High resolution solid state sup (13) C NMR study of the alpha and beta crystalline forms of poly(butylene terephthalate).

Uniaxially strained poly(butylene terephthalate) (PBT) undergoes a reversible alpha to beta crystalline phase transition, where the fiber repeat distance is increased in the beta phase. The question of what are the intrachain conformations of PBT in both relaxed alpha and strained beta crystalline phases has not yet received a completely satisfactory answer. We have performed variable temperature sup (13) C CPMAS NMR measurements on both crystalline phases of PBT in the hope that their high resolution solid state spectra would permit us to compare the crystalline chain conformations in the alpha and beta phases. In the spectra recorded between ambient temperature and ca. 90-100C all resonances are broadened by contributions from the amorphous PBT chains.