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 γ, 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(γ) < 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 γ in such a way that τα(Ta) ∼ 1/γ at and beyond the yield point. In our experiment, 10–5 s–1 < γ < 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.