Kurt Kremer - Academia.edu (original) (raw)

Papers by Kurt Kremer

European Physical Journal-special Topics, Apr 1, 2017

We present large-scale molecular dynamics simulations for a coarse-grained model of polymer melts... more We present large-scale molecular dynamics simulations for a coarse-grained model of polymer melts in equilibrium. From detailed Rouse mode analysis we show that the time-dependent relaxation of the autocorrelation function (ACF) of modes p can be well described by the effective stretched exponential function due to the crossover from Rouse to reptation regime. The ACF is independent of chain sizes N for N/p < N e (Ne is the entanglement length), and there exists a minimum of the stretching exponent as N/p → N e. As N/p increases, we verify the crossover scaling behavior of the effective relaxation time τ eff,p from the Rouse regime to the reptation regime. We have also provided evidence that the incoherent dynamic scattering function follows the same crossover scaling behavior of the mean square displacement of monomers at the corresponding characteristic time scales. The decay of the coherent dynamic scattering function is slowed down and a plateau develops as chain sizes increase at the intermediate time and wave length scales. The tube diameter extracted from the coherent dynamic scattering function is equivalent to the previous estimate from the mean square displacement of monomers.

Journal of Chemical Physics, Mar 4, 2019

To study the cooling behavior and the glass transition of polymer melts in bulk and with free sur... more To study the cooling behavior and the glass transition of polymer melts in bulk and with free surfaces, a coarse-grained weakly semi-flexible polymer model is developed. Based on a standard bead spring model with purely repulsive interactions, an attractive potential between non-bonded monomers is added such that the pressure of polymer melts is tuned to zero. Additionally, the commonly used bond bending potential controlling the chain stiffness is replaced by a new bond bending potential. For this model, we show that the Kuhn length and the internal distances along the chains in the melt only very weakly depend on the temperature, just as for typical experimental systems. The glass transition is observed by the temperature dependency of the melt density and the characteristic non-Arrhenius slowing down of the chain mobility. The new model is set to allow for a fast switch between models, for which a wealth of data already exists.

Journal of Chemical Physics, May 3, 2016

We present a coarse-graining strategy that we test for aqueous mixtures. The method uses pair-wis... more We present a coarse-graining strategy that we test for aqueous mixtures. The method uses pair-wise cumulative coordination as a target function within an iterative Boltzmann inversion (IBI) like protocol. We name this method coordination iterative Boltzmann inversion (C-IBI). While the underlying coarse-grained model is still structure based and, thus, preserves pair-wise solution structure, our method also reproduces solvation thermodynamics of binary and/or ternary mixtures. Additionally, we observe much faster convergence within C-IBI compared to IBI. To validate the robustness, we apply C-IBI to study test cases of solvation thermodynamics of aqueous urea and a triglycine solvation in aqueous urea.

Macromolecular Chemistry and Physics, Nov 5, 2018

This article aims to stimulate research on non-equilibrium macromolecular systems, as nowadays a ... more This article aims to stimulate research on non-equilibrium macromolecular systems, as nowadays a large toolbox to synthesize tailored macromolecules is available. A large variety of characterization methods covering a broad spectrum of length and timescales allows researchers to follow and also manipulate macromolecular systems on their paths toward equilibrium. These possibilities are paralleled by the development of new concepts of the statistical physics of non-equilibrium phenomena in macromolecular systems as well as new models and algorithms for computer simulation.

Journal of Chemical Physics, Apr 20, 2016

We present a detailed study of the static and dynamic behavior of long semiflexible polymer chain... more We present a detailed study of the static and dynamic behavior of long semiflexible polymer chains in a melt. Starting from previously obtained fully equilibrated high molecular weight polymer melts [Zhang et al. ACS Macro Lett. 3, 198 (2014)] we investigate their static and dynamic scaling behavior as predicted by theory. We find that for semiflexible chains in a melt, results of the mean square internal distance, the probability distributions of the end-to-end distance, and the chain structure factor are well described by theoretical predictions for ideal chains. We examine the motion of monomers and chains by molecular dynamics simulations using the ESPResSo++ package. The scaling predictions of the mean squared displacement of inner monomers, center of mass, and relations between them based on the Rouse and the reptation theory are verified, and related characteristic relaxation times are determined. Finally we give evidence that the entanglement length Ne,P P A as determined by a primitive path analysis (PPA) predicts a plateau modulus, G 0 N = 4 5 (ρkBT /Ne), consistent with stresses obtained from the Green-Kubo relation. These comprehensively characterized equilibrium structures, which offer a good compromise between flexibility, small Ne, computational efficiency, and small deviations from ideality provide ideal starting states for future non-equilibrium studies.

Bioinspiration & Biomimetics, Sep 13, 2016

Polymer nanocomposites render a range of outstanding materials from natural products such as silk... more Polymer nanocomposites render a range of outstanding materials from natural products such as silk, sea shells and bones, to synthesized nanoclay or carbon nanotube reinforced polymer systems. In contrast to the fast expanding interest in this type of material, the fundamental mechanisms of their mixing, phase behavior and reinforcement, especially for higher nanoparticle content as relevant for bio-inorganic composites, are still not fully understood. Although polymer nanocomposites exhibit diverse morphologies, qualitatively their mechanical properties are believed to be governed by a few parameters, namely their internal polymer network topology, nanoparticle volume fraction, particle surface properties and so on. Relating material mechanics to such elementary parameters is the purpose of this work. By taking a coarse-grained molecular modeling approach, we study an range of different polymer nanocomposites. We vary polymer nanoparticle connectivity, surface geometry and volume fraction to systematically study rheological/mechanical properties. Our models cover different materials, and reproduce key characteristics of real nanocomposites, such as phase separation, mechanical reinforcement. The results shed light on establishing elementary structure, property and function relationship of polymer nanocomposites.

Journal of Physical Chemistry B, Sep 15, 2021

Most of the artwork and cultural heritage objects are stored in museums under conditions that are... more Most of the artwork and cultural heritage objects are stored in museums under conditions that are difficult to monitor. While advanced technologies aim to control and prevent the degradation of cultural heritage objects in line with preventive conservation measures, there is much to be learned in terms of the physical processes that lead to the degradation of the synthetic polymers that form the basis of acrylic paints largely used in contemporary art. In museums, stored objects are often exposed to temperature and relative humidity fluctuations as well as airborne pollutants such as volatile organic compounds (VOCs). The glass transition of acrylic paints is below room temperature; while low temperatures may cause cracking, at high temperatures the sticky surface of the paint becomes vulnerable to pollutants. Here we develop fully atomistic models to understand the structure of two types of acrylic copolymers and their interactions with VOCs and water. The structure and properties of acrylic copolymers are slighlty modified by incorporation of a monomer with a longer side chain. With favorable solvation free energies, once absorbed, VOCs and water interact with the polymer side chains to form hydrogen bonds. The cagelike structure of the polymers prevents the VOCs and water to diffuse freely below the glass transition temperature. In addition, our model forms the foundation for developing mesoscopic and continuum models that will allow us to access longer time and length scales to further our understanding of the degradation of artwork.

Advanced theory and simulations, May 16, 2018

Syndiotactic polystyrene (sPS) exhibits complex polymorphic behavior upon crystallization. Comput... more Syndiotactic polystyrene (sPS) exhibits complex polymorphic behavior upon crystallization. Computational modeling of polymer crystallization has remained a challenging task because the relevant processes are slow on the molecular time scale. We report herein a detailed characterization of sPS-crystal polymorphism by means of coarse-grained (CG) and atomistic (AA) modeling. The CG model, parametrized in the melt, shows remarkable transferability properties in the crystalline phase. Not only is the transition temperature in good agreement with atomistic simulations, it stabilizes the main α and β polymorphs, observed experimentally. We compare in detail the propensity of polymorphs at the CG and AA level and discuss finite-size as well as box-geometry effects. All in all, we demontrate the appeal of CG modeling to efficiently characterize polymer-crystal polymorphism at large scale.

Annual Review of Condensed Matter Physics, Mar 10, 2020

In this review we summarize recent theoretical and computational developments in the field of sma... more In this review we summarize recent theoretical and computational developments in the field of smart responsive materials, together with complementary experimental data. A material is referred to as smart responsive when a slight change in external stimulus can drastically alter its structure, function, or stability. Because of this smart responsiveness, these systems are used for the design of advanced functional materials. The most characteristic properties of smart polymers will be discussed, especially polymer properties in solvent mixtures. We will show how a multi-scale simulation approach can shed light on the intriguing experimental observations. Special emphasis will be given to two symmetric phenomena: co-non-solvency and co-solvency. The first phenomenon is associated with the collapse of polymers in two miscible good solvents, while the later is associated with the swelling of polymers in poor solvent mixtures. Furthermore, we will discuss when the standard Flory-Huggins type mean-field polymer theory can (or can not) be applied to understand these complex solution properties. We will also point towards future directions-how smart polymer properties can be used for the design principles of advanced functional materials.

Journal of Chemical Physics, Apr 3, 2007

We report adaptive resolution molecular dynamics simulations of a flexible linear polymer in solu... more We report adaptive resolution molecular dynamics simulations of a flexible linear polymer in solution. The solvent, i.e., a liquid of tetrahedral molecules, is represented within a certain radius from the polymer's center of mass with a high level of detail, while a lower coarse-grained resolution is used for the more distant solvent. The high resolution sphere moves with the polymer and freely exchanges molecules with the low resolution region through a transition regime. The solvent molecules change their resolution and number of degrees of freedom on-the-fly. We show that our approach correctly reproduces the static and dynamic properties of the polymer chain and surrounding solvent.

Journal of Chemical Physics, Dec 8, 2005

We present a new adaptive resolution technique for efficient particle-based multiscale molecular ... more We present a new adaptive resolution technique for efficient particle-based multiscale molecular dynamics (MD) simulations. The presented approach is tailor-made for molecular systems where atomistic resolution is required only in spatially localized domains whereas a lower mesoscopic level of detail is sufficient for the rest of the system. Our method allows an on-the-fly interchange between a given molecule's atomic and coarse-grained level of description, enabling us to reach large length and time scales while spatially retaining atomistic details of the system. The new approach is tested on a model system of a liquid of tetrahedral molecules. The simulation box is divided into two regions: one containing only atomistically resolved tetrahedral molecules, the other containing only one particle coarse-grained spherical molecules. The molecules can freely move between the two regions while changing their level of resolution accordingly. The coarse-grained and the atomistically resolved systems have the same statistical properties at the same physical conditions.

Macromolecules, Aug 29, 2019

Polymer melts undergoing large deformation by elongation are studied by molecular dynamics simula... more Polymer melts undergoing large deformation by elongation are studied by molecular dynamics simulations of bead-spring chains in melts. By applying a primitive path analysis to strongly deformed polymer melts, the role of topological constraints in highly entangled polymer melts is investigated and quantified. We show that the overall, large scale conformations of the primitive paths (PPs) of stretched chains follow affine deformation while the number and the distribution of entanglement points along the PPs do not. Right after deformation, PPs of chains retract in both directions parallel and perpendicular to the elongation. Upon further relaxation we observe a long-lived clustering of entanglement points. Together with the delayed relaxation time this leads to a metastable inhomogeneous distribution of topological constraints in the melts.

Journal of Chemical Physics, Jan 15, 1999

We present the results of a combined experimental ͑neutron scattering͒ and theoretical ͑computer ... more We present the results of a combined experimental ͑neutron scattering͒ and theoretical ͑computer simulation͒ effort to investigate structural properties of polycarbonate melts and glasses in the wave vector regime of Qр2.2 Å Ϫ1 . The experimental part consists of advanced spin polarized scattering experiments, allowing us to extract the coherent scattering for protonated and deuterated samples. The simulations employ recently developed novel mapping procedures, which allow us to efficiently equilibrate complex polymer melts, thereby reproducing the experiment in much closer detail than earlier attempts.

Springer eBooks, Apr 15, 2007

Topological constraints, entanglements, dominate the viscoelastic behavior of high molecular weig... more Topological constraints, entanglements, dominate the viscoelastic behavior of high molecular weight polymeric liquids. To give a microscopic foundation of the phenomenological tube, recently a method for identifying the so called primitive path mesh that characterizes the microscopic topological state of (computer generated) conformations of long-chain polymer networks, melts and solutions was introduced. Here we give a short account of this approach and compare this to long time simulations.

Journal of Chemical Physics, Oct 12, 2020

Equilibration of polymer melts containing highly entangled long polymer chains in confinement or ... more Equilibration of polymer melts containing highly entangled long polymer chains in confinement or with free surfaces is a challenge for computer simulations. We approach this problem by first studying polymer melts based on the soft-sphere coarse-grained model confined between two walls with periodic boundary conditions in two directions parallel to the walls. Then, we insert the microscopic details of the underlying bead-spring model. Tuning the strength of the wall potential, the monomer density of confined polymer melts in equilibrium is kept at the bulk density even near the walls. In a weak confining regime, we observe the same conformational properties of chains as in the bulk melt showing that our confined polymer melts have reached their equilibrated state. Our methodology provides an efficient way of equilibrating large polymer films with different thicknesses and is not confined to a specific underlying microscopic model. Switching off the wall potential in the direction perpendicular to the walls enables to study free-standing highly entangled polymer films or polymer films with one supporting substrate.

Journal of Chemical Physics, Jul 8, 2002

We discuss the hydrodynamic radius RH of polymer chains in good solvent, and show that the leadin... more We discuss the hydrodynamic radius RH of polymer chains in good solvent, and show that the leading order correction to the asymptotic law RH ∝ N ν (N degree of polymerization, ν ≈ 0.59) is an "analytic" term of order N -(1-ν) , which is directly related to the discretization of the chain into a finite number of beads. This result is further corroborated by exact calculations for Gaussian chains, and extensive numerical simulations of different models of good-solvent chains, where we find a value of 1.591 ± 0.007 for the asymptotic universal ratio RG/RH , RG being the chain's gyration radius. For Θ chains the data apparently extrapolate to RG/RH ≈ 1.44, which is different from the Gaussian value 1.5045, but in accordance with previous simulations. We also show that the experimentally observed deviations of the initial decay rate in dynamic light scattering from the asymptotic Benmouna-Akcasu value can partly be understood by similar arguments.

arXiv (Cornell University), Aug 9, 2018

We demonstrate that hierarchical backmapping strategies incorporating generic blob-based models c... more We demonstrate that hierarchical backmapping strategies incorporating generic blob-based models can equilibrate melts of high-molecular-weight polymers, described with chemically specific, atomistic, models. The central idea behind these strategies, is first to represent polymers by chains of large soft blobs (spheres) and efficiently equilibrate the melt on mesoscopic scale. Then, the degrees of freedom of more detailed models are reinserted step by step. The procedure terminates when the atomistic description is reached. Reinsertions are feasible computationally because the fine-grained melt must be re-equilibrated only locally. To develop the method, we choose a polymer with sufficient complexity. We consider polystyrene (PS), characterized by stereochemistry and bulky side groups. Our backmapping strategy bridges mesoscopic and atomistic scales by incorporating a blob-based, a moderately CG, and a united-atom model of PS. We demonstrate that the generic blob-based model can be parameterized to reproduce the mesoscale properties of a specific polymer -here PS. The moderately CG model captures stereochemistry. To perform backmapping we improve and adjust several fine-graining techniques. We prove equilibration of backmapped PS melts by comparing their structural and conformational properties with reference data from smaller systems, equilibrated with less efficient methods.

We derive a kinetic Monte Carlo algorithm to simulate flow-induced nucleation in polymer melts. T... more We derive a kinetic Monte Carlo algorithm to simulate flow-induced nucleation in polymer melts. The crystallisation kinetics are modified by both stretching and orientation of the amorphous chains under flow, which is modelled by a recent non-linear tube theory. Rotation of the crystallites under flow is modelled by a simultaneous Brownian dynamics simulation. Our kinetic Monte Carlo approach is highly efficient at simulating nucleation and is tractable even at low under-cooling. The simulations predict enhanced nucleation under both transient and steady state shear. Furthermore the model predicts the growth of shish-like elongated nuclei for sufficiently fast flows, which grow by a purely kinetic mechanism.

European Physical Journal B, Jan 23, 2008

The relation between atomistic structure, architecture, molecular weight and material properties ... more The relation between atomistic structure, architecture, molecular weight and material properties is a basic concern of modern soft material science. This by now goes far beyond standard properties of bulk materials. A typical additional focus is on surface or interface aspects or on the relation between structure and function in nanoscopic molecular assemblies. This all implies a thorough understanding on many length and correspondingly time scales ranging from (sub)-atomic to macroscopic. At this point computer simulations are playing an increasingly important, if not the central role. Traditionally simulations have been separated in two main groups, namely simplified models to deal with generic or universal aspects of polymers, i.e. critical exponents, and those employing classical force field simulations with (almost) all atomistic detail, i.e. for the diffusion of small additives in a small "sample". Still characteristic problems, which require huge systems and/or long times in combination with a chemistry specific model, cannot be tackled by these methods alone. More recently with the development of scale bridging or multi scale simulation techniques, these different approaches have been combined into an emerging rather powerful tool. It is the purpose of this contribution to give a few examples of how such an approach can be used to understand specific material properties.

arXiv (Cornell University), Jul 23, 2020

Polymorphism rationalizes how processing can control the final structure of a material. The rugge... more Polymorphism rationalizes how processing can control the final structure of a material. The rugged free-energy landscape and exceedingly slow kinetics in the solid state have so far hampered computational investigations. We report for the first time the free-energy landscape of a polymorphic crystalline polymer, syndiotactic polystyrene. Coarse-grained metadynamics simulations allow us to efficiently sample the landscape at large. The free-energy difference between the two main polymorphs, α and β, is further investigated by quantumchemical calculations. The two methods are in line with experimental observations: they predict β as the more stable polymorph at standard conditions. Critically, the free-energy landscape suggests how the α polymorph may lead to experimentally observed kinetic traps. The combination of multiscale modeling, enhanced sampling, and quantum-chemical calculations offers an appealing strategy to uncover complex freeenergy landscapes with polymorphic behavior.