Crystallization of poly(aryl ether ketone ketone) copolymers containing terephthalate/isophthalate moieties

In a previous study, we have investigated the structure, crystallization, and morphology of poly(aryl ether ketone ketone), PEKK, copolymers prepared from diphenyl ether (DPE), terephthalic acid (T), and isophthalic acid (I) with T/I ratios from 100/0 to 50/50. These materials were considered as having ‐DPE‐T‐DPE‐T‐ (TT) and/or ‐DPE‐T‐DPE‐I‐ (TI) “phthalate diads.” In this work, we continue the study of this copolymer series with six different T/I ratios (40/60, 30/70, 20/80, 15/85, 10/90, and 0/100), which are viewed as having TI and/or ‐DPE‐I‐DPE‐I‐ (II) “diads.” The I moieties (1,3‐linked isomers) were always found to be incorporated in the crystals and acted as “entropy or symmetry” defects that effectively decreased the equilibrium melting temperature Tmo and the rate of crystallization. However, the retardation of crystallization in PEKK 0/100 (the homopolymer with pure II diads) was significantly less than expected, which was attributed to the segregation of I moieties between the chains leading to a reduction of total entropy in the unit cells. The evidence of segmental segregation in PEKK 0/100 was seen in x‐ray diffraction patterns, where several extra reflections were seen that could only be indexed by the published unit cell modified with a larger c‐axis dimension (3.048 nm, corresponding to the length of six phenyl residues or 1.524 nm, the length of three phenyl residues). The composition of 15/85 was found to have the lowest value of Tmo and the slowest crystallization rate. Upon heating, the “II” crystals (T/I from 30/70 to 0/100) exhibited the conventional double‐melting behavior rather than the triple‐melting behavior as in the “TI” crystals (50/50 to 40/60). No indication of the second polymorph form 2 was found in “II” crystals.