Autohesion Mechanisms at Interfaces Between Random Copolymer Melts: Mesoscopic Simulations with Realistic Coarse-Grained Models.

The increasing of welding time during the interdiffusion of a pair of non reacting random copolymer melts favors the strength rate of healing at the interface. Furthermore, the diffusion kinetic during the interpenetration of copolymer chains across the interface is strongly dependant on molecular weight. In this paper we perform mesoscopic simulations with realistic coarse grain models to study the autohesion mechanism across the interface between slightly entangled styrene-butadiene random copolymer melts.

Toward a Better Understanding of the Poly(glycerol sebacate)-Water Interface.

Molecular simulations were conducted to provide a better description of the poly(glycerol sebacate) (PGS)-water interface. The density and the glass-transition temperature as well as their dependencies on the degree of esterification were examined in close connection with the available experimental data. The work of adhesion and water contact angle were calculated as a function of the degree of esterification. A direct correlation was established between the strength of the hydrogen bond network in the interfacial region and the change in the water contact angle with respect to the degree of esterification. The interfacial region was described by local density profiles and orientations of the water molecules.

Probing Enzymatic PET Degradation: Molecular Dynamics Analysis of Cutinase Adsorption and Stability.

Understanding the mechanisms influencing poly(ethylene terephthalate) (PET) biodegradation is crucial for developing innovative strategies to accelerate the breakdown of this persistent plastic. In this study, we employed all-atom molecular dynamics simulation to investigate the adsorption process of the LCC-ICCG cutinase enzyme onto the PET surface. Our results revealed that hydrophobic, π-π, and H bond interactions, specifically involving aliphatic, aromatic, and polar uncharged amino acids, were the primary driving forces for the adsorption of the cutinase enzyme onto PET. Additionally, we observed a negligible change in the enzyme's tertiary structure during the interaction with PET (RMSD = 1.35 Å), while its secondary structures remained remarkably stable. Quantitative analysis further demonstrated that there is about a 24% decrease in the number of enzyme-water hydrogen bonds upon adsorption onto the PET surface. The significance of this study lies in unraveling the molecular intricacies of the adsorption process, providing valuable insights into the initial steps of enzymatic PET degradation.

Development of anisotropic force fields for homopolymer melts at the mesoscale.

With the aim of producing realistic coarse-grained models of homopolymers, we introduce a tabulated backbone-oriented anisotropic potential. The parameters of the model are optimized using statistical trajectory matching. The impact of grain anisotropy is evaluated at different coarse-graining levels using cis-polybutadiene as a test case. We show that, at the same time, tuning the aspect ratio of the grains can lead to a better density and structure and may reduce the unphysical bond crossings by up to 90%, without increasing the computation time too much and thereby jeopardizing the main advantage of coarse-grained models.

Molecular interactions at the metal-liquid interfaces.

We reported molecular simulations of the interactions among water, an epoxy prepolymer diglycidic ether of bisphenol A (DGEBA), and a hardener isophorone diamine (IPDA) on an aluminum surface. This work proposes a comprehensive thermodynamic characterization of the adhesion process from the calculation of different interfacial tensions. The cross-interactions between the atoms of the metal surface and different molecules are adjusted so as to reproduce the experimental work of adhesion. Water nanodroplets on the metal surface are then simulated to predict their contact angle. Liquid-vapor surface tensions of the epoxy prepolymer (DGEBA) and hardener (IPDA) and the solid-vapor surface tension of the aluminum surface are also calculated to provide the solid-liquid interfacial tension that remains very difficult to obtain from the mechanical definition.

Associated molecular liquids at the graphene monolayer interface.

We report molecular simulations of the interaction between a graphene sheet and different liquids such as water, ethanol, and ethylene glycol. We describe the structural arrangements at the graphene interface in terms of density profiles, number of hydrogen bonds (HBs), and local structuration in neighboring layers close to the surface. We establish the formation of a two-dimensional HB network in the layer closest to the graphene. We also calculate the interfacial tension of liquids with a graphene monolayer and its profile along the direction normal to the graphene to rationalize and quantify the strengthening of the intermolecular interactions in the liquid due to the presence of the surface.

Assessing the derivation of time parameters from branched polymer coarse-grain model.

The parameterization of rheological models for polymers is often obtained from experiments via the top-down approach. This procedure allows us to determine good fitting parameters for homogeneous materials but is less effective for polymer mixtures. From a molecular simulation point of view, the timescales needed to derive those parameters are often accessed through the use of coarse-grain potentials. However, these potentials are often derived from linear model systems and the transferability to a more complex structure is not straightforward. Here, we verify the transferability of a potential computed from linear polymer simulations to more complex molecular shapes and present a type of analysis, which was recently formulated in the framework of a tube theory, to a coarse-grain molecular approach in order to derive the input parameters for a rheological model. We describe the different behaviors arising from the local topological structure of molecular sub-units. Coarse-grain models and mean-field based tube theory for polymers form a powerful combination with potentially important applications.

Strain induced crystallization of polymers at and above the crystallization temperature by coarse-grained simulations.

We examine the behavior of short and long polymers by means of coarse-grained computer simulations of a by-polyvinyl alcohol inspired model. In particular, we focus on the structural changes in the monomer and polymer scales during cooling and the application of uni-axial true strain. The straining of long polymers results in the formation of a semi-crystalline system at temperatures well above the crystallization temperature, which allows for the study of strain induced crystallization.

Multi-scale modeling of the polymer-filler interaction.

We report mesoscopic simulations of the interaction between a silica nanoparticle and cis-1,4-polybutadiene chains with realistic coarse-(CG) grained models. The CG models are obtained with a bottom-up Bayesian method based on trajectory matching of atomistic configurations of the system. We then investigate the structural properties of the interfacial region as a function of the grafting density and polymer chain length. We take advantage of the realistic CG models to explore the dynamics of the nanoparticle over a period of 10 microseconds. We show that the dynamics of the nanoparticle is affected by the grafting density and the polymer chain length of the grafted chains.

Backbone oriented anisotropic coarse grains for efficient simulations of polymers.

Despite the fact that anisotropic particles have been introduced to describe molecular interactions for decades, they have been poorly used for polymers because of their computing time overhead and the absence of a relevant proof of their impact in this field. We first report a method using anisotropic beads for polymers, which solves the computing time issue by considering that beads keep their principal orientation alongside the mean local backbone vector of the polymer chain, avoiding the computation of torques during the dynamics. Applying this method to a polymer bulk, we study the effect of anisotropic interactions vs isotropic ones for various properties such as density, pressure, topology of the chain network, local structure, and orientational order. We show that for different classes of potentials traditionally used in molecular simulations, those backbone oriented anisotropic beads can solve numerous issues usually encountered with isotropic interactions. We conclude that the use of backbone oriented anisotropic beads is a promising approach for the development of realistic coarse-grained potentials for polymers.

Calculation of the interfacial tension of the graphene-water interaction by molecular simulations.

We report the calculation of the solid-liquid interface tension of the graphene-water interaction by using molecular simulations. Local profiles of the interfacial tension are given through the mechanical and thermodynamic definitions. The dependence of the interfacial tension on the graphene area is investigated by applying both reaction field and Ewald summation techniques. The structure of the interfacial region close to the graphene sheet is analyzed through the profiles of the density and hydrogen bond number and the orientation of the water molecules. We complete this study by plotting the profiles of the components of the pressure tensor calculated by the Ewald summation and reaction field methods. We also investigate the case of a reaction field version consisting in applying a damped shifted force in the case of the calculation of the pressure components.

Importance of the tail corrections on surface tension of curved liquid-vapor interfaces.

We report molecular simulations of the liquid-vapor cylindrical interface of methane. We apply the truncated Lennard-Jones potential and specific long-range corrections for the surface tension developed especially for cylindrical interfaces. We investigate the impact of the cutoff on the radial density profile, the intrinsic and long-range correction parts to the surface tension, and Tolman length. We also study the curvature dependence of the surface tension as a function of the cutoff used. In this work we shed light that both density and Tolman length are cutoff-dependent whereas the total surface tension is slightly curvature and cutoff dependent.

Computer modelling of the surface tension of the gas-liquid and liquid-liquid interface.

This review presents the state of the art in molecular simulations of interfacial systems and of the calculation of the surface tension from the underlying intermolecular potential. We provide a short account of different methodological factors (size-effects, truncation procedures, long-range corrections and potential models) that can affect the results of the simulations. Accurate calculations are presented for the calculation of the surface tension as a function of the temperature, pressure and composition by considering the planar gas-liquid interface of a range of molecular fluids. In particular, we consider the challenging problems of reproducing the interfacial tension of salt solutions as a function of the salt molality; the simulations of spherical interfaces including the calculation of the sign and size of the Tolman length for a spherical droplet; the use of coarse-grained models in the calculation of the interfacial tension of liquid-liquid surfaces and the mesoscopic simulations of oil-water-surfactant interfacial systems.

Monte Carlo simulations of the static friction between two grafted polymer brushes.

A configurational bias Monte Carlo method has been developed to study the static friction between grafted polymers immersed in a good solvent. Simple models using the soft quadratic potential from a dissipative particle dynamics study have been used to model polyzwitterionic brushes at physiological pressures (up to 7.5 MPa). Three models of decreasing rigidity have been used to model the friction between the brushes by calculating the tangential component of the pressure induced by a mismatch in the registry of the two grafting surfaces. The static friction coefficient can be calculated for three model systems and the slip between the layers occurs at a much lower values of shear force for the more flexible polymer layer. A moderate increase in the flexibility of the chains reduces the friction coefficient by a factor of ca. 20. Tilting the layer directors of the brushes also increases the static friction between the layer when the top, tilted layer is displaced in the direction away from the tilt. Non-equilibrium dynamics techniques for the same model were performed using dissipative particle dynamics and the limiting extremes of the Stribeck curve corresponding to the boundary lubrication regime and the hydrodynamic lubrication regime were observed for these flat surfaces. As expected, µk is significantly lower than µs for the same system. The dynamical friction coefficients in the model are in good agreement with those observed in the experiment and the ratio of µk/µs of between 0.11 and 0.5 observed in the simulations is in reasonable agreement with the value of 0.5 normally observed for these seen for these systems.

Can we approach the gas-liquid critical point using slab simulations of two coexisting phases?

In this paper, we demonstrate that it is possible to approach the gas-liquid critical point of the Lennard-Jones fluid by performing simulations in a slab geometry using a cut-off potential. In the slab simulation geometry, it is essential to apply an accurate tail correction to the potential energy, applied during the course of the simulation, to study the properties of states close to the critical point. Using the Janecek slab-based method developed for two-phase Monte Carlo simulations [J. Janec̆ek, J. Chem. Phys. 131, 6264 (2006)], the coexisting densities and surface tension in the critical region are reported as a function of the cutoff distance in the intermolecular potential. The results obtained using slab simulations are compared with those obtained using grand canonical Monte Carlo simulations of isotropic systems and the finite-size scaling techniques. There is a good agreement between these two approaches. The two-phase simulations can be used in approaching the critical point for temperatures up to 0.97 TC∗ (T∗ = 1.26). The critical-point exponents describing the dependence of the density, surface tension, and interfacial thickness on the temperature are calculated near the critical point.

Development of DPD coarse-grained models: From bulk to interfacial properties.

A new Bayesian method was recently introduced for developing coarse-grain (CG) force fields for molecular dynamics. The CG models designed for dissipative particle dynamics (DPD) are optimized based on trajectory matching. Here we extend this method to improve transferability across thermodynamic conditions. We demonstrate the capability of the method by developing a CG model of n-pentane from constant-NPT atomistic simulations of bulk liquid phases and we apply the CG-DPD model to the calculation of the surface tension of the liquid-vapor interface over a large range of temperatures. The coexisting densities, vapor pressures, and surface tensions calculated with different CG and atomistic models are compared to experiments. Depending on the database used for the development of the potentials, it is possible to build a CG model which performs very well in the reproduction of the surface tension on the orthobaric curve.

Structure and rheology of star polymers in confined geometries: a mesoscopic simulation study.

Mesoscopic simulations of star polymer melts adsorbed onto solid surfaces are performed using the dissipative particle dynamics (DPD) method. A set of parameters is developed to study the low functionality star polymers under shear. The use of a new bond-angle potential between the arms of the star creates more rigid chains and discriminates between different functionalities at equilibrium, but still allows the polymers to deform appropriately under shear. The rheology of the polymer melts is studied by calculating the kinetic friction and viscosity and there is good agreement with experimental properties of these systems. The study is completed with predictive simulations of star polymer solutions in an athermal solvent.

Superpermittivity of nanoconfined water.

Nowadays, it is well established that the physical properties of confined liquids strongly differ from those in bulk phase. While dynamical and structural properties were strongly explored, dielectric properties are poorly studied despite their importance in the understanding and the modelling of molecular mechanism in a number of nano-applications such as nanofluidics, nanofiltration, and nanomedicine. Among them, the dielectric permittivity is probably one of the most important. The lack of knowledge about it strongly limits our ability to model fluid-material interactions and more generally our understanding of the behaviour of confined fluids. Recently, the dielectric permittivity of confined water in silica, Metal Organic Frameworks, and graphene materials was found to be slightly higher than the permittivity of water in bulk phase. In this work, the permittivity of water and dichloromethane confined in carbon nanotubes was predicted by means of molecular dynamics simulations. The static dielectric constant was found to be 700, i.e., 10-fold higher than the bulk value. This superpermittivity has, for origin, the excluded volume and the presence of an unconfined direction leading to a pre-orientation of water molecules close to the pore wall and an increase in dipolar fluctuations.

The gas-liquid surface tension of argon: a reconciliation between experiment and simulation.

We present a simulation of the liquid-vapor interface of argon with explicit inclusion of the three-body interactions. The three-body contributions to the surface tension are calculated using the Kirkwood-Buff approach. Monte Carlo calculations of the long-range corrections to the three-body contribution are calculated from the radial distribution function g((2))(z1, cos θ12, r12). Whereas the effective two-body potentials overestimate the surface tension by more than 15%, the inclusion of the three-body potential provides an excellent agreement with the experimental results for temperatures up to 15 K below the critical temperature. We conclude that the three-body interactions must be explicitly included in accurately modelling the surface tension of argon.

Crossover in structure and dynamics of a primary alcohol induced by hydrogen-bonds dilution.

Primary alcohols show a prominent Debye process in the dielectric relaxation located at a timescale longer than the main structural relaxation. Böhmer and co-workers studied dilution effects of the hydrogen bonding network of n-butanol (BuOH) with n-bromobutane (BuBr) to better understand the origin of this process. Interestingly, this work has evidenced a crossover in Debye relaxation time (τD) for a critical concentration in BuBr xc = 0.5. By using molecular dynamics simulations and NMR experiments we propose here to explore further dilution effects on the dipolar and translational dynamics. Moreover, we discuss the relation between structural and dynamical properties in the context of a detailed study of the microstructure and the H-bond network. The overall results are consistent with the existence of a topological change in the liquid structure occurring at about xc = 0.5 from a hypernetted percolating network to independent nanodomains of n-butanol molecules embedded in the n-bromobutane phase.