Time-resolved emission of dye probes in a shock-compressed polymer

Simultaneous impact velocity and time-resolved emission measurements are made on shocked poly-methylmethacryalate doped with rhodamine 640 (R640) dye. Planar single-stage shocks in the 0-16 GPa range are produced using a laser-driven flyer plate apparatus. This method allows for reproducible measurements of the time-resolved dye emission intensity, redshift, and spectral width monitored with 1 ns time resolution. The redshift is used to probe the shock front, which has a two-part viscoelastic structure. The maximum dye emission redshift under shock loading stops increasing above similar to 6 GPa. In static high-pressure measurements, the redshift continues to increase up to at least 10 GPa. The smaller redshift seen in shock experiments is not an effect of the shock temperature increase, and it is attributed to incomplete polymer configurational relaxation frustrated by the short duration (<15 ns) of the shock. The viscous relaxation behind the shock front has a part whose rate increases with increasing shock pressure and a part that does not. It is this latter part that causes the shock-induced emission redshift to stop increasing above 6 GPa, and the differences between the two types of relaxation are explained in the context of the free-energy needed to surmount configurational barriers. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4765687]