Poly(hexamethylene terephthalate)—II. The crystal structure of forms I and II,from electron and x-ray diffraction,and packing analyses

Form I of poly(hexamethylene terephthalate), poly(6GT), has been shown, through electron diffraction of micro-single crystals, and fibre X-ray diffraction to belong to the triclinic system. The unit cell containing only one chain has dimensions $\text{a = 5.217(8), b = 5.284(12), c = 15.738(16)}\text{A}\text{?}\text{(}\text{fibre axis}\text{), α = 129.4(2), β = 97.6(2)}\text{and}\text{γ = 95.6(2)°}$. The space group is PT. The crystal structure of poly(6GT) form I has been established by the model compound approach and confirmed using 19 independent electron diffraction intensities. Upon refinement of the scale and temperature factors, the agreement index R reaches 0.172, with an overall isotropic temperature factor $\text{B = 3.0}\text{A}\text{?}{}_2$. Calculation of dynamical amplitudes, and also of structure amplitudes which allow for crystal bending, confirms that the electron diffraction data are suitable for a conventional structure analysis. The chain of the polyester has the same fully extended conformation as its related model compound hexamethylene glycol dibenzoate. The planar chain in the single cell is at 35° to the α-axis. The unit-cell dimensions of poly(6GT) form II differ from those of form I by the doubling of the b-dimension. The two chains in the double unit-cell are at 31 and 39° respectively from the a-axis. The agreement index R is 0.161 for the 19 reflections of the single cell and R = 0.196 when the 10 reflections causing the doubling of b are included.