Glass transition and mixing thermodynamics of a binary eutectic system

Abstract

A quantitative evaluation of the contribution of mixing thermodynamics to glass transition is performed for a binary eutectic benzil and m-nitroaniline system. The microcalorimetric measurements of the enthalpy of mixing give small and positive values, typically ~200 J mol(-1) for the equimolar mixture. The composition dependence of the glass transition temperature, T(g), is found to show a large and negative deviation from the ideal mixing rule. The Gordon-Taylor and Couchman-Karasz models are subsequently applied to interpret the T(g) behavior, however, only a small fraction of the deviation is explained. The analyses of the experimental results manifest quantitatively the importance of the mixing thermodynamics in the glass transition in miscible systems.

Unveiling the Dependence of Glass Transitions on Mixing Thermodynamics in Miscible Systems

“…The system of 12PDA vs. PG has a remarkably large and positive T g deviation with the maximum Δ T g / T g being as high as 14.5%. In contrast, the system of BZL vs. MNA has a large and negative T g deviation with the maximum of Δ T g / T g of −2.3% 21 . For systems with smaller positive or negative Δ H mix , Δ T g / T g becomes smaller.…”

“…The relative T g shift, Δ T g / T g , for the systems of o - vs. m - MMB, o - vs. m - DBB, and 12PDA vs. PG are shown in Fig. 5 , where Δ T g is the T g difference between the measured values and the ones calculated in terms of the linear average of the pure components' T g s. Two more typical systems recorded in our recent work, benzil (BZL) vs. m -nitroaniline (MNA) 21 and methyl m -toluate (MMT) vs. methyl o -toluate (MOT) 24 , are also included in Fig. 5 to represent the mixing scenarios of larger positive Δ H mix and ideal mixing, respectively.…”

“…The mixing thermodynamics for the BZL vs. MNA system ( Fig. 5 ) is recently measured showing the Δ H mix maximum of ~200 J/mol 21 , while the MMT vs. MOT system is regarded as an ideal mixing case with Δ H mix = 0 24 25 . Accordingly, the five cases in Fig.…”

“…The results in this work allow us to check the presumably quantitative correlation between T g and Δ H mix in binary mixtures. In addition to the experimental measurements reported here, more T g 8 21 25 29 32 and Δ H mix values 21 22 23 33 34 35 36 37 are collected. Fig.…”

“…Although the two models have successfully explained some experimental measurements 18 19 , there are a number of systems that show pronounced discrepancy between experimental measurements and the theoretical calculation made by the models 6 13 20 . Alternatively, mixing thermodynamics is argued to determine the glass transition behaviors in mixtures, and studies found that positive enthalpy of mixing generally corresponds to negative T g deviation 5 , and even a value of Δ H mix as low as 200 J/mol can result in a remarkably negative T g shift 21 . However, the evaluation of the weighted contributions to the T g shifts from both mixing thermodynamics and the properties of pure components have not been available.…”