Crystallization kinetics and equilibrium melting temperature of poly(ether ketone ketone) with high terephthalate content utilizing fast scanning calorimetry

Abstract

Fast scanning calorimetry (FSC) was used to analyze the crystallization kinetics and determine the equilibrium melting temperature (Tmo) of KEPSTAN PEKK 8001 (PEKK 80/20) with a terephthalate to isophthalate (T/I) ratio of 80/20. Quantitative 1H, qualitative 13C and 2D HSQC NMR spectroscopy for KEPSTAN 8001 and KEPSTAN 7002 were collected to confirm linear topology and comonomer composition. Utilizing FSC, the isothermal crystallization kinetics for PEKK 80/20 are reported for the first time over a large temperature range from 200 °C to 310 °C utilizing an interrupted isothermal crystallization method. The crystallization kinetics of PEKK 80/20 exhibits parabolic behavior with the most rapid crystallization occurring at 260 °C. Compared to earlier studies of PEKK copolymers with lower T/I ratios (i.e., PEKK 70/30 and 60/40) using conventional DSC, PEKK 80/20 is observed to crystallize up to 1 to 2 orders of magnitude faster (i.e., at rates inaccessible with conventional DSC). The Tmo of PEKK 80/20 was determined utilizing the zero-entropy-production temperature (TZEP) allowing for the first time an accurate determination of the equilibrium melting temperature of 382 °C. This value was compared to a traditional Hoffman-Weeks linear extrapolation over a range of heating rates (500 K/s – 20,000 K/s) to arrest melt-reorganization upon heating. Due to unavoidable crystalline reorganization during slow heating scans with conventional DSC, this new value for Tmo using FSC is significantly higher than the commonly referenced literature value of 368 °C. The double melting peak behavior observed after isothermal crystallization of PEKK 80/20 is discussed and understood to be due to reorganization during heating at relatively slow heating rates, as commonly observed with conventional DSC.