Unveiling the dependence of glass transitions on mixing thermodynamics in miscible systems.

Wenkang Tu,Xin Li,Shaohua Jin,Yongjun Tian,Yunxi Wang,Li-Min Wang,Peng Zhang

doi:10.1038/srep08500

Wenkang Tu, Xin Li + Show 5 more

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https://doi.org/10.1038/srep08500

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Abstract

The dependence of the glass transition in mixtures on mixing thermodynamics is examined by focusing on enthalpy of mixing, ΔHmix with the change in sign (positive vs. negative) and magnitude (small vs. large). The effects of positive and negative ΔHmix are demonstrated based on two isomeric systems of o- vs. m- methoxymethylbenzene (MMB) and o- vs. m- dibromobenzene (DBB) with comparably small absolute ΔHmix. Two opposite composition dependences of the glass transition temperature, Tg, are observed with the MMB mixtures showing a distinct negative deviation from the ideal mixing rule and the DBB mixtures having a marginally positive deviation. The system of 1, 2- propanediamine (12PDA) vs. propylene glycol (PG) with large and negative ΔHmix is compared with the systems of small ΔHmix, and a considerably positive Tg shift is seen. Models involving the properties of pure components such as Tg, glass transition heat capacity increment, ΔCp, and density, ρ, do not interpret the observed Tg shifts in the systems. In contrast, a linear correlation is revealed between ΔHmix and maximum Tg shifts.

Highlights

IntroductionThe dependence of the glass transition in mixtures on mixing thermodynamics is examined by focusing on enthalpy of mixing, DHmix with the change in sign (positive vs. negative) and magnitude (small vs. large)
The dependence of the glass transition in mixtures on mixing thermodynamics is examined by focusing on enthalpy of mixing, DHmix with the change in sign and magnitude
The system of 1, 2- propanediamine (12PDA) vs. propylene glycol (PG) with large and negative DHmix is compared with the systems of small DHmix, and a considerably positive Tg shift is seen

Summary

Introduction

The dependence of the glass transition in mixtures on mixing thermodynamics is examined by focusing on enthalpy of mixing, DHmix with the change in sign (positive vs. negative) and magnitude (small vs. large). Mixing thermodynamics is argued to determine the glass transition behaviors in mixtures, and studies found that positive enthalpy of mixing generally corresponds to negative Tg deviation[5], and even a value of DHmix as low as 200 J/mol can result in a remarkably negative Tg shift[21]. We studied the glass transitions in binary glass forming molecular systems, and systematic comparison is shown with typical scenarios of small vs large and positive vs negative DHmix. The results are expected to benefit the understanding of the glass transition as well as the precise evaluation of Tg in poor glass formers

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