Effect of supercritical CO₂ aging on the microstructure and mechanical properties of PEEK

Polyetheretherketone (PEEK) is a promising material for oil and gas (O&G) downhole applications, having replaced metal parts owing to its excellent mechanical properties and corrosion resistance. In operations related to carbon capture and storage (CCS), CO₂-based geothermal systems, and enhanced oil recovery (EOR), carbon dioxide (CO₂) is injected into wells under conditions of high pressure and temperature, resulting in supercritical states (scCO₂). Herein, the effect of scCO₂ on the microstructural and mechanical characteristics of PEEK was evaluated, particularly under conditions that simulated the extreme environments encountered in O&G downhole applications. Aging experiments were conducted on PEEK samples under scCO₂ exposure at 150 °C and 100 bar in addition to gaseous CO₂, argon, and vacuum conditions to delineate the specific impacts of each agent. Thermal analysis via differential scanning calorimetry (DSC) unveiled a secondary crystal peak formation at 197 °C exclusively under scCO₂ exposure, indicating significant microstructural changes with a 9% increase in crystallinity. Dynamic mechanical analysis (DMA) showed a notable increase of 6 °C in glass-transition temperature (T_g) under scCO₂ exposure, suggesting enhanced thermal stability and reduced molecular mobility. In addition, mechanical tests revealed a 25% increase in yield strength, 8% increase in ultimate strength, and a considerable reduction in elongation at break due to enhanced crystallinity and limited amorphous phase mobility.