Improving oxygen barrier properties of poly(ethylene terephthalate) by incorporating isophthalate. II. Effect of crystallization

The present study examined crystallization of poly(ethylene terephthalate) (PET) and a series of random and blocky copolymers in which up to 30% of the terephthalate was replaced with isophthalate. Isothermal crystallization kinetics and direct observation of the spherulitic morphology revealed that the blocky copolymers crystallized more rapidly than PET, at least in part, as the result of enhanced spherulite nucleation. The statistical copolymers with 10 and 20% isophthalate achieved almost the same level of crystallinity as that of the blocky copolymers. The statistical copolymers with 10% isophthalate crystallized almost as fast as PET, although the statistical copolymer with 20% isophthalate crystallized much more slowly. Crystallization substantially reduced the oxygen permeability. Analysis of oxygen-transport parameters in terms of a two-phase structural model that considered a dispersion of lower-permeability spherulites in an amorphous matrix of higher permeability revealed that dedensification of the PET interlamellar amorphous regions was responsible for the unexpectedly high oxygen solubility of crystallized PET. In contrast, copolymerization with isophthalate prevented dedensification of the interlamellar amorphous regions. As a result, crystallization was more effective in reducing the oxygen permeability. It was speculated that segregation of kinked isophthalate units to the amorphous regions of the spherulite relieved constraint on the interlamellar amorphous chain segments. (c) 2005 Wiley Periodicals, Inc.