Weak links between fast mobility and local structure in molecular and atomic liquids

Abstract

We investigate by Molecular-Dynamics simulations the fast mobility -the rattling amplitude of the particles temporarily trapped by the cage of the neighbors -in mildly supercooled states of dense molecular (linear trimers) and atomic (binary mixtures) liquids. The mixture particles interact by the Lennard-Jones potential. The non-bonded particles of the molecular system are coupled by the more general Mie potential with variable repulsive and attractive exponents in a range which is characteristic of small n-alkanes and n-alcohols. Possible links between the fast mobility and the geometry of the cage (size and shape) are searched. The correlations on a per-particle basis are rather weak. Instead, if one groups either the particles in fast-mobility subsets or the cages in geometric subsets, the increase of the fast mobility with both the size and the asphericity of the cage is revealed. The observed correlations are weak and differ in states with equal relaxation time. Local forces between a tagged particle and the first-neighbour shell do not correlate with the fast mobility in the molecular liquid. It is concluded that the cage geometry alone is unable to provide a microscopic interpretation of the known, universal link between the fast mobility and the slow structural relaxation. We suggest that the particle fast dynamics is affected by regions beyond the first neighbours, thus supporting the presence of collective, extended fast modes.

Anisotropy of the monomer random walk in a polymer melt: local-order and connectivity effects

“…The main motivation is the pursuit of the microscopic origin of the universal correlation between the mean square amplitude of the cage rattling (related to the Debye-Waller factor) and the relaxation and transport, as found in simulations of polymers [13,14,15], binary atomic mixtures [14,16], colloidal gels [17] and antiplasticized polymers [18,19], and supported by the experimental data concerning several glassformers in a wide fragility range (20 ≤ m ≤ 191) [13,20,21,16,22]. From this respect, the local structure due to the first neighbours was recently found to correlate poorly with the rattling amplitude in the cage of the closest neighbours, and the structural relaxation, in liquids of linear trimers [23,24] and atomic mixtures [23]. On the other hand, extended modes ranging up to about the fourth shell do correlate with the rattling amplitude in the cage and the structural relaxation [25,26].…”

“…The monomers escape from the cage on average within the time τ α (orange circles of Fig.1). We define, as in previous works [13,14,15,16,17,20,21,23,25,44,46,71,72], the structural relaxation time by the equation F s (q max , τ α ) = φ with φ = e −1 where q max is the maximum of the static structure factor and F s is the self-part of the intermediate scattering function [1]. It is worth noting that our polymer model complies with the temperature-time superposition principle, resulting in a constant (and moderate) stretching of F s (q max , t) [13].…”

Short-time elasticity of polymer melts: Tobolsky conjecture and heterogeneous local stiffness

“…End monomers are bonded to a single one. The different connectivity alters the arrangement of the nonbonded (blue) nearest monomers . [Color figure can be viewed in the online issue, which is available at http://wileyonlinelibrary.com.…”

Cage effect in supercooled molecular liquids: Local anisotropies and collective solid-like response

“…Instead, Fig.6 evidences the strong correlation between the surface and the volume of VPs. It follows from the good packing and the subsequent relatively narrow width of the distribution of the asphericity [67,68]. To show that we recast Eq.10 as S = 36 π (a + 1)…”