Temperature of polymethyl methacrylate in the secondary shock wave

The brightness temperature of polymethyl methacrylate loaded by a shock wave reflected from a transparent LiF or sapphire window was measured by a pyrometric method. The pressure range in the primary wave was 20–40 GPa, and that in the reflected wave was 30–78 GPa. At the intensity of the primary shock wave in the range of low pressures (19.2–20.8 GPa), the temperature values in the reflected wave lie on a single Hugoniot within the experimental error, as opposed to what might be expected from the gas-dynamic calculation for the case of loading by two shock waves. As the primary shock wave pressure increases to 38–41 GPa, the reflected wave temperature ((2.4–2.6) $\cdot$ 10$^3$ K) becomes smaller than on the single Hugoniot (3.1 $\cdot$ 10$^3$ K), but remain larger than those calculated by the equation of state (2.3 $\cdot$ 10$^3$ K) under the assumption that the composition of polymethyl methacrylate is inert. It is concluded that this form of the temperature dependence is due to the depolymerization and subsequent decomposition of polymethyl methacrylate.