Structure of terephthalate polymers I—Infra-red spectra and molecular structure of poly(ethylene terephthalate)

Abstract

The ‘rigid’ crystalline model for poly(ethylene terephthalate) is not wholly consistent with the inra-red spectra of PET and model compounds, including polarized spectra in the far infra-red (600-20 cm−1). The results indicate that the extent of rotation of the glycol fragment about the terephthalate framework in the PET crystal appears to be greater than in the above model. The space group P1̄ is, however, conserved. There appears to be more rotational hindrance in the crystal than in the amorphous polymer about the CH2O and CO bonds and the existence of the discrete trans and gauche glycol configurations has been confirmed. The degree of crystallinity of PET is correlated with the trans glycol residue and a planar terephthalate framework. Conversely the amorphous content is correlated with the gauche form of the glycol residue and an accompanying distortion of the terephthalate framework from Vh to C2v symmetry. Assignments for the vibrational modes of the terephthalate framework have been made on the basis of Vh and C2v symmetries for the trans and gauche isomers in PET, respectively, using information from the related poly(cyclohexane-1,4-dimethylene terephthalate). The assignments of the vibrational modes for the ethylene glycol fragment on the basis of C2h symmetry (trans isomer) and C2 symmetry (gauche isomer) are also given together with assignments for the principal stretching, deformation and torsional modes of the backbone chain for each configuration. These are found to be in reasonable agreement with calculated values.