This presentation will cover a comprehensive program in crosslinked polymer networks at the US Army Research Laboratory, where we investigate the rate dependent mechanical response of these materials 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 and tissue simulants for assessing protective equipment. For rigid networks we have focused on highly crosslinked epoxies and Ring Opening Metathesis Polymerized (ROMP) resins based on polymeric dicyclopentadiene (pDCPD) copolymerized with various norbornene derivatives. For the ROMP resins, a critical molecular weight between crosslinks (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 mode I fracture toughness, poor elongation to break during quasi-static tensile deformation, and substantial radial fracture during high velocity impact. By formulating DCPD copolymers we were able to make crosslinked resins with a glass transition temperature (Tg) of 150 oC that maintained ductility deep in the glassy regime at quasi-static to ballistic strain rates, which is in stark contrast with crosslinked epoxy resins that are typically brittle even with Tg as low as 50 oC. 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 mechanical response of crosslinked polymer networks

254th National Meeting (2017)

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