Performance evolution and structure–property relationships of poly(aryl ether ketone)s under high-temperature supercritical carbon dioxide exposure

Abstract

Supercritical carbon dioxide (S-CO₂) is an emerging working fluid for high-efficiency energy systems, but its high diffusivity and solvent-like character can trigger swelling, plasticization, and property drift in polymer insulation and sealing components. Here, we use the ketone/ether ratio as a molecular-structure variable to clarify why different poly(aryl ether ketone)s (PAEKs) respond differently to high-temperature S-CO₂ exposure. PEEK, PEK, and PEKK were aged at 10 MPa under two temperatures (150 °C and 200 °C) and two exposure durations (120 h and 480 h), followed by coupled swelling, spectroscopic, thermal, crystalline rearrangement, mechanical, and dielectric characterization. All three PAEKs retained strong dimensional stability, with mass changes below 1.7% and linear dimensional changes below 1.5%. FTIR and TGA/DTG revealed no detectable chemical degradation, indicating that the aging response was governed mainly by physical rearrangement. However, the mechanical outcomes diverged sharply with chain structure. PEEK and PEK showed hardness loss and increased elongation at break under severe exposure, consistent with S-CO₂-assisted free-volume expansion and plasticization. In contrast, ketone-rich PEKK showed the lowest mass change (0.88%) and the highest first-heating crystallinity (39.34%) after 200 °C/480 h aging, while retaining hardness, tensile stability, and dielectric breakdown resistance. Overall, these results support a crystalline-domain-locking interpretation for ketone-rich PAEKs, in which chain rigidity and exposure-induced crystalline consolidation jointly help resist coupled swelling-plasticization damage in high-temperature S-CO₂ systems.