Mechanism of structural evolution during thermal stretching of PEEK

Abstract

Poly (ether ether ketone) (PEEK) is widely utilized due to its outstanding properties. Elucidating the conformational changes of PEEK under mechanical stress and their impact on crystalline morphology will offer an in-depth and systematic insight into the processing behavior of the poly (arylether ketone ketone) family. This study utilizes in-situ WAXD/SAXS and FTIR analyses, complemented by DSC characterization, to systematically explore the mechanism of aggregated state structure evolution in PEEK films subjected to high-temperature stress fields. Findings from the in-situ WAXD/SAXS tests indicate that the PEEK molecular chains orient along the direction of applied stress, which induces crystal fragmentation or melting. During the strain-hardening stage of thermal stretching process, the molecular orientation increases linearly with strain, along with the formation of form II crystalline structures. Higher strain and elevated stretching temperatures favor the formation of form II crystals. According to in-situ FTIR measurements, the movement of ether bonds during stretching triggered conformational changes that strengthened intermolecular interactions within the PEEK molecular chains. The DSC analysis demonstrated that form II crystals possess a higher melting temperature and enhanced stability. This work not only enriches the theoretical system of poly (arylether ketone ketone) series, but also provides a detailed theoretical basis for regulating the processing performance of PEEK.