In situ x-ray diffraction of high density polyethylene during dynamic drive: Polymer chain compression and decomposition

Polymers are widely implemented materials often used for safety and security in our daily lives. However, we do not have a thorough understanding of the phase transformations of these materials when exposed to extreme condition, high strain rate events. To study polymers in these severe environments, we performed plate impact experiments that impart a one-dimensional shockwave, at high pressures and temperatures, to observe the materials response. Generally, velocimetry is the experimental diagnostic used to capture the movement of high velocity interfaces to then calculate the mechanical variables associated with shock compression. However, when a material undergoes a phase transition, such as between two crystalline phases or melting to decomposition during shock compression, velocimetry does not explicitly indicate what structural change occurred. At the Dynamic Compression Sector at the Advanced Photon Source, gas gun driven compression has been paired with x-ray diffraction, providing the ability to study both the shock parameters and structural changes concurrently. We explored high density polyethylene (HDPE) both on its reactants Hugoniot (< 10.12 GPa) and within its transition region (24.3 GPa) where HDPE is hypothesized to decompose from reactants to products. With x-ray diffraction, we observed both polymer chain compression and crystal structure reorientation from orthorhombic to monoclinic above 6 GPa as well as either a possible phase change or chemical decomposition at 24.3 GPa. With these experiments, we were able to determine the crystalline and amorphous Hugoniot loci at pressures below the polymer transition region.