The crystal structure of poly(trimethylene terephthalate) by X-ray and electron diffraction

Abstract

The crystal structure of poly(trimethylene terephthalate), [C 6H 4COO(CH 2) 3OCO] n has been determined by a combination of electron diffraction, X-ray diffraction and packing analyses. Single crystals having a parallelogram shape were grown from a nitrobenzene solution. Seen in the electron microscope they consist of platelets about 80 Å thick. Single crystal electron diffraction and X-ray fibre diffraction patterns show that the chain molecules are normal to the base plane of the single crystal. For both single crystal and fibre the unit cell of poly(trimethylene terephthalate) is triclinic with parameters: a = 4.637 A ̊ , b = 6.266 A ̊ , c = 18.64 A ̊ (fibre axis), α = 98.4°, β = 93.0°, γ = = 111.1°. The space group is P1, and the calculated crystalline density: 1.387 g/cm 3. Conformational energies were calculated for the monomeric repeating unit using semi-empirical methods. The starting geometry of the residue was derived from previous studies on relevant model compounds. The several plausible molecular models thus obtained were examined by packing and X-ray analyses. The structure consists of rigid planar terephthaloyl residues alternating with a more flexible trimethylene sequence. This OCH 2CH 2CH 2O segment of the chain has a trans-gauche-gauche-trans conformation.