Comparison of statistical and blocky copolymers of ethylene terephthalate and ethylene 4,4′-bibenzoate based on thermal behavior and oxygen transport properties

Poly(ethylene terephthalate) (PET) was blended with a frustrated liquid-crystalline polymer, poly(ethylene terephthalate-co-4,4'-bibenzoate) (PETBB55), in the weight ratio 70:30. Under the melt conditions used for blending, NMR analysis showed that some transesterification had occurred. Accordingly, the blended product resembled a blocky copolymer more closely than it did a physical blend. A random copolymer with the same composition was synthesized for comparison. The study examined the effect of the comonomer distribution (blocky vs random) on the thermal behavior and oxygen transport properties of the glassy and cold-drawn polymers. The glass-transition temperatures and the crystallization behavior suggested that the PETBB55 blocks phase-separated as very small domains. Higher levels of orientation, as indicated by higher densities and higher trans glycol fractions, were achieved by the cold drawing of the blocky copolymer. It was speculated that the cold drawing of the blocky copolymer at temperatures up to the glasstransition temperature of the PETBB55 blocks produced highly oriented PETBB55 domains. Constraints imposed by connections between PET and the PETBB55 blocks prevented the relaxation of the continuous PET phase, even at temperatures well above the glass-transition temperature of the PET blocks. In this sense, the blocky copolymer embodied the concept of a self-reinforcing polymer. As a result, an improved oxygen barrier was obtained over a wider range of cold-draw temperatures with the blocky copolymer. (C) 2002 Wiley Periodicals, Inc.