Crystal Morphology and Phase Identifications in Poly(aryl ether ketone)s and Their Copolymers. 1. Polymorphism in PEKK

Poly(ether ketone ketone) (PEKK) with all-para linkages in phenylene groups is one member of the poly(aryl ether ketone) family. This polymer has been reported to exhibit two polymorphisms (forms I and II) based on wide-angle X-ray diffraction (WAXD) experiments. The form I crystal structure has been determined to have two-chain orthorhombic packing with a = 0.767 nm, b = 0.606, and c = 1.008 nm. For form II, however, two different structures have been proposed: both are orthorhombic, but with different size and symmetry.^1,2 Through our morphological observations, two different crystal forms have been isolated. Using electron diffraction (ED) experiments combined with WAXD, we have found that form II of PEKK should have a two-chain orthorhombic lattice. This unit cell dimension is consistent with the form II cell reported by Blundell et al.,¹ namely, a = 0.417 nm, b = 1.134 nm, and c = 1.008 nm. Two maps of the formation of these forms with crystallization conditions are also discussed.