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, T g ≈ 423 K for PC. Indeed, in the frequency range of our experiment (10 –2 and 10 3 Hz), the dielectric response of the stretched PC at room temperature superimposes to the dielectric response of PC at a temperature T a (γ̇) < T g, which is a function of strain rate. Specifically we observe that at T a the dominant relaxation time τ α ( T a ) of PC at rest is related to γ̇ in such a way that τ α ( T a ) ∼ 1/γ̇ at and beyond the yield point. In our experiment, 10 –5 s –1 < γ̇ < 10 –3 s –1, the temperature shifts T g – T a 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.