Polarization fluctuations near the glass transition

Measurements of polarization fluctuations were performed for an epoxy glass former. The voltage noise produced by polarization fluctuations of the sample filling a capacitor was acquired via a home-made very high input impedance current–voltage converter, in series with an ultra low noise pre-amplifier, achieving high sensitivity and accuracy in the range 0.1–1000 Hz. The temperature and frequency dependence of polarization noise was investigated above and below the glass transition temperature Tg. The sample was driven to the glassy state with different cooling rate and then isothermally aged, while the noise spectral density was measured at different times and compared with the Johnson–Nyquist noise determined by the sample impedance measured by conventional dielectric spectroscopy. At thermodynamic equilibrium the polarization noise agreed with the predictions of the fluctuation–dissipation theorem linking noise spectral density to susceptibility. On the contrary, a strong violation of the theorem was observed after a fast cooling below Tg: an intense polarization noise was detected, with a power spectral density following an inverse power law frequency behavior, whose intensity was decreasing with aging time. At the same time, the amplitude of polarization fluctuations showed a non-Gaussian distribution, whose width reduced during the aging process, up to recover the Gaussian statistics on approaching the equilibrium.