The yield stress of a polymer increases with strain rate but the fracture stress is almost constant, inducing a ductile to brittle transition. At 20 °C polystyrene (PS) is a brittle, glassy polymer. Under tensile loading, fracture of PS is preceded by crazing, a dilational plastic deformation mechanism that leads to local energy dissipation ahead of the propagating crack. Plastic deformation can also induce shear bands. If crazing and shear banding can be made extensive, plastic deformation can dissipate large amounts of energy, and if occurring at very high loading rates, substantial sample heating can lower both the crazing stress and the yield stress leading to further adiabatic heating and plastic flow rather than brittle behavior.<br/>Chains near the surface regions of free standing glassy polymer films have a higher mobility and lower entanglement density lowering the crazing and yield stresses. We investigate PS and PMMA films of various molecular weights (30-400 kg/mol) and thicknesses (30-300 nm), impacted by μ-size spherical projectiles at incident velocities of 350-800 m/s. Additionally, we explore polymer grafted nanoparticle films (~ 1-5% v/v NPs) in order to observe the influence of the 100’s of chains covalently anchored to each NP on the deformation. We find unprecedented energy absorption for entangled PS films and even greater energy absorption for entangled PS grafted to NPs. The observed deformation is comprised of extensive multiaxis crazing, tangential craze widening with strong adiabatic heating of the impact region leading to plastic flow of a viscoelastic melt and record high specific energy absorption at strain rates of ~ 10<sup>7</sup>/s.

Specific energy absorption of freestanding glassy homopolymer and homopolymer grafted nanoparticle thin films at extreme strain rates

256th National Meeting (2018)

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