Effects of 2 nm size added heterogeneity on non-exponential dielectric response, and the dynamic heterogeneity view of molecular liquids

Abstract

To investigate how non-exponential response could vary under different conditions, we studied the effects of adding 2 nm size polyhedral oligomeric silsesquioxane (POSS) to diglycidyl ether of bisphenol-A, whose molecules have the same terminal (epoxide) dipoles as the tentacle-like side chains attached to the silsesquioxane core of the POSS molecule. Dielectric relaxation spectra show that, on initial addition, the POSS nano-heterogeneity decreases the non-exponential response parameter β, which is consistent with the dynamic heterogeneity view, but it also decreases the relaxation time τ(m), which is inconsistent with that view. The variations in β and τ(m) with the composition have a thermal equivalence. Despite the lack of translational diffusion required for dynamic heterogeneity, plastic crystals show non-exponential response and non-Arrhenius dynamics. Measurements of β and τ(m) seem more appropriate than using probe molecules or modeling nonlinear response data as a sum of linear responses for testing the dynamic heterogeneity view. Data on molecular liquid mixtures is not generally consistent with this view, and adding a solute does not always decrease β. Studies of mixtures of different size rigid molecules with identical dipolar groups, including polymers, may be useful for comparing the relative effects of temperature and molecular size on β and τ(m).

Effect of adding nanometre-sized heterogeneities on the structural dynamics and the excess wing of a molecular glass former

“…3 ). This increased stretching of the exponential decay under addition of POSS resembles the results from dielectric-spectroscopy recently reported by Johari and Khouri for different glass formers and POSS molecules where it was attributed to an increase of heterogeneity in the system 28 29 . Interestingly, the corresponding α relaxation time, 〈 τ α 〉 only shows a slight increase (i.e., a slowing down of the α mode) when adding POSS as will be treated in more detail in the discussion section.…”

“…In the present work, following the previous work by Johari and Khouri 28 29 we investigate the influence of structural heterogeneity on molecular dynamics by adding POSS molecules with a size comparable to that of typical dynamic heterogeneous domains of 1–3 nanometers 30 31 to the classical molecular glass former glycerol. We investigate the influence of these heterogeneities on the α relaxation and, in addition, also explore possible effects on the EW.…”

“…The intermolecular reactivity of these ligands allows the molecules to be dispersed in various complex fluids 30 . Indeed, the influence of added POSS molecules on glassy dynamics has been investigated in several previous works 28 29 33 34 .…”

Cooperativity and Heterogeneity in Plastic Crystals Studied by Nonlinear Dielectric Spectroscopy

“…For example, in the plasticcrystalline state strong correlations seem to lead to a much weaker slowing down of glassy dynamics than in conven tional glass formers. Moreover, an explanation of the fieldinduced variation of the permittivity of cyclo-octanol and ortho-carborane within the heterogeneity scenario as devel oped for supercooled liquids is not straightforward, which may indicate that molecular motions in PCs are intrinsically nonexponential [21,22,53], Finally, we find no nonlinear behavior of the secondary relaxations of cyclo-octanol. Thus, while sharing many characteristics of glassy dynam ics with canonical glass formers, on a microscopic level PCs may behave quite different, shedding new light on cooperativity and heterogeneity in glassy systems in general.…”

“…For example, it was argued that heterogeneity in PCs may be of less importance than in structural glass formers, implying that the dynamics of a single molecule may be intrinsically nonexponential [21,22], Moreover, the intermolecular interactions leading to cooperativity should be affected by fixing the molecules on lattice positions, and indeed the deviations from Arrhenius behavior in PCs are generally weaker than in canonical glass formers [2,23]. Within Angell's strong-fragile classification scheme [24], this implies that PCs are strong glass formers [23], despite the fact that exceptions are known [25,26].…”