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The crystal structure of PEEK has been determined by wide angle X-ray diffraction from the melt spun, drawn and annealed fibers. The unit cell is orthorhombic; a=7.80Å, b=5.92Å, c (fiber axis)=10.00Å. The space group of the crystal is Pbcn. Two molecular chains pass through the a-b plane of the unit cell; one through center and the other through corner. The ether groups and ketone groups of the chains are arranged in a zig-zag manner in the b-c plane. But there is no intramolecular order in the arrangement of them. The bond angle between them is about 126.5°. The planes of phenylene groups are alternately at 37° and -37° to the b-c plane.By using the atomic coordinates determined above, the refractive indices of each crystal axis are calculated: nA=1.507, nB=1.732, nC=1.936. Then average refractive index is 1.728, and the intrinsic birefringence of c-axis is 0.321. The average refractive index and birefringence of fibers measured by interference microscope are 1.707 and 0.199, respectively. Thus the calculated values seem to be reasonable.
Shimizu et al. (Tue,) studied this question.