Reproducible Infrastructure for Modeling Thermoplastic Polymer Fusion Welding

Aircraft materials must balance the demands of being light-weight and fuel efficient with being safe, durable, and strong. Carbon-fiber reinforced polymer composites are able to meet both of these requirements, and as a result, they are being utilized more and more in aircraft manufacturing. Thermoset polymers are typically used as the matrix material in these composites; however, thermoset polymers require long curing times in large autoclave ovens in order to form chemical cross-links, which once present, cannot be melted upon further heating. As a result, joining smaller thermoset composite parts together to build larger structures requires using mechanical fasteners, which act as stress risers and potential corrosion sites, or adhesives, which require intensive surface preparation. Because of these long curing times, and challenges of joining parts together, interest is growing in replacing thermosets with thermoplastic polymers as the matrix material. Unlike thermosets, thermoplastics can be heated to melt, and cooled to re-solidify. This feature of thermoplastics offers the ability to utilize fusion welding joining between parts instead of mechanical fasteners or adhesives. Before thermoplastic carbon-fiber composites can become widely adopted in critical aircraft parts, it is needed to develop a deeper understanding between material processing, properties and performance in order to meet the stringent requirements of Federal Aviation Administration (FAA) certification and regulations. Molecular dynamics (MD) offers the ability to gain atomistic insight into fusion welding of polymer materials, where polymer diffusion and entanglements can be observed and analyzed in ways inaccessible to experimental methods. However, computational limitations of MD simulations, combined with the large length scales and slow relaxation times of polymers are a significant obstacle. Furthermore, standardized methods, and the tools needed to reproducibly perform fusion welding simulation workflows are not readily available. In this work, coarse-grained models of high-performance thermoplastics poly(ether ketone ketone) (PEKK) and poly(phenylene sulfide) (PPS) are derived from atomistic models in order to reasonably access the length and time scales needed to form polymer diffusion and entanglements across a polymer-polymer interface. An open-source software package is designed in an effort to make thermoplastic fusion welding modeling more accessible while also being transparent, reproducible, usable by others, and extensible to other materials and polymer models. Fusion welds are performed using a coarse-grain model of PPS at several chain lengths, and we perform tensile test simulations to understand the mechanical properties of the welded interface.