Crystal Structure of Poly(glycolic acid) β Form
Deyu Niu, Hai Wang, Bo Liu, Pengwu Xu, Kohji Tashiro, Chunming Yang, Piming Ma
Abstract
Crystal structure of the β form of poly(glycolic acid) (PGA, −[CH 2 OCO] 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 Å, b = 5.02 Å, c = 20.5 Å (chain axis), α = 90°, β = 88°, and γ = 90° with the space group symmetry P 1̅, 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 β form transforms to the α form of a planar-zigzag conformation above 80 °C with the exothermic enthalpy change, indicating that this crystal form is thermodynamically unstable, as supposed by the preliminarily performed molecular dynamics calculation.