Structural Relaxation during the Strain-rate Switching of Polymeric Glasses

In the flow state of a polymeric glass under deformation, when the strain rate is abruptly lowered, stress undershoot occurs. This cannot be explained by a simple transition of the system between two minima in the potential energy landscape (PEL). We stretch poly(methyl methacrylate) glass films at a strain rate of 10^-5 s^-1 at a temperature Tg-20K and measure in situ the segmental relaxation time using a photobleaching technique. The segmental relaxation time provides access to the position of the system on the PEL (or more generally, a state variable). After the yield, in the steady flow region, we switch the applied strain rate to 10^-6 s^-1 and observe the stress undershoot. The segmental relaxation time shows a delay in the response during the strain rate switching. We have extracted the characteristic timescale associated with this memory/delay, which is ~ t_a, the segmental relaxation time in the undeformed state. After the initial delay, the segmental relaxation time reaches a steady state value that depends upon the applied strain rate and is independent of the previous mechanical history. These results support the approach, taken in several models, that postulates an internal variable that does not respond instantaneously to a change in the strain rate.