FT-IR specular reflection and FT-Raman study of the structure of PET

Samples of PET with different states of crystallinity, prepared by annealing and/or drawing amorphous sheet, were examined by front-surface reflection FT-IR and FT-Raman spectroscopy. The spectra were analyzed in detail by means of spectral subtraction and curve fitting in order to obtain information on the molecular conformation. The strain-induced crystalline (SIC) material produced by drawing has a quite different structure from the true crystalline material produced by thermal annealing. In the latter the glycol groups are known to exist entirely in the form of trans conformers, and the carbonyl,groups are coplanar with the benzene ring and in the trans conformation. The SIC structure possesses an extended trans glycol structure, but the carbonyl groups are shown to have: the same conformation as in amorphous material, believed to involve a distribution of out-of-plane conformations. As a result, the SIC structure cannot achieve the same degree of molecular packing as the true crystalline structure. Furthermore, it is shown that on thermal annealing the material passes through an intermediate structure similar to the SIC structure.