Neural Networks Reveal the Impact of the Vibrational Dynamics in the Prediction of the Long-Time Mobility of Molecular Glassformers.

Two neural networks (NN) are designed to predict the particle mobility of a molecular glassformer in a wide time window ranging from vibrational dynamics to structural relaxation. Both NNs are trained by information concerning the local structure of the environment surrounding a given particle. The only difference in the learning procedure is the inclusion (NN A) or not (NN B) of the information provided by the fast, vibrational dynamics and quantified by the local Debye-Waller factor. It is found that, for a given temperature, the prediction provided by the NN A is more accurate, a finding which is tentatively ascribed to better account of the bond reorientation. Both NNs are found to exhibit impressive and rather comparable performance to predict the four-point susceptibility χ4(t) at τα, a measure of the dynamic heterogeneity of the system.

Nanoscale Elastoplastic Wrinkling of Ultrathin Molecular Films.

Ultrathin molecular films deposited on a substrate are ubiquitously used in electronics, photonics, and additive manufacturing methods. The nanoscale surface instability of these systems under uniaxial compression is investigated here by molecular dynamics simulations. We focus on deviations from the homogeneous macroscopic behavior due to the discrete, disordered nature of the deformed system, which might have critical importance for applications. The instability, which develops in the elastoplastic regime above a finite critical strain, leads to the growth of unidimensional wrinkling up to strains as large as 0.5. We highlight both the dominant wavelength and the amplitude of the wavy structure. The wavelength is found to scale geometrically with the film length, λ∝L, up to a compressive strain of ε≃0.4 at least, depending on the film length. The onset and growth of the wrinkling under small compression are quite well described by an extended version of the familiar square-root law in the strain ε observed in macroscopic systems. Under large compression (ε≳0.25), we find that the wrinkling amplitude increases while leaving the cross section nearly constant, offering a novel interpretation of the instability with a large amplitude. The contour length of the film topography is not constant under compression, which is in disagreement with the simple accordion model. These findings might be highly relevant for the design of novel and effective wrinkling and buckling patterns and architectures in flexible platforms for electronics and photonics.

Mutual Information in Molecular and Macromolecular Systems.

The relaxation properties of viscous liquids close to their glass transition (GT) have been widely characterised by the statistical tool of time correlation functions. However, the strong influence of ubiquitous non-linearities calls for new, alternative tools of analysis. In this respect, information theory-based observables and, more specifically, mutual information (MI) are gaining increasing interest. Here, we report on novel, deeper insight provided by MI-based analysis of molecular dynamics simulations of molecular and macromolecular glass-formers on two distinct aspects of transport and relaxation close to GT, namely dynamical heterogeneity (DH) and secondary Johari-Goldstein (JG) relaxation processes. In a model molecular liquid with significant DH, MI reveals two populations of particles organised in clusters having either filamentous or compact globular structures that exhibit different mobility and relaxation properties. In a model polymer melt, MI provides clearer evidence of JG secondary relaxation and sharper insight into its DH. It is found that both DH and MI between the orientation and the displacement of the bonds reach (local) maxima at the time scales of the primary and JG secondary relaxation. This suggests that, in (macro)molecular systems, the mechanistic explanation of both DH and relaxation must involve rotation/translation coupling.

Open and Anisotropic Soft Regions in a Model Polymer Glass.

The vibrational dynamics of a model polymer glass is studied by Molecular Dynamics simulations. The focus is on the "soft" monomers with high participation to the lower-frequency vibrational modes contributing to the thermodynamic anomalies of glasses. To better evidence their role, the threshold to qualify monomers as soft is made severe, allowing for the use of systems with limited size. A marked tendency of soft monomers to form quasi-local clusters involving up to 15 monomers is evidenced. Each chain contributes to a cluster up to about three monomers and a single cluster involves a monomer belonging to about 2-3 chains. Clusters with monomers belonging to a single chain are rare. The open and tenuous character of the clusters is revealed by their fractal dimension df<2. The inertia tensor of the soft clusters evidences their strong anisotropy in shape and remarkable linear correlation of the two largest eigenvalues. Owing to the limited size of the system, finite-size effects, as well as dependence of the results on the adopted polymer length, cannot be ruled out.

Coincident Correlation between Vibrational Dynamics and Primary Relaxation of Polymers with Strong or Weak Johari-Goldstein Relaxation.

The correlation between the vibrational dynamics, as sensed by the Debye-Waller factor, and the primary relaxation in the presence of secondary Johari-Goldstein (JG) relaxation, has been investigated through molecular dynamics simulations. Two melts of polymer chains with different bond length, resulting in rather different strength of the JG relaxation are studied. We focus on the bond-orientation correlation function, exhibiting higher JG sensitivity with respect to alternatives provided by torsional autocorrelation function and intermediate scattering function. We find that, even if changing the bond length alters both the strength and the relaxation time of the JG relaxation, it leaves unaffected the correlation between the vibrational dynamics and the primary relaxation. The finding is in harmony with previous studies reporting that numerical models not showing secondary relaxations exhibit striking agreement with experimental data of polymers also where the presence of JG relaxation is known.

Fast Vibrational Modes and Slow Heterogeneous Dynamics in Polymers and Viscous Liquids.

Many systems, including polymers and molecular liquids, when adequately cooled and/or compressed, solidify into a disordered solid, i.e., a glass. The transition is not abrupt, featuring progressive decrease of the microscopic mobility and huge slowing down of the relaxation. A distinctive aspect of glass-forming materials is the microscopic dynamical heterogeneity (DH), i.e., the presence of regions with almost immobile particles coexisting with others where highly mobile ones are located. Following the first compelling evidence of a strong correlation between vibrational dynamics and ultraslow relaxation, we posed the question if the vibrational dynamics encodes predictive information on DH. Here, we review our results, drawn from molecular-dynamics numerical simulation of polymeric and molecular glass-formers, with a special focus on both the breakdown of the Stokes-Einstein relation between diffusion and viscosity, and the size of the regions with correlated displacements.

Vibrational scaling of the heterogeneous dynamics detected by mutual information.

The correlations detected by the mutual information in the propensities of a molecular viscous liquid are studied by molecular-dynamics simulations. Dynamic heterogeneity is evidenced and two particle fractions with different mobility and relaxation identified. The two fractions exhibit the scaling of their relaxation in terms of the rattling amplitude of the particle trapped in the cage of the first neighbours 〈u2〉 . The scaling master curve does not differ from the one found for bulk systems, thus confirming identical results previously reported in other systems with strong dynamic heterogeneity as thin molecular films. The excitation of planar and globular structures at short and long times with respect to structural relaxation, respectively, is revealed. Some of the globular structures are different from the ones evidenced in atomic mixtures. States with equal 〈u2〉 are found to have identical time dependence of several quantities, referring to both bulk and the two fractions with heterogeneous dynamics, at least up to the structural relaxation time [Formula: see text].

Mutual information does not detect growing correlations in the propensity of a model molecular liquid.

The dynamical spatial correlations detected by the mutual information (MI) in the isoconfigurational particle displacements of a monodisperse molecular viscous liquid are studied via molecular-dynamics simulations by changing considerably both the molecular mobility and the degree of dynamical heterogeneity. Different from atomic liquids, the MI correlation length does not grow on approaching the glass transition by considering the liquid both in full detail as a collection of monomers and as a coarse-grained ensemble of molecular centers of mass. In the detailed picture, it is found that: (i) the MI correlations between monomers are largely due to inter-molecular correlations, (ii) the MI length scale is numerically identical, within the errors, to the correlation length scale of the displacement direction, as drawn by conventional correlation functions. The time evolution of the MI spatial correlations complies with the scaling between the fast vibrational dynamics and the long-time relaxation. Our findings suggest that the characteristics of the MI length scale are markedly system-dependent and not obviously related to dynamical heterogeneity.

Criticality of O(N) symmetric models in the presence of discrete gauge symmetries.

We investigate the critical properties of the three-dimensional antiferromagnetic RP^{N-1} model, which is characterized by a global O(N) symmetry and a discrete Z_{2} gauge symmetry. We perform a field-theoretical analysis using the Landau-Ginzburg-Wilson (LGW) approach and a numerical Monte Carlo study. The LGW field-theoretical results are obtained by high-order perturbative analyses of the renormalization-group flow of the most general Φ^{4} theory with the same global symmetry as the model, assuming a gauge-invariant order-parameter field. For N=4 no stable fixed point is found, implying that any transition must necessarily be of first order. This is contradicted by the numerical results that provide strong evidence for a continuous transition. This suggests that gauge modes are not always irrelevant, as assumed by the LGW approach, but they may play an important role to determine the actual critical dynamics at the phase transition of O(N) symmetric models with a discrete Z_{2} gauge symmetry.