Crystal Morphology and Phase Identifications in Poly(aryl ether ketone)s and Their Copolymers. 2. Poly(oxy-1,4-phenylenecarbonyl-1,3-phenylenecarbonyl-1,4-phenylenene)

The crystal structure and morphology of poly(oxy-1,4-phenylenecarbonyl-1,3-phenylenecarbonyl-1,4-phenylene) [PEKK(I)] containing one meta linkage in the three phenylene repeat units have been studied. Only one crystal form was found based on wide-angle X-ray diffraction (WAXD) and electron diffraction (ED) experiments. The crystal unit cell possesses a two-chain orthorhombic lattice with a = 0.766 nm, b = 0.611 nm, and c = 1.576 nm. It is particularly interesting that the c-axis contains three phenylene groups. Through our morphological observations, lamellar crystals are edge-on in most thin films. The crystal growth direction is along the b-axis, and the a-axis is perpendicular to the film surface. The flat-on lamellae can only be observed in the “eye” of the spherulites. The crystal unit cell dimensions change with the isothermal crystallization temperatures. In particular, the c-axis shrinks drastically with increasing temperature. This can be attributed to the conformation changes in meta-linked phenylene groups, which is evident by comparing the data for PEKK(I), PEKK with all-para-linked phenylene groups, and poly(ether ether ketone) (PEEK).