Impact of Processing Conditions on the Crystallization of an Alternated Tere/Iso PEKK Copolymer

The crystallization behavior of a poly(ether ketone ketone) copolymer constituted of alternated terephthaloyl (T) and isophthaloyl (I) units (PEKK 50/50) is studied as a function of temperature by simultaneous wide- and small-angle X-ray scattering (WAXS and SAXS). Three samples, having different thermal or processing histories, are compared, and the evolution of their crystalline morphology, crystallinity, and crystal form is established in detail over a wide temperature window, from ∼30 °C up to the full melting at 345 °C, which is compared to the thermal behavior observed by differential scanning calorimetry. Whatever the initial state of this copolymer, amorphous, crystalline form I or form II, it finishes in form I above ∼280 °C. We show that the previously reported form III is a defective form I (named here form I′). We also show, for the first time, the existence of a defective form II (named form II′). Form II′ transforms first to form I′ and finally to form I above ∼305 °C. The thermal expansion of the crystal cell of the defective form I′ is clearly higher than the one of the most perfect form I. This is very similar to the behavior of defective phases of other semicrystalline polymers such as polyamide 11 and some fluorinated copolymers, for instance. Moreover, the full analysis of the WAXS spectra gave very valuable information about the amorphous fractions of the polymer chains, their relative density, and their evolution as a function of temperature. It turns out that the sample issued directly from the synthesis process exhibits the most dense and constrained amorphous phase coexisting with crystals of form II′. Finally, it is worth noting that whatever the thermal or processing history of this PEKK, its thermal behavior above ∼280 °C becomes independent of thermal history and therefore is always predictable, which is an advantage in industrial applications.