Polyethylene terephthalate rotational isomerism

Grime, D., and I. M. Ward The assignment of infrared absorptions and rotational isomerism in polyethylene terephthalate and related compounds. Trans. Faraday Soc. 54, 959—971 (1958). 

Rotational isomerism arises because of the alternative conformations of a molecule that can result from the possibility of hindered rotation about the many single bonds in the structure. Spectroscopic techniques [4] developed in small molecules have been extended to polymers, and as an example we illustrate (Figure 1.7) the alternative trans and gauche conformations in the glycol residue of polyethylene terephthalate [5] the former is a crystalline conformation, but the latter is present in amorphous regions. 

Figure 1.7 Polyethylene terephthalate in the crystalline trans conformation (a) and in the gauche conformation present in amorphous regions (b). (Adapted from Crime, D. and Ward, I.M. (1958) The assignment of infra-red absorptions and rotational isomerism in polyethylene terephthalate and related compounds. Trans. Faraday Soc., 54, 959. Copyright (1958) Royal Society of Chemistry.)...

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