No abstract

The stress-induced crystalline α⇌β phase transition found in poly(butylene terephthalate) and its copolymers with poly(tetramethylene oxide) has been studied by Fourier transform infrared spectroscopy coupled with mechanical measurements. The phase transformation behavior was explained in terms of a cooperative model which considered both intermolecular as well as intramolecular interactions within the crystal. It was shown that the strength of the intramolecular interactions increased with length of the hard segments and that the strength of the intermolecular interactions increased with perfection and lateral size of the crystals. The intermolecular interaction was assumed to be dominated by the interaction between neighboring terephthalate groups. The “mean” intramolecular energy was estimated at 0.40 Kcal/mole. This calculation was based on the potential energy of rotations of a carbonyl group about a benzene-carbonyl bond. Cooperativity between chains diminished when the surface to volume ratio increased above 2 x 10-2 Å-1.