Distinguishing different classes of secondary relaxations from vapour deposited ultrastable glasses

Abstract

Secondary relaxations persistent in the glassy state after structural arrest are especially relevant for the properties of the glass. A major thrust in research in dynamics of glass-forming liquids is to identify what secondary relaxations exhibit a connection to the structural relaxation and are hence more relevant. Via the Coupling Model, secondary relaxations having such connection have been identified by properties similar to the primitive relaxation of the Coupling Model and are called the Johari-Goldstein (JG) β-relaxations. They involve the motion of the entire molecule and act as the precursor of the structural α-relaxation. The change in dynamics of the secondary relaxation by aging an ordinary glass is one way to understand the connection between the two relaxations, but the results are often equivocal. Ultrastable glasses, formed by physical vapour deposition, exhibit density and enthalpy levels comparable to ordinary glasses aged for thousands of years, as well as some particular molecular arrangement. Thus, ultrastable glasses enable the monitoring of the evolution of secondary processes in case aging does not provide any definitive information. Here, we study the secondary relaxation of several ultrastable glasses to identify different types of secondary relaxations from their different relationship with the structural relaxation. We show the existence of two clearly differentiated groups of relaxations: those becoming slower in the ultrastable state and those becoming faster, with respect to the ordinary unaged glass. We propose ultrastability as a way to distinguish between secondary processes arising from the particular microstructure of the system and those connected in properties to and acting as the precursor of the structural relaxation in the sense of the Coupling Model.

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

“…12 These secondary relaxations are called the Johari-Goldstein (JG) b-relaxations in order to distinguish them from the other trivial ones. 12 In contrast to the a-relaxation time t a , the 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 . The data from these three cases are extracted from C. Rodríguez-Tinoco et al 15 and shown in Fig.…”

“…[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 . Later we shall show how the two general properties are derived from theoretical considerations.…”

“…1 Adapted from ref. 15. Arrhenius plot of t b in USG and OG of (A) toluene, (B) etoricoxib, and (C) telmisartan.…”

“…On the other hand, the secondary relaxations of maltose octa-acetate, carvedilol and celecoxib, also discussed in ref. 15, involve intramolecular degrees of freedom, bear no connection to the a-relaxation, 15 do not belong to the class of JG b-relaxations, and are not considered in this paper.…”

“…On the other hand, the relaxation strength of the JG b-relaxation, De b , is significantly reduced in USG in toluene to such a level that it would require several thousands of years of annealing for an ordinary glass to achieve the same state. [13][14][15] The reduction of De b indicates that the JG b-relaxation does respond to densification in the USG like the a-relaxation. 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.…”