Characterizing Thermal Degradation in Semi-Crystalline Thermoplastic Composites

Abstract

Semi-crystalline thermoplastic composites, such as carbon fiber-reinforced polyetheretherketone (PEEK) and polyetherketoneketone (PEKK), are processed and consolidated while melted at high temperatures. During cool-down, polymer chains fold into lamellar structures at the nanoscale to form crystalline morphology. These lamellar structures radiate from a nucleus, creating spherulitic structures in bulk polymers and transcrystallinity in fiber-reinforced polymers. A certain amount of thermal degradation occurs when the thermoplastic matrix is melted, and the amount of degradation is a function of several parameters, such as melting temperature, time at melt, and whether the material is processed in an inert environment such as nitrogen. One form of degradation that occurs in the matrix is cross-linking and oxidation. In this case, the polymer chain breaks and new bonds form between chains or within the chain. Moreover, thermal degradation affects crystallization, lamellar thickness and spacing, and the overall degree of crystallinity. The spacing between lamellar structures can be measured through Small Angle X-ray Scattering (SAXS) at the nanoscale, while the degree of crystallinity can be found using Wide Angle X-ray Scattering (WAXS). To study thermal degradation, the effects of melting temperature, environmental condition, and reprocessing were investigated using samples of neat PEEK and carbon-fiber PEKK prepreg. These samples were thermally cycled multiple times, with repeats performed for each condition. Degree of crystallinity, spacing, and lamellar thickness were measured using an X-ray scattering system. To study the underlying physics and correlations, a probabilistic machine-learning framework was used for regression. Using this approach, different thermal-degradation mechanisms for neat resin and prepreg samples were demonstrated at the nanoscale. The differences were explained in terms of crystallinity and nucleation around fibers in prepreg. This framework provides a holistic understanding of crystal formation and degradation, which ultimately affects the reprocessability and end-part properties of semi-crystalline thermoplastic composites.