Quantitative Structural Study of Cold-Crystallized PEKK

Poly(ether ketone ketone) (PEKK) is a semicrystalline polymer investigated for highly demanding applications in aerospace, transportation, electronics, and oil & gas industries. The properties required for these applications, such as thermomechanical and chemical stability, are intimately linked to the crystalline state of the material. PEKK exhibits a polymorphism that depends, among other factors, on its thermal history. The control and quantification of the crystalline state of PEKK is the subject of this study. Amorphous PEKK (T/I = 60/40, where T stands for terephthalic and I for isophthalic units) films were cold-crystallized at various crystallization temperatures, TC, from 180 to 280 °C in a ventilated oven. Based on a quantitative analysis of small-angle X-ray scattering (SAXS), wide-angle X-ray scattering (WAXS), and differential scanning calorimetry (DSC) data, we propose for the first time a methodology to obtain detailed information about the crystalline state of PEKK: total crystallinity, relative amount and stability of crystalline forms, form I and form II, and melting enthalpy of 100% crystallized PEKK. The evolutions of each crystalline form and the total crystallinity with TC were deduced. The amount of each crystalline form, form I and form II, can be tuned by controlling the heating rate to 280 °C. The evolution of the crystalline lamellar thickness and periodicity as well as the crystalline amount and cell parameters with TC were interpreted and discussed in relation to the published results on PEEK, PEKK (100% T), and PEKK (100% I). Finally, the melting enthalpy of a 100% crystallized PEKK copolymer (T/I = 60/40), ΔHm100% = (202 ± 20) J/g, was estimated.