Manifestations of crystallization in the processing of high-performance thermoplastic composites

The processing of semi-crystalline thermoplastic matrix composites is a complicated process that requires an in-depth understanding of the complete processing cycle, from heating and melt processing to solidification. Because the cooling step is when the microstructure develops in these materials, and microstructure highly influences the material's performance, the solidification process is of primary concern when designing end-use properties. Additionally, the stability of these materials, both during and after processing, is now a prime concern as far as utilizing these materials in demanding and often intolerant aerospace applications and has caused these materials to fall from grace. Previous studies have focused on many of the individual aspects of thermoplastic composite processing, yet none has focused on overall processing characterization and how semi-crystallinity plays a role. In this investigation, three model material systems were utilized, by first characterizing their crystallization kinetics and then applying these results when characterizing their processing and solidification behavior. Specifically, the three model material systems used were polyphenylene sulfide (PPS), polyetherketonketone (PEKK), and new-thermoplastic polyimide (new-TPI). They exhibited varied kinetic behavior with fast, "normal", and slow crystallization kinetics respectively, which was then applied to examining their contrasting processing behavior. Through this work, the irreversible behavior of thermoplastics, as manifested by a loss in crystallizability, was experienced and further probed. A broad based investigation was conducted to quantify and further understand this relatively unrecognized phenomenon by introducing the concept of "bulk-equivalency", by which the neat polymer material is not the same material once it is used as a matrix. Overall, this investigation brings together several issues in thermoplastics into a processing overview by continually focusing on the structure-processing relationship during and after processing and also opens avenues for further research into non-linear thermoplastic behavior.