Crystal Structure of Poly(glycolic acid) Form

Crystal structure of the beta form of poly(glycolic acid) (PGA, -[CH2OCO](n)-), the existence of which was recently discovered by our research group, has been revealed by the quantitative analysis of two-dimensional wide-angle X-ray diffraction (2D-WAXD) data measured for the uniaxially oriented sample. The two helical chains of (7/2) conformation [s(21*1/2) by IUPAC nomenclature] of TTG torsional sequences are packed in the triclinic unit cell of a = 8.72 & Aring;, b = 5.02 & Aring;, c = 20.5 & Aring; (chain axis), alpha = 90 degrees, beta = 88 degrees, and gamma = 90 degrees with the space group symmetry P1, where (7/2) indicates the seven monomeric units turn twice in a repeating period, and T and G are trans and gauche bonds, respectively. The beta form transforms to the alpha form of a planar-zigzag conformation above 80 degrees C with the exothermic enthalpy change, indicating that this crystal form is thermodynamically unstable, as supposed by the preliminarily performed molecular dynamics calculation.