First-principles study on the specific heat of glass at glass transition with a case study on glycerol

TL;DR

This paper introduces a first-principles computational method using DFT-based MD simulations to accurately calculate the specific heat of glass at transition, applied to glycerol, and clarifies the energy contributions to the specific heat jump.

Contribution

It develops a non-empirical, first-principles approach to determine the specific heat of glass at transition, addressing longstanding theoretical challenges.

Findings

Successfully calculated the specific heat jump for glycerol.

Identified structural energy as the main contributor to the heat capacity change.

Verified the empirical relationship between fragility and specific-heat jump.

Abstract

The standard method to determine the transition temperature (Tg) of glass transition is the jump in the specific heat. Despite this importance, standard theory for this jump is lacking. The difficulties encompass from lack of proper treatments of specific heat of liquids, hysteresis, to the timescale issue. The first part of this paper provides a non-empirical method to calculate specific heat of glass. The method consists of molecular dynamics (MD) simulations based on density-functional theory (DFT) and thermodynamics methods. The total-energy approach based on DFT enables us to calculate specific heat, irrespective of solids or liquids. A serious problem for glass-transition states is involvement of complicated energy dissipation processes, which is resolved by adiabatic MD simulations. The problems of hysteresis and the timescale issue are alleviated by restricting the scope of calculations to equilibrium states only. The second part of this paper describes an application of the theory to the specific-heat jump (Delta Cp) of glycerol in order to show the validity of the methods. Despite severe limitations due to the small size of supercells, a reasonable value for the specific-heat jump is obtained. By decomposing Delta Cp into contributions of the structural energy, phonon, and thermal expansion parts, we have a sound interpretation for the Delta Cp: the major contribution to Delta Cp comes from the change in the structural energy. From this, a neat energy diagram about the glass transition is obtained. An outcome of this study is verification of the empirical relationship between the fragility and specific-heat jump. These two energies are scaled by the ratio k=Tg/ Delta T, where Delta T is the width of the transition, through which the two quantities are interrelated. This is useful for organizing various relationships that are found empirically.