Enthalpy and high temperature relaxation kinetics of stable vapor-deposited glasses of toluene

TL;DR

This study investigates the structure, stability, and transformation kinetics of vapor-deposited toluene glasses using fast scanning calorimetry, revealing how deposition rate and film thickness influence their properties and stability.

Contribution

It provides new insights into the kinetic stability and thermodynamic properties of vapor-deposited toluene glasses, highlighting the effects of deposition rate and film thickness on their transformation behavior.

Findings

Transformation follows zero-order kinetics at high heating rates

Transformation rate increases with deposition rate in a sub-linear manner

Thinner films (<250 nm) show increased transformation rates linked to surface roughness

Abstract

Stable non-crystalline toluene films of micrometer and nanometer thicknesses were grown by vapor deposition at distinct rates and probed by Fast Scanning Calorimetry. Fast scanning calorimetry is shown to be extremely sensitive to the structure of the vapor-deposited phase and was used to characterize simultaneously its kinetic stability and its thermodynamic properties. According to our analysis, transformation of vapor -deposited samples of toluene during heating with rates in excess 10^5 K/s follows the zero-order kinetics. The transformation rate correlates strongly with the initial enthalpy of the sample, which increases with the deposition rate according to sub-linear law. Analysis of the transformation kinetics of vapor deposited toluene films of various thicknesses reveal a sudden increase in the transformation rate for films thinner than 250 nm. The change in kinetics correlates with the surface roughness scale of the substrate, which is interpreted as evidence for kinetic anisotropy of the samples. The implications of these findings for the formation mechanism and structure of vapor deposited stable glasses are discussed.