POLYMORPHISM AND CRYSTAL-STRUCTURE IDENTIFICATION IN POLY(ARYL ETHER KETONE KETONE)S

Poly(aryl ether ketone ketone)s (PEKKs) are prepared via a Friedel-Cra fts acylation reaction with diphenyl ether and terephthalic acid (T) a nd/or isophthalic acid (I). The crystal structure, polymorphism and mo rphology of three PEKK homopolymers have been investigated using trans mission electron microscopy (TEM), electron diffraction (ED) and wide angle X-ray diffraction (WAXD) experiments. The PEKKs studied consist of either all para-phenylene linkages [PEKK(T)], one meta-phenylene li nkage [PEKK(I)] or alternating T and I isomers [PEKK(T/I)]. PEKK(T) an d PEKK(T/I) show crystalline polymorphism depending upon the crystalli zation conditions and methods. In PEKK(T) two forms (I and II) can be observed, and their unit cell dimensions and symmetry groups are deter mined via ED experiments. A possible epitaxial relationship between th ese two forms is proposed when they coexist. PEKK(I) possesses only on e crystal unit cell which belongs to the same category as the form I s tructure in PEKK(T). The c-axis dimension of the unit cell has been de termined as three phenylenes having a zig-zag conformation, with the m eta-phenylene lying on the backbone plane. PEKK(T/I) shows a complicat ed polymorphism. In addition to forms I and II, as in the case of PEKK (T), a possible form III is also observed.