2018 
Kriuchevskyi, I., Wittmer, J. P., Meyer, H., Benzerara, O., & Baschnagel, J. (2018). Shearstress fluctuations and relaxation in polymer glasses. Physical Review E, 97(1).
Abstract: We investigate by means of molecular dynamics simulation a coarsegrained polymer glass model focusing on (quasistatic and dynamical) shearstress fluctuations as a function of temperature T and sampling time Delta t. The linear response is characterized using (ensembleaveraged) expectation values of the contributions (time averaged for each shear plane) to the stressfluctuation relation mu(sf) for the shear modulus and the shearstress relaxation modulus G(t). Using 100 independent configurations, we pay attention to the respective standard deviations. While the ensembleaveraged modulus mu(sf) (T) decreases continuously with increasing T for all Delta t sampled, its standard deviation delta mu(sf) (T) is nonmonotonic with a striking peak at the glass transition. The question of whether the shear modulus is continuous or has a jump singularity at the glass transition is thus ill posed. Confirming the effective timetranslational invariance of our systems, the Delta t dependence of mu(sf) and related quantities can be understood using a weighted integral over G(t).


2017 
Dolgushev, M., Wittmer, J. P., Johner, A., Benzerara, O., Meyer, H., & Baschnagel, J. (2017). Marginally compact hyperbranched polymer trees. Soft Matter, 13(13), 2499–2512.
Abstract: Assuming Gaussian chain statistics along the chain contour, we generate by means of a proper fractal generator hyperbranched polymer trees which are marginally compact. Static and dynamical properties, such as the radial intrachain pair density distribution rho(pair)(r) or the shearstress relaxation modulus G(t), are investigated theoretically and by means of computer simulations. We emphasize that albeit the selfcontact density rho(c) = rho(pair)(r approximate to 0) similar to log(N/S)/root S diverges logarithmically with the total mass N, this effect becomes rapidly irrelevant with increasing spacer length S. In addition to this it is seen that the standard Rouse analysis must necessarily become inappropriate for compact objects for which the relaxation time tau p of mode p must scale as tau(p) similar to (N/p)(5/3) rather than the usual square power law for linear chains.


Kriuchevskyi, I., Wittmer, J. P., Benzerara, O., Meyer, H., & Baschnagel, J. (2017). Numerical determination of shear stress relaxation modulus of polymer glasses. Eur. Phys. J. E, 40(4), 6 pp.
Abstract: Focusing on simulated polymer glasses well below the glass transition, we confirm the validity and the efficiency of the recently proposed simpleaverage expression G(t) = mu(A)h(t) for the computational determination of the shear stress relaxation modulus G(t). Here, mu(A) = G(0) characterizes the affine shear transformation of the system at t = 0 and h(t) the meansquare displacement of the instantaneous shear stress as a function of time t. This relation is seen to be particulary useful for systems with quenched or sluggish transient shear stresses which necessarily arise below the glass transition. The commonly accepted relation G(t) = c(t) using the shear stress autocorrelation function c(t) becomes incorrect in this limit.


2015 
Weysser, F., Benzerara, O., Johner, A., & Kulic, I. M. (2015). Topological energy storage of work generated by nanomotors. Soft Matter, 11(4), 732–740.
Abstract: Most macroscopic machines rely on wheels and gears. Yet, rigid gears are entirely impractical on the nanoscale. Here we propose a more useful method to couple any rotary engine to any other mechanical elements on the nanoand microscale. We argue that a rotary molecular motor attached to an entangled polymer energy storage unit, which together form what we call the “tanglotron” device, is a viable concept that can be experimentally implemented. We derive the torqueentanglement relationship for a tanglotron (its “equation of state”) and show that it can be understood by simple statistical mechanics arguments. We find that a typical entanglement at low packing density costs around 6kT. In the high entanglement regime, the free energy diverges logarithmically close to a maximal geometric packing density. We outline several promising applications of the tanglotron idea and conclude that the transmission, storage and backconversion of topological entanglement energy are not only physically feasible but also practical for a number of reasons.


Wittmer, J. P., Xu, H., Benzerara, O., & Baschnagel, J. (2015). Fluctuationdissipation relation between shear stress relaxation modulus and shear stress autocorrelation function revisited. Molecular Physics, 113(1718), 2881–2893.
Abstract: The shear stress relaxation modulus G(t) may be determined from the shear stress (tau) over cap (t) after switching on a tiny step strain gamma or by inverse Fourier transformation of the storage modulus G'(omega) or the loss modulus G ''(omega) obtained in a standard oscillatory shear experiment at angular frequency.. It is widely assumed that G(t) is equivalent in general to the equilibrium stress autocorrelation function C(t) = beta V


2013 
Mortazavi, B., Benzerara, O., Meyer, H., Bardon, J., & Ahzi, S. (2013). Combined molecular dynamicsfinite element multiscale modeling of thermal conduction in graphene epoxy nanocomposites. Carbon, 60, 356–365.
Abstract: We developed a multiscale scheme using molecular dynamics (MD) and finite element (FE) methods for evaluating the effective thermal conductivity of graphene epoxy nanocomposites. The proposed hierarchical multiscale approach includes three different scales. First, we used MD simulations for the investigation of thermal conduction in graphene epoxy assembly at atomic scale. Our results suggest that thermal conductivity of single layer graphene decline by around 30% in epoxy matrix for two different hardener chemicals. Using MD, we also calculated thermal boundary conductance (TBC) between crosslinked epoxy and graphene sheet. In the next step, using the results obtained by the MD method, we developed FE based representative volume elements (RVE) of the nanocomposite in order to evaluate the thermal conductivity at the microscale. Finally,,nanocomposite effective thermal conductivity was obtained using FE homogenization of an ensemble of microscale RVEs. The validity of the proposed approach was confirmed by comparing predicted results with experimental results in the literature. (C) 2013 Elsevier Ltd. All rights reserved.


2012 
Solar, M., Meyer, H., Gauthier, C., Fond, C., Benzerara, O., Schirrer, R., et al. (2012). Mechanical behavior of linear amorphous polymers: Comparison between molecular dynamics and finiteelement simulations. Physical Review E, 85(2).
Abstract: This paper studies the rheology of weakly entangled polymer melts and films in the glassy domain and near the rubbery domain using two different methods: molecular dynamics (MD) and finite element (FE) simulations. In a first step, the uniaxial mechanical behavior of a bulk polymer sample is studied by means of particlebased MD simulations. The results are in good agreement with experimental data, and mechanical properties may be computed from the simulations. This uniaxial mechanical behavior is then implemented in FE simulations using an elastoviscoelastoviscoplastic constitutive law in a continuum mechanics (CM) approach. In a second step, the mechanical response of a polymer film during an indentation test is modeled with the MD method and with the FE simulations using the same constitutive law. Good agreement is found between the MD and CM results. This work provides evidence in favor of using MD simulations to investigate the local physics of contact mechanics, since the volume elements studied are representative and thus contain enough information about the microstructure of the polymer model, while surface phenomena (adhesion and surface tension) are naturally included in the MD approach.


2011 
Solar, M., Meyer, H., Gauthier, C., Benzerara, O., Schirrer, R., & Baschnagel, J. (2011). Molecular dynamics simulations of the scratch test on linear amorphous polymer surfaces: A study of the local friction coefficient. Wear, 271(1112), 2751–2758.
Abstract: This work presents a mechanical analysis of tangential contact using molecular dynamics simulations. Scratch tests with a conical tip on amorphous polymer surfaces were simulated at various temperatures, scratching velocities, roughnesses of the tip and with various interactions between the tip and the monomers. The local friction coefficient for the different contact conditions (temperature, tip roughness and scratching velocity) was calculated by studying the apparent friction and the contact asymmetry. The results are in good agreement with experimental data from tests on classical polymer surfaces on a microscopic scale. (C) 2011 Elsevier B.V. All rights reserved.


2010 
Durand, M., Meyer, H., Benzerara, O., Baschnagel, J., & Vitrac, O. (2010). Molecular dynamics simulations of the chain dynamics in monodisperse oligomer melts and of the oligomer tracer diffusion in an entangled polymer matrix. J. Chem. Phys., 132(19), 194902.
Abstract: The apparent analogy between the selfdiffusion of linear oligomers in monodisperse systems, 2 up to 32 monomers, and their tracer diffusion in an entangled polymer matrix of length 256 is investigated by molecular dynamics simulations at constant pressure. Oligomers and polymers are represented by the same coarsegrained (beadspring) model. An analysis based on the Rouse model is presented. The scaling relationship of the selfdiffusion coefficient D with the chain length N written as D proportional to Nalpha is analyzed for a wide range of temperatures down to the glass transition temperature Tg. Near Tg, the heterogeneous dynamics is explored by the selfpart of the van Hove distribution function and various nonGaussian parameters. For the selfdiffusion in a monodisperse system a scaling exponent alpha(T)>1 depending on temperature is found, whereas for the tracer diffusion in an entangled matrix alpha=1 is obtained at all temperatures, regardless of the oligomer length. The different scaling behavior between both systems is explained by a different monomer mobility, which depends on chain length for monodisperse systems, but is constant for all tracers in the polymer matrix. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3420646]


Solar, M., Meyer, H., Gauthier, C., Benzerara, O., Pelletier, H., Schirrer, R., et al. (2010). Molecular dynamics simulations as a way to investigate the local physics of contact mechanics: a comparison between experimental data and numerical results. J. Phys. DAppl. Phys., 43(45), 455406.
Abstract: In this work, a mechanical analysis of normal contact using molecular dynamics (MD) simulations is presented. Conical indentation on amorphous polymer surfaces was simulated at various temperatures and indentation rates under displacement or load control. The results are qualitatively compared with experimental data from tests on epoxy materials with different glass transition temperatures (Tg), and show good agreement with experiments. Moreover, MD simulations of nanoindentation tests allow us to estimate the mechanical properties of the polymer films studied as in experimental nanoindentation tests, which demonstrates the relevance of this approach.

