The interpretation of transition phenomena in polymethylene terephthalate polymers

<jats:title>Abstract</jats:title><jats:p>A series of polymethylene terephthalate polymers has been investigated by measurement of dynamic mechanical properties, nuclear magnetic resonance spectra, infra‐red spectra and X‐ray diffraction.</jats:p><jats:p>These polymers show three transitions, α, β, and γ; α corresponds to the melting point, β is clearly the glass transition. Consideration of dynamic loss results alone suggests that the γ transition is connected with motions in the aliphatic part of the chain and that the β transition is connected in some way with the phenylene carbonyl bond.</jats:p><jats:p>The NMR results show that the γ transition is restricted to very small intramolecular motions, whereas the β transition is associated with considerable motions of the molecules in the amorphous regions.</jats:p><jats:p>Infra‐red data suggests that an additional preferred configuration of the OCH<jats:sub>2</jats:sub>CH<jats:sub>2</jats:sub>O group exists in the amorphous regions of polyethylene terephthalate. Since crystallisation sets in above the β transition it is reasonable to imply that this transition is associated with at least this configurational change.</jats:p><jats:p>Detailed X‐ray and optical examination of single crystals of cyclic tris(ethylene terephthalate) suggests that in the cyclic trimer molecules the carbonyl groups are placed in <jats:italic>cis</jats:italic> positions with respect to the benzene rings. Thus, the configurational changes may also involve the phenylene carbonyl bond.</jats:p><jats:p>The most satisfactory explanation of these transitions, taking into account all the data, is hat the γ transition involves some very restricted rotation of the glycol residue and the β transition a considerable rotation of this residue, also involving paraphenylene linkages in the amorphous material.</jats:p>