The Crystal Structure and the Refractive Index of Drawn and Annealed Poly (Ether-Ether-Ketone) (Peek) Fibers

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.