Nonlinear thermal response at the glass transition

Publisher Website
Google Scholar
Abstract
In linear response, the complex heat capacity is equal to the entropy compliance divided by temperature. The fluctuation dissipation theorem gives a relation to the correlation function (time domain) or spectral density (frequency domain) of entropy fluctuations. Another formal description of frequency dependent heat capacity at the glass transition is based on the Tool–Narayanaswamy–Moynihan approach of structural recovery. In the limit of small temperature perturbations both models are able to describe measured complex heat capacity. We have measured the response at medium temperature amplitudes, when first nonlinearities appear, and have compared the results with the behavior expected from both approaches. We have studied higher harmonics in the temperature amplitude range 0.1–15 K. At the glass transition of poly(vinyl acetate) (PVAc) we observe 1% of higher harmonics at temperature amplitudes of 1 K. Even harmonics can be described quantitatively by the temperature dependence of first harmonic (heat capacity) up to temperature amplitudes of 6 K. This is not possible for odd harmonics. The remaining small odd harmonics support, on a qualitative basis, the fluctuation approach to glass transition.