Abstract
The gas permeability and free volume of a series of polyester copolymers were studied. The free volume was studied using the positron annihilation lifetime spectroscopy (PALS) technique to indicate the relative size and concentration of free volume cavities in the copolymers. The homopolymers were poly(ethylene terephthalate) (PET) and poly(1,4 cyclohexamethylene terephthalate) (PCT). The copolymers were prepared by reacting terephthalic acid with varying amounts of ethylene glycol and 1,4 cyclohexanedimethanol to produce a copolymer series containing 11.8, 30.8, 31.8, 60.9, 68.9, and 80.8 mol% PCT. The logarithm of the permeability to oxygen and carbon dioxide was found to increase linearly with vol% PCT. The mean free volume cavity size and relative concentration (as indicated by the PALS parameters τ 3 and I 3, respectively) were found to increase approximately linearly with vol% PCT. The results are modelled using the Cohen-Turnbull theory for transport in polymers and discussed in terms of various methods of calculating, from PALS data, the free volume fraction that is important to transport properties.
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