Materials Day 2019 – Materials Modeling: Across scales, across materials

Program

Abstracts

Prof. Dr. Elie Raphael – Dynamics of thin polymer films

Thin polymer films have striking dynamical properties that differ from their bulk counterparts. With the simple geometry of a stepped polymer film on a substrate, we probe mobility above and below the glass transition temperature Tg. Above Tg, the entire film flows, whereas below Tg only the near-surface region responds to the excess interfacial energy. An analytical thin-film model for flow limited to the free surface region shows excellent agreement with sub-Tg data. The system transitions from whole-film flow to surface localized flow over a narrow temperature region near the bulk Tg. The experiments and model provide a measure of surface mobility in a simple geometry. This fine control of the glassy rheology is of key interest to nano lithography among numerous other applications. We also present recent results of thin film leveling using molecular dynamics simulations. Finally, we present recent results on the capillary relaxation of a square pattern at the free surface of a viscoelastic polymer film, using molecular dynamics simulations of a coarse-grained polymer model.

Prof. Dr. Tanja Schilling – Multi-scale modeling far from thermal equilibrium

Complex microscopic many-body processes are often interpreted in terms of “reaction coordinates”, i.e. in terms of the evolution of a small set of coarse-grained, ensemble averaged variables. Under stationary conditions, the evolution of such coordinates is described by the generalized Langevin equation. In contrast, if the dynamics is not stationary, it is not a priori clear which form the equation of motion for an averaged observable has. We employ the formalism of time-dependent projection operator techniques to derive the equation of motion for a non-equilibrium trajectory-averaged observable as well as for its auto-correlation function. We consider, in particular, Hamiltonians and observables which depend on time explicitly as e.g. in systems under external driving.The equation of motion which we obtain is similar in structure to the generalized Langevin equation, but it exhibits a time-dependent memory kernel as well as a fluctuating force that implicitly depends on the initial conditions of the process. We show how to construct the memory kernel from experimental (or simulation) data. As a numerical example, the procedure is then applied to crystal nucleation from a supercooled Lennard-Jones melt.

Prof. Dr. Liesbeth Janssen – Glassy dynamics of vitrimers

Vitrimers are a new class of polymers which unite the high mechanical performance of thermosets with the malleability and recyclability of thermoplastics. Their facile processability stems from their ‘superstrong’ (sub-Arrhenius) viscosity growth upon cooling, making them malleable over a broad temperature range. We use coarse-grained simulations and mode-coupling theory (MCT) to describe this unusual sub-Arrhenius behavior in a vitrimeric star-polymer system. We can directly link the observed glassy dynamics to anomalous changes in the microstructure, shedding new light on the structural origins of glassy fragility.

Prof. Dr. Emmanuela Zaccarelli – In silico synthesis of microgels: structure, elasticity and effective interactions in bulk and at liquid-liquid interfaces

Microgels are soft particles individually made by cross-linked polymer networks which are nowadays widely used as a colloidal model system because of their swelling properties and their responsivity to external control parameters such temperature or pH. While extensively used as model systems in experimental, their numerical investigation lagged behind due to the inherently complex and multiscale nature of the particles. In this talk I will illustrate the protocol that we recently developed to synthesized microgels in-silico, providing a realistic description of the particles, in particular their characteristic inhomogeneous core-corona structure and their swelling behavior. I will also report on the calculation of their elastic properties and effective interactions in bulk and at liquid-liquid interfaces. The numerical results will be compared to available experiments. Our work aims to establish a clear link between the microscopic network properties and the resulting microgel-microgel interactions, paving the way for a deeper understanding of the behaviour of microgel suspensions.

Prof. Dr. Ludovic Berthier – Equilibrium simulations of supercooled liquids beyond laboratory timescale

Computer simulations give unique insights into the microscopic behavior of disordered and amorphous materials, but their typical timescales are orders of magnitude shorter than the experimentally relevant ones. In particular, simulations of supercooled liquids performed with standard techniques cover at most 4-5 decades of viscous slowing down, far behind the 13 decades commonly accessible in experimental studies. Recently, we have closed this enormous gap for a class of realistic models of liquids, which we can successfully equilibrate beyond laboratory time scales by means of a swap Monte Carlo algorithm. For some models, we achieve over 10 orders of magnitude speedup in equilibration timescale. This exciting numerical advance allows us to address some outstanding questions concerning the formation and properties of glasses in a dynamical range that remains inaccessible in experiments, such as the relevance of an entropy crisis underlying glass formation, the kinetics of ultrastable glasses, and the mechanical properties of realistic amorphous solids.

Prof. Dr. David Rodney – Ab initio modeling of dislocations in metals

The plastic deformation of metals is governed by the glide of dislocations. Understanding the mobility of these defects requires atomic-scale modeling to capture their core structure. Approaches based ab initio density functional theory (DFT) calculations have made tremendous progress these past few years, in part thanks to an increase in computing power, but also because of methodological developments, including methods to correct for elastic interactions between periodic images. In this talk, we will discuss recent advances in dislocation plasticity based on ab initio calculations, mainly in body centered cubic (BCC) and hexagonal close packed (HCP) metals. We will see that the calculations allow to interpret dislocation behaviors observed in in-situ transmission electron microscopy. We will consider the case of pure metals, as well as the effect of alloying, highlighting how interstitial atoms can restructure dislocation cores and profoundly alter the plastic behavior. More generally, we will the strong connection between the dislocation core properties at the atomic scale and macroscopic plastic behaviors.

Prof. Dr. Jörg Neugebauer – Ab initio descriptors to design materials with superior mechanical properties

Modern engineering materials have evolved from simple single phase materials to nano-composites that employ dynamic mechanisms down to the atomistic scale. The structural and thermodynamic complexity of this new generation of structural materials presents a challenge to their design since experimental trial-and-error approaches as successfully used in the past are often no longer feasible. Ab initio approaches provide perfect tools to new design routes but face serious challenges: Finite temperature free energies of the various phases are almost degenerate, requiring advanced theoretical formalisms that accurately capture all relevant entropic contributions. In addition, their hierarchical nature with respect to length and time makes it challenging to explore the large range of chemical compositions. We have therefore developed a python based framework pyiron that allows in a highly automated way to combine accurate finite temperature first principles calculations with big data analytics. The flexibility and the predictive power of these automated approaches will be discussed for examples ranging from the design of ductile Mg alloys, over describing the finite temperature behavior of high entropy alloys, to the discovery of general rules for interstitials in metals.

Prof. Dr. Evelyne Van Ruymbeke – From complex polymer architectures to supramolecular polymeric assemblies: understanding their flow properties with the help of molecular models

Understanding and tailoring the flow properties of polymer melts or concentrated solutions based on the knowledge of their molecular architecture represents a very active field of research, which still requires us to address important and fundamental questions. To this end, coarse-grained models such as models based on the tube theory have been developed, which allow us to describe the dynamics of the macromolecules at a mesoscopic scale. By studying the viscoelastic properties of model polymers, from linear to complex architecture, the main relaxation mechanisms behind their flow behavior are identified and accounted for in the models. Today, while the viscoelastic response of a polymer melt under a small deformation is quite well understood and can be described at a quantitative level, elucidating the molecular origin of their flow properties under large deformation remains a real challenge.

We can go one step further and study the rheology of entangled macromolecular self-assemblies built from supramolecular polymers. These systems are very modulable, exhibiting reversible structural changes with temperature or deformation, and are thermorheologically complex. Often characterized by two distinct dynamics, these systems offer a large potential for the development of new materials.