The conformations of poly(butylene-terephthalate) and poly(butylene-2,6-naphthalate) chains in their α and β crystalline polymorphs

Both thermoplastic polyesters, poly(butylene-terephthalate) (PBT) and poly(butylene-2,6-naphthalate) (PBN), have been observed to undergo solid–solid phase transitions between α and β crystalline polymorphs. Fiber repeat distances determined by X-ray diffraction in the β polymorphs of PBT and PBN are larger than those observed in their α polymorphs. This has generally been attributed to an extension of the butylene glycol portion of both polyesters, where the O–CH2–↷ϕ–CH2–CH2 and (CO)–O–↷ψ–CH2–CH2 bonds are transformed from ϕ=G to S and ψ=G to T or S to T conformations, respectively, when PBT or PBN transform from their α to their β polymorphs. However, this interpretation is not consistent with solid-state 13C NMR observations made on the α and β crystalline forms of PBT and PBN. In both instances, only very small differences (0.4–0.8 ppm) in the resonance frequencies of the butylene carbon resonances are observed between the 13C NMR spectra recorded for both polymorphs. Similar small differences in the aromatic carbon resonance frequencies are observed between their α and β polymorphs. Thus, we conclude that the extension of both PBT and PBN chains, as they are transformed from their α to their β crystalline polymorphs, is more likely a consequence of an increase in the coplanarity of their ester groups and their phenyl or naphthyl rings, rather than conformational extensions of their butylene glycol fragments. A search for PBT and PBN conformations, both of which match the fiber repeat distances observed for their α and β crystalline polymorphs and are consistent with the closely similar 13C chemical shifts observed in both of their crystalline phases, lends further support to this suggestion.