STRUCTURAL STUDY OF LINEAR POLYESTERS .1. CRYSTAL-STRUCTURE OF POLY(TRIMETHYLENE SEBACATE), ESTABLISHED FROM X-RAY AND ELECTRON-DIFFRACTIONDATA

The crystal structure of the poly(trimethylene sebacate) polyester, PE (3,8), was arrived at through a combined analysis of its electron, pow der, and fiber diffraction patterns. PE(3,8) has an orthorhombic unit cell of dimensions a = 5.032, b = 7.532, and c = 31.33 Angstrom (fiber axis) and belongs to the P2(1)2(1)2(1) space group. There are two che mical units per fiber repeat and the two chains within the unit cell a re positioned on 2(1) screw axes, parallel to the c direction. The fib er repeat is 2.50 Angstrom shorter than that for an all-trans conforma tion. The non-trans torsion angles are located in the glycolic moiety of the chemical repeat. A conformational analysis revealed several ene rgy minima which, after minimization, yielded 18 potentially acceptabl e conformations. The most likely conformation for the -O-(CH2)(3)-O-fl exible segment of the polyester is g(+)tg(-)g(+), while the sebacate g roup is in the all-trans conformation. The final structure and its ori entation and position within the unit cell were arrived at by a compar ison of the theoretical and experimental intensities recorded on singl e crystal electron diffractograms and on X-ray diffraction powder patt erns. The proposed structure;is confirmed by the small values taken by the conventional R factor, R = 0.108 for 19 powder diffraction arcs a nd R = 0.136 (R(w) = 0.096) for 12 independent spots on the electron d iffraction recording.