New insights into the development of ordered structure in poly(ethylene terephthalate).: 1.: Results from external reflection infrared spectroscopy

Front-surface external reflection infrared spectroscopy was used to study a set of samples of poly(ethylene terephthalate) (PET) corresponding to various states of order: highly amorphous, drawn at 80 degreesC to different draw ratios, and thermally crystallized under different conditions. Kramers-Kronig transformation provided high-quality spectra that included an accurate representation of the most intense bands in the spectrum, which are generally saturated or distorted in transmission and internal reflection spectra. Factor analysis indicated the presence of three principal components in the spectra, and by taking linear combinations of the three principal factors, it was possible to generate three distinct physically meaningful basis spectra designated G, TC, and TX. The G spectrum corresponds to a gauche conformation of the ethylene glycol moiety, which is predominant in the amorphous state, while the other two correspond to a trans glycol conformation, The TC spectrum corresponds to the true crystalline state of PET, in which the carbonyl groups are coplanar with and in an all trans arrangement with respect to the benzene rings. The TX spectrum, on the other hand, corresponds to a less ordered trans structure in which the peaks associated with the terephthaloyl moiety of the molecule resemble those observed for the amorphous phase, where the carbonyl groups are either noncoplanar or cis and trans with respect to the benzene rings. However, the TX spectrum is a major contributor in the spectra of the drawn samples. This indicates that drawing at 80 degreesC produces a structure in which gauche conformers are converted into extended trans sequences, but the terephthaloyl conformation remains rather disordered. In other words, the development of order involves two processes that do not necessarily occur simultaneously. This provides new insight into the nature of the widely reported "intermediate" phase in PET and into the complex behavior of some of the trans peaks in the infrared spectrum. Detailed analysis of the basis spectra, including curve fitting, has also made it possible to suggest more precise assignments for some of the bands in the IR spectrum.