Spectroscopic Characterization of the α⇌β Crystalline Phase Transition in Poly(Butylene Terephthalate) and its Copolymers with Poly(Tetramethylene Oxide).

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.

Crystal Structure and Vibrational Spectra of Poly(trimethylene terephthalate) from Periodic Density Functional Theory Calculations

“…The present work opens the way to different future possibilities: on one hand, the simple case of PTT demonstrated the reliability of periodic DFT calculations for the investigation of polyesters and they could be extended to the investigation of more subtle phenomena in similar systems, such as for example the polymorphic transitions promoted in poly(butylene terephthalate) by mechanical deformation. − On the other hand, based on the very good results obtained for PTT and other different polymers, the same methodology can be extended to any other crystalline polymers, both to give an interpretation of the properties of the material and to carry out an investigation of the molecular phenomena involved or to support experimental techniques in the development and characterization of innovative polymeric systems.…”

“…These polymers show peculiar properties such as even–odd effects depending on the number of methylene units between aromatic rings, resulting in a different conformation and different mechanical behavior for individual members of this family, and even some subtle polymorphic transitions. Indeed, the second member of this family in order of technological importance, poly(butylene terephthalate) (PBT), shows a reversible transition between different crystal structures upon mechanical deformation due to a variation in the conformation of the chain. − On the other hand, poly(trimethylene terephthalate), PTT, the aromatic polyester containing three methylene units between the aromatic rings, has received much less attention with respect to PET and PBT. This is mainly due to the fact that it has become available at low cost only recently, paving the way to its technological application for commercial production.…”