Contrasting two different interpretations of the dynamics in binary glass forming mixtures

Abstract

In a series of papers on binary glass-forming mixtures of tripropyl phosphate (TPP) with polystyrene (PS), Kahlau et al. [J. Chem. Phys. 140, 044509 (2014)] and Bock et al. [J. Chem. Phys. 139, 064508 (2013); J. Chem. Phys. 140, 094505 (2014); and J. Non-Cryst. Solids 407, 88-97 (2015)] presented the data on the dynamics of the two components studied over the entire composition range by several experimental methods. From these sets of data, obtained by multiple experimental techniques on mixtures with a large difference ΔT ≈ 200 K between the glass transition temperatures of two starting glass formers, they obtained two α-relaxations, α1 and α2. The temperature dependence of the slower α1 is Vogel-Fulcher like, but the faster α2 is Arrhenius. We have re-examined their data and show that their α2-relaxation is the Johari-Goldstein (JG) β-relaxation with Arrhenius T-dependence admixed with a true α2-relaxation having a stronger temperature dependence. In support of our interpretation of their data, we made dielectric measurements at elevated pressures P to show that the ratio of the α1 and α2 relaxation times, τ(T,P)/τ(T,P), is invariant to variations of T and P, while τ(T,P) is kept constant. This property proves unequivocally that the α2-relaxation is the JG β-relaxation, the precursor of the α1-relaxation. Subsequently, the true but unresolved α2-relaxation is recovered, and its relaxation times with much stronger temperature dependence are deduced, as expected for the α-relaxation of the TPP component. The results are fully compatible with those found in another binary mixture of methyltetrahydrofuran with tristyrene and PS with ΔT ≈ 283 K, even larger than ΔT ≈ 200 K of the mixture of TPP with PS, and in several polymer blends. The contrast between the two very different interpretations brought out in this paper is deemed beneficial for further progress in this research area.

Why is the change of the Johari–Goldstein β-relaxation time by densification in ultrastable glass minor?

“…It also has the immediate consequence that t b is also a function of r g /T, if t a is a function of r g /T, although the two functions t b = f b (r g /T) and t a = F a (r g /T) are different. This is an example of the dynamic and thermodynamics properties of the JG b-relaxation similar to those of the a-relaxation, [20][21][22][23][24][25][26][27] which follows from the approximate relation, 23,27 t a r g =T ð Þ% t…”

“…The property of the invariance of the ratio t a (T,P)/t b (T,P) to variations of T and P while keeping t a (T,P) constant [20][21][22][23][24][25][26][27] is key to arriving at the results in Section (A). It also has the immediate consequence that t b is also a function of r g /T, if t a is a function of r g /T, although the two functions t b = f b (r g /T) and t a = F a (r g /T) are different.…”

“…As mentioned in the Introduction, to explain the properties of the JG b-relaxation in USG compared with the OG we shall apply two general dynamic and thermodynamic properties of the a-relaxation and JG b-relaxation established before from studies at elevated pressures in molecular glass-formers. [20][21][22][23][24][25][26][27] The application is justified by the fact that density is increased at elevated pressure, and the increase can be comparable to or even larger than that in USG. Through the application, we are able to explain why the Johari-Goldstein (JG) b-relaxation time, t b , in USG of toluene, 13 etoricoxib 14 and telmisartan 15 is about an order of magnitude longer than that in OG, and has nearly the same activation energy E b .…”

“…These changes are comparable to or larger than those achieved by producing USG. Found in general in many molecular glassformers [20][21][22][23][24][25][26][27][28] is the approximate invariance of t b (T,P) to variations of P and T provided t a (T,P) are kept constant. Examples of experimental evidence for this property in OG are shown in Fig.…”

“…Notwithstanding, the much smaller changes of t b and E b by densification in USG are in stark contrast to the behavior of the a-relaxation time t a , and this remarkable experimental observation deserves an explanation. In this paper, we provide an explanation based on two general properties established from the studies of glasses and liquids at elevated pressures [20][21][22][23][24][25][26][27] and applied it to USG. The increase in density of the glasses formed under high pressure can be even larger than that in USG, and hence the general properties should apply.…”