Dielectric Spectroscopy of a Stretched Polymer Glass: Heterogeneous Dynamics and Plasticity.

We study the dielectric relaxation of polycarbonate (PC) at room temperature under imposed strain rate γ̇, above the yield stress, and up to 13% strain. We find that the dielectric response of stretched PC behaves as if it was heated up at a temperature just below its glass transition temperature, Tg ≈ 423 K for PC. Indeed, in the frequency range of our experiment (10-2 and 10³ Hz), the dielectric response of the stretched PC at room temperature superimposes to the dielectric response of PC at a temperature Ta(γ̇) < Tg, which is a function of strain rate. Specifically we observe that at Ta the dominant relaxation time τα(Ta) of PC at rest is related to γ̇ in such a way that τα(Ta) ∼ 1/γ̇ at and beyond the yield point. In our experiment, 10-5 s-1 < γ̇ < 10-3 s-1, the temperature shifts Tg - Ta are of a few kelvin. The mechanical rejuvenation modifies the dielectric response at frequencies smaller than 10 Hz, whereas for higher frequencies the spectrum is only slightly modified. [ABSTRACT FROM AUTHOR]