Understanding temperature modulated calorimetry through studies of a model system

TL;DR

This paper investigates the fundamental aspects of temperature modulated calorimetry using a model system with complex relaxation dynamics, clarifying its relation to entropy production, aging, and memory effects in glassy materials.

Contribution

It provides a detailed analysis of temperature modulated calorimetry's capabilities and limitations, highlighting its relation to relaxation times, aging, and the Kovacs effect in out-of-equilibrium systems.

Findings

Temperature modulated calorimetry relates to entropy production.

It can observe aging and memory effects in glasses.

It complements standard calorimetry but does not detect all relaxation phenomena.

Abstract

Temperature Modulated calorimetry is widely used but still raises some fundamental questions. In this paper we study a model system as a test sample to address some of them. The model has a nontrivial spectrum of relaxation times. We investigate temperature modulated calorimetry at constant average temperature to precise the meaning of the frequency-dependent heat capacity, its relation with entropy production, and how such measurements can observe the aging of a glassy sample leading to a time-dependent heat capacity. The study of the Kovacs effect for an out-of-equilibrium system shows how temperature modulated calorimetry could contribute to the understanding of this memory effect. Then we compare measurements of standard scanning calorimetry and temperature-modulated calorimetry and show how the two methods are complementary because they do not observe the same features. While it can probe the time scales of energy transfers in a system, even in the limit of low frequency temperature modulated calorimetry does not probe some relaxation phenomena which can be measured by scanning calorimetry, as suggested by experiments with glasses.