Chemical Changes in Liquid Benzene Multiply Shock Compressed to 25 GPa

Shock wave experiments utilizing stepwise-loading, with peak stresses ranging between 4 and 25 GPa, were performed to examine the dynamic high pressure response of liquid benzene at thermodynamic conditions not attainable in single shock experiments. Time-resolved Raman spectroscopy was used to monitor the molecular and chemical changes on sub-mu s time scales. Up to 20 GPa, the Raman modes showed pressure-induced shifting and broadening but no indication of a chemical change. At 24.5 GPa, however, the Raman modes become indistinguishable from an increasing background within 40 ns after the sample attained peak pressure, indicating a chemical change. A thermodynamically consistent equation of state (EOS) was developed to calculate the relevant thermodynamic variables in multiply shock compressed liquid benzene. Idealized molecular configurations were used in combination with the thermodynamic quantities in the shocked state to calculate the intermolecular separation between benzene molecules and to ascertain the likelihood of pi-orbital overlap. These idealized calculations show that sufficient energy and pi-orbital overlap exist in multiply shock compressed liquid benzene to permit intermolecular bonding at 24.5 GPa. Analysis of the Raman spectra, using the thermodynamic and intermolecular separation calculations, suggests that benzene undergoes polymerization through cycloaddition reactions. The rapid rate of polymerization is attributed to the benzene remaining in a liquid state on the sub-mu s experimental time scale. The results from the present work demonstrate the importance of time, pressure, temperature, and phase in chemical changes associated with pi-bonded molecules.