a relaxation. Changes in the fraction of gauche-glycol conformations result ing from copolymerization were confirmed by Fourier transform infrared spec troscopy. (C) 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 3 9: 1900-1910, 2001.