Polymorphism and crystal structure identification in poly(aryl ether ketone ketone)s

Poly(aryl ether ketone ketone)s (PEKKs) are prepared via a Friedel-Crafts acylation reaction with diphenyl ether and terephthalic acid (T) and/or isophthalic acid (I). The crystal structure, polymorphism and morphology of three PEKK homopolymers have been investigated using transmission electron microscopy (TEM), electron diffraction (ED) and wide angle X-ray diffraction (WAXD) experiments. The PEKKs studied consist of either all pura-phenylene linkages [PEKK(T)], one meta-phenylene linkage [PEKK(I)] or alternating T and I isomers [PEKK(T/I)]. PEKK(T) and PEKK(T/I) show crystalline polymorphism depending upon the crystallization conditions and methods. In PEKK(T) two forms (I and II) can be observed, and their unit cell dimensions and symmetry groups are determined via ED experiments. A possible epitaxial relationship between these two forms is proposed when they coexist. PEKK(I) possesses only one crystal unit cell which belongs to the same category as the form I structure in PEKK(T). The c-axis dimension of the unit cell has been determined as three phenylenes having a zig-zag conformation, with the meta-phenylene lying on the backbone plane. PEKK(T/I) shows a complicated polymorphism. In addition to forms I and II, as in the case of PEKK(T), a possible form III is also observed.