Extreme Plastic Deformation of Glassy Polymer Thin Films at Ballistic Strain Rates

Polystyrene (PS), is a brittle, glassy solid at room temperature and absorbs little energy during deformation. When confined in a very thin film, the behavior of high molecular weight polymers is modified. Here we show unexpected ductile deformation mechanisms resulting in unprecedented energy absorption by high molecular weight thin freestanding PS films at extreme deformation rates created by impact of micron sized projectiles. We use a laser-induced projectile impact testing apparatus to launch 3.7 micron diameter silica projectiles to impact the substrate-free thin films mounted across a TEM grid to induce high strain rate deformation (~ 10⁷ s^-1). For the range of film thicknesses investigated, the ratio of the projectile diameter to the film thickness, D/h, varies from approximately 50:1 to 25:1 to 13:1. The impact produces axisymmetric tensile loading of a thin membrane with the principal stresses along the radial and tangential directions of a circular impact region. The supersonic projectiles initiate deformation zones, crazing and adiabatic heating leading to extensive plastic flow of a viscoelastic melt prior to perforation and film rupture. We investigate films of 75, 150, 290 and 550nm and find a strong thickness dependence of the specific energy absorption. This suggests that the less entangled near-surface region of free standing films likely plays an increased role in nucleating deformation zones and crazes as the film thickness decreases. While other materials adiabatically heat during rapid deformation, only polymers have a load-bearing viscoelastic melt state due to chain entanglements and strong frictional forces from the sliding of the long and aligning chains. These ductile deformation processes result in record high specific energy absorption (3 MJ/kg for the 75 nm PS film at 800 m/s) at extreme strain rates in what is normally considered a brittle material.