(311a) Representing Stereoisomer Effects in a Coarse-Grained Model of Thermoplastic Polymer Poly(etherketoneketone)

Thermoplastic polymers are gaining traction as replacements for thermosets as the matrix phase in aerospace composite materials due to their superior mechanical properties, recyclability, and improved joining through fusion welding. In parts manufactured from fusion welding, entanglements across the interface are the primary load bearing mechanisms. It is therefore important to understand how entanglements develop during fusion welding at the molecular level. Molecular dynamics simulations give unique insight into the fusion welding process, but are limited by the long time scales required for modeling polymer diffusion and entanglement. In this work, we address the time and length scale challenges by using multi-state iterative Boltzmann inversion (MSIBI) to create a coarse-grain model of poly(etherketoneketone) (PEKK) which is a thermoplastic utilized in aerospace composite materials. PEKK monomers can take the form of two possible stereoisomers where the difference is the para- and meta- bonding position between consecutive ketone groups. The relative amount of para and meta linkages has significant effects on the properties of PEKK, including melting temperature, and crystallization kinetics, both of which play key roles during the fusion welding process. We highlight the challenges with representing these atomistic linkages within a coarse model, and demonstrate an approach that accounts for these stereoisomer effects. We show that the coarse model accurately recreates structural distributions across a range of relevant thermodynamic states and accurately represents key structural chain metrics such as radius of gyration, end-to-end distance, and persistence length. In sum, we parameterize and validate a coarse-grained model of PEKK that enables entanglement dynamics to be interrogated in simulated fusion welds. This research was supported by the National Aeronautics and Space Administration (NASA) under the University Leadership Initiative program; grant number 80NSSC20M0165.