Dynamics of polymer response to nanosecond shock compression

The high strain rate mechanical dynamics of a polymer under shock compression, poly-methyl methacrylate (PMMA, MW 996 000), were probed in experiments that combined two measurement techniques. The km s(-1) impacts with laser-launched flyer plates were characterized by a fast interferometer, and the polymer dynamics were characterized by the nanosecond emission redshifting of Rhodamine 6G dye in PMMA. The thicknesses and velocities of the flyer plates were varied to create shock durations in the 5-22 ns range and pressures in the 2.6-9.2 GPa range. The strain rates were on the order of 10(7) s(-1). With shock durations greater than 10 ns, a two-part rise of the redshift transients was observed, which was interpreted with reference to the well-known viscoelastic shock compression paradigm. However, the initial compression stage, lasting similar to 10 ns, was found to be only partially elastic. With an elastic compression, after a shock the density should promptly return to its initial value. But even with the shortest-duration 5 ns shocks, the density relaxation after the shock ended was quite small. The critical shear stress where PMMA loses strength and the viscous compression process begins, appeared to be considerably larger with nanosecond shocks than with microsecond shocks. The loss of strength always took similar to 10 ns, and could not be accelerated by increasing the shock pressure, suggesting the rate of strength loss was limited by the need for collective motions of the 30-40 nm polymer chains. The rates and amplitudes of the viscous density relaxation process increased with shock duration and shock pressure. (C) 2014 AIP Publishing LLC.