Influence of Entanglements on Ultrahigh Strain Rate Deformation of Polystyrene Microprojectiles

Micron-sized polystyrene spherical projectiles made from three different molecular weight polymers spanning from below to well above the entanglement molecular weight were launched against a rigid substrate, causing ultrahigh strain rate deformation. Scanning electron microscopy, focused ion beam cross sections, and atomic force microscopy were used to elucidate the deformation mechanisms from the observed morphologies of the deformed specimens and to evaluate the important role of entanglements in the various deformation processes that dissipate the kinetic energy. Due to adiabatic heating from shock compression, the temperature at the bottom region of the polymer projectile is elevated above the glass transition temperature, enabling viscoplastic flow. The cooler portions of the unentangled sample undergo strain localization via micro-shear banding and brittle fracture, while the two entangled samples resist fracture and exhibit extensive viscoplastic flow and, in the upper, cooler regions, extensive shear banding followed by crazing. The interaction of molecular entanglements, temperature, shear rate, and total shear strain on melt viscosity influences the amount of additional plastic work and adiabatic heating occurring in the sample and hence the extent of lateral spreading of the projectile over the substrate.