Blocky bromination of poly(ether ketone ketone) as a means to preserve crystallizability and rapid crystallization kinetics

Abstract

In this work, the post-polymerization bromination of poly(ether ketone ketone) (PEKK, KEPSTAN 8001, T/I = 80/20) with N-bromosuccinimide is reported for the first time. Two microstructures are synthesized – a random microstructure through functionalization in the homogeneous solution state and a blocky microstructure through functionalization in the heterogeneous gel state from a benign solvent, diphenyl acetone. The resultant gel exhibits a nanoscale fibrillar-network morphology with porosity and surface areas of 85.5% ± 3 and 152 ± 12 m² g⁻¹, respectively. Degrees of bromination between 12–62 mol% were obtained for both the random and blocky brominated PEKK. At similar degrees of bromination, blocky brominated PEKK exhibited increased glass transition temperatures, greater crystallizability, higher melting temperatures and faster crystallization kinetics as compared to the randomly brominated analogs. The crystallization kinetics were analyzed over a range of isothermal crystallization temperatures (210 °C–290 °C) utilizing fast scanning calorimetry. For the random microstructure, the isothermal crystallization temperature resulting in the fastest crystallization kinetics decreased to lower temperatures with increasing mol% bromination, attributed to a comonomer confinement effect. In contrast, the temperature of fastest crystallization for the blocky microstructures remained relatively constant with increasing mol% bromination and similar to that of pure, unfunctionalized PEKK. This behavior was attributed to the longer runs of pristine PEKK units between brominated monomers compared to the random analogs, resulting in a negligible confinement effect. SAXS/WAXS analysis showed that the electron density of the amorphous phase is affected by the copolymer microstructure. It is shown that blocky functionalization of PEKK in the gel state preserves crystallizability and high melting temperatures with minor decreases in crystallization rates. This finding further demonstrates blocky functionalization in the gel state is a facile way to effectively block up functionality while preserving long runs of crystallizable segments along the chains.