New insights into the development of ordered structure in poly(ethylene terephthalate), II Results from transmission infrared spectroscopy of thin films

To complement our earlier work involving external reflection infrared spectroscopy of PET, a set of thin PET films with different crystalline structures (amorphous, strain-induced crystallinity, thermally induced crystallinity) was prepared and analyzed in detail in the transmission mode. In analogy with the reflection work, factor analysis was used to generate three basis spectra corresponding to three distinct structures: G = an arrangement involving gauche glycol conformers and disordered terephthalate groups; TX = an arrangement involving trans glycol conformers and disordered terephthalate groups (probably a mixture of cis and trans conformers); and TC = the all-trans arrangement found in the true crystalline phase. The TX structure is believed to play an important role in the widely reported “intermediate” phase of PET, which is particularly significant in cold-drawn samples. The transmission spectra confirm the validity of the reflection spectra and also provide more detailed information. In addition, analysis of two biaxially oriented industrial films has provided further information on the geometry of certain vibrational modes. Overall, the results lead to a better understanding of the complex crystalline structure of PET and its relationship to the infrared spectrum.