Crystal structure of the β 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
Å, b = 5.02 Å, c = 20.5
Å (chain axis), α = 90°, β = 88°, and γ
= 90° with the space group symmetry P1̅,
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.