Correlation between fragility and free volume void size at glass transition temperature

Qingli Ma,Yong Wang,Youlin Gu,Nanxiang Zhao,Sheng Luo,Lei Wang,Jiajie Fang,Yihua Hu

doi:10.1063/5.0069962

Qingli Ma, Yong Wang + Show 6 more

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https://doi.org/10.1063/5.0069962

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Abstract

Despite the tremendous endeavors devoted to exploiting the nature of glass transition, the factors that control the steepness index of viscosity near glass transition, that is, fragility, remain unclear. In this study, we demonstrate that, for polymeric and small molecular weight organic glass formers, fragility increases upward with increasing size of the free volume void at the glass transition temperature. This changing trend indicates that fragility is governed by the properties of the segments or molecular clusters in the free volume void rather than by the properties of the entire polymer chains. The physics behind the relationship between fragility and free volume void at the glass transition temperature is consistent with the physics behind the relationship between fragility and the molecular weight as well as the mechanics of the relationship between fragility and size of the cooperative units. This relationship also provides new insights into the understanding of the nature of the glass transition of polymeric and small molecular weight organic glass formers.

Highlights

The major issue in understanding glass transition is that without a significant structural change, the dynamics of a material may be retarded by several to tens of orders of magnitude within a very narrow temperature range.1 The steepness index of this retardation process is an important parameter for describing the properties of glass-forming liquids and understanding the nature of glass transition
∂T/Tg T=Tg scitation.org/journal/adv where x is the relaxation time τ or the viscosity η, and the glass transition temperature Tg is the temperature at which τ = 100 s or η = 1013 Pa s
Fragility has been correlated with numerous physical parameters of glass-forming liquids, including glass transition temperature, molecular weight, nonergodicity factor, and Poisson’s ratio, a general relationship between fragility and the physical parameters of glass-forming liquids remains lacking, thereby inhibiting the understanding of the nature of glass transition

Summary

Introduction

The major issue in understanding glass transition is that without a significant structural change, the dynamics of a material may be retarded by several to tens of orders of magnitude within a very narrow temperature range.1 The steepness index of this retardation process is an important parameter for describing the properties of glass-forming liquids and understanding the nature of glass transition. This study aims to relate fragility and the size of the free volume void at the glass transition temperature.

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Conclusion