Viscous Liquids and the Glass Transition. II. Secondary Relaxations in Glasses of Rigid Molecules

Abstract

The dielectric loss factor and dielectric permittivity of 8–16 mol% solutions of chlorobenzene, o-dichlorobenzene, and 1-chloronaphthalene in cis-decalin; 50–60 mol% mixtures of pyridine with chlorobenzene, bromobenzene, 1-chloronaphthalene, and toluene; 50–60 mol% mixtures of tetrahydrofuran with bromobenzene and 1-chloronaphthalene; the pure liquids cis-decalin, o-terphenyl, iso-propylbenzene, propylene carbonate; and two fused salt systems, 45 mol% Ca(NO3)2–KNO3 mixture and Ca(NO3)2·4H2O have been measured from 50 Hz to 1 × 105 Hz from − 196° in the vitreous state to about 30° above their respective glass transition temperatures. The Tg's of the organic glasses have been measured by DTA. With the exception of propylene carbonate, all glasses show the presence of one secondary relaxation between − 196° and their respective Tg's either as a peak or shoulder in a tanδ–temperature plot at a single frequency, or in the dielectric loss spectrum. Arrhenius plots of the frequency of maximum loss against temperature in the main relaxation region for all systems are nonlinear, with the activation energy at the lowest temperature of our measurements ranging from 55 kcal/mol to 70 kcal/mol. The Arrhenius plots in the secondary relaxation region are linear and have activation energies between 5 and 12 kcal/mol. These glasses, most of which are composed of rigid molecules, show a remarkable similarity in their dielectric behavior to amorphous polymers. The results confirm the prediction made by one of the authors that the occurence of secondary relaxations is an intrinsic property of the glassy state.

A robust comparison of dynamical scenarios in a glass-forming liquid

“…The aim of this exercise is to elucidate whether, in addition to the expected translational diffusion of the molecular centre of mass, all molecules are subjected to: either two additional (and distinct) relaxation processes affecting the entire ensemble (HG); or if some molecules are moving faster than others giving rise to the so-called islands of mobility (HT). 38,39 Mathematically the HG can be described by a multiplicative ansatz of dynamical processes in the time domain and, as such, it affects all molecules on the time scale of the a-relaxation. The HT, on the other hand, is based on the arithmetic sum of two processes, each one affecting a certain percentage of molecules at a-relaxation time scales.…”

Emergence of glassy-like dynamics in an orientationally ordered phase

“…[4][5][6][7][8][9][10][11][12][13] This JG process is associated with local, noncooperative motions of the molecular unit as a whole, thus different from secondary relaxations -due to intramolecular degrees of freedom (e.g., side-chain motions in polymers) and indeed it was found even in single rigid molecules. 4,12 Despite the large quantity of available experimental and simulations studies, the nature of the local JG relaxation is still far from being understood. According to the original work, 4,12 β-relaxation appears as a consequence on the nonuniformity of the glassy state (islands of mobility) involving only local regions in which molecules can diffuse.…”

“…4,12 Despite the large quantity of available experimental and simulations studies, the nature of the local JG relaxation is still far from being understood. According to the original work, 4,12 β-relaxation appears as a consequence on the nonuniformity of the glassy state (islands of mobility) involving only local regions in which molecules can diffuse. An alternative homogeneous explanation attributes the secondary relaxation phenomena to small-angle reorientations of all the molecules.…”

“…It is worth recalling that some models even claim the nonexistence of the JG relaxation for strong glasses. 20,21 In spite of the theoretical prediction that "amorphous packing" is the cause of the ubiquity of JG secondary relaxation in disordered systems, 4 a new universal mechanism for this process has been recently proposed, related to the existence of dynamic heterogeneity and of molecular motions on different length scales 22 and thus applicable, in principle, also to structurally ordered systems. Because no ultimate theoretical model for the glassy dynamics has been proposed so far, indeed experimental studies in simple systems allowing for the extraction of some fundamental features may be of particular importance.…”

Fragility and elasticity: Description of flow in highly viscous liquids

“…For G, one takes the measured high-frequency value from Table I. If necessary, one can add a Gaussian to describe an eventual Johari-Goldstein peak, 12 which requires three more parameters, height, position, and width. In this way, f c V c and consequently I 2 were obtained for the six glass formers in Table I for which dynamical shear data close to T g exist in the literature.…”