This presentation will cover a comprehensive program in crosslinked polymer networks at the Army Research Laboratory, where we investigate the rate dependent mechanical response over a broad range of crosslink density. For soft networks we have investigated the role of chain entanglement on the mechanical response, and are developing these materials as testing media for the assessment of military protective equipment. For rigid materials we have focused on highly crosslinked epoxies and Ring Opening Metathesis Polymerized (ROMP) resins based on polymeric dicyclopentadiene (pDCPD). For the ROMP resins, the molecular weight between crosslinks (Mc) was systematically varied by co-polymerizing (DCPD) with either a chain extender that increases Mc, or a di-functional crosslinker, which decreases Mc. A critical Mc of 500 ± 100 g/mol was identified that dictated the failure process. Resins with Mc above this value were ductile, deep in the glassy regime, over a broad range of strain rates. Resins with Mc below this value were brittle, exhibiting low fracture toughness, poor elongation to break, and substantial radial fracture during high velocity impact. We were able to formulate crosslinked resins with a glass transition temperature (Tg) of 150 C that maintained ductility deep in the glassy regime at quasi-static to ballistic strain rates, which is in stark contrast with crosslinked epoxy resins. Molecular dynamics simulations showed that resins with lower Mc were more prone to forming molecular scale nano-voids. We postulate that the higher chain stiffness in low Mc resins inhibits relaxation and plastic deformation in the vicinity of these nanovoids, causing embrittlement. The experimental and computational results suggests that when Mc approaches the persistence length of the crosslinking chains, that the glassy network becomes brittle. These ROMP copolymers offer the potential for new adhesives, resins, and fiber reinforced composites that circumvent the traditional trade-off between structural performance and energy dissipation.

Rate dependent mechanics of crosslinked polymers

255th National Meeting (2018)

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