Determination of the crystal structure of poly-(p-phenylene terephthalate) from power diffraction data. A computer modelling approach

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Abstract
The crystal structure of poly-(p-phenylene terephthalate), PPT, has been determined using a combination of molecular modelling and X-ray powder diffraction. The polymer is found to crystallize in the monoclinic space group, P21/a, with unit-cell parameters a= 7.98 Å, b= 5.33 Å, c= 12.65 Å and β= 98.98 ° at 25 °C. The crystal packing favours intermolecular phenyl-edge to phenyl-face and carbonyl-carbon to carbonyl-oxygen interactions over other possibilities. The method used to predict the structure consisted of several distinct stages. Initially, a database of structural information relevant to the component units of the polymer was assembled. A molecular modelling program was used to construct and manipulate model crystals based on this information and energy calculations employed to discard those which were energetically unlikely, or sterically impossible. Finally, powder diffraction patterns were simulated for the remaining models and compared with experimental data. Likely candidates for the crystal structure were then iteratively refined, to improve the agreement between simulation and experiment. The method is particularly useful for solving structures of polymers which, as in the present case, are unable to form large single crystals or oriented fibres.

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