Martin Kröger | Swiss Federal Institute of Technology (ETH) (original) (raw)

Papers by Martin Kröger

The Journal of Chemical Physics, 2001

Results are presented for the structural and thermodynamic properties of the orthorhombic phase o... more Results are presented for the structural and thermodynamic properties of the orthorhombic phase of crystalline polyethylene (PE) from detailed atomistic molecular-dynamics (MD) simulations in the NPT statistical ensemble. Two different PE crystal systems have been investigated: (a) paraffins consisting of an odd number of carbon atoms such as C23H48 and (b) infinite length PE monocrystals, at temperatures ranging from T=150

Macromolecules, 2003

Results are presented from 300 ns long atomistic molecular dynamics (MD) simulations of polyethyl... more Results are presented from 300 ns long atomistic molecular dynamics (MD) simulations of polyethylene (PE) melts, ranging in molecular length from C 78 to C250. Above C156, the self-diffusion coefficient D is seen to exhibit a clear change in its power-law dependence on the molecular weight (M), significantly deviating from a Rouse (where D ∼ M-1) toward a reptation-like (where D ∼ M-2.4) behavior. The mean-square displacement (msd) of chain segments and the dynamic structure factor is also calculated and the crossover from the Rouse to entangled behavior is again observed above C156. A novel strategy is also developed for projecting atomistic chain configurations to their primitive paths and thereby mapping simulation trajectories onto the reptation model. Results for the friction factor , the zero-shear rate viscosity η0 and the self-diffusion coefficient D are found to be internally consistent and in agreement with experimental rheological data.

Rheologica Acta, 2008

Numerical simulations have been performed to evaluate the accuracy of the multimode Giesekus mode... more Numerical simulations have been performed to evaluate the accuracy of the multimode Giesekus model in predicting the flow behavior of a rheologically well characterized low-density polyethylene melt in a lubricated cross-slot channel. Specifically, the fidelity of the numerical results is established by detailed comparison with flow-induced birefringence measurements in a new optical rheometer with lubricated side walls that allows the creation of ideal two-dimensional flow kinematics that lead to the elimination of end effects commonly encountered in flow birefringence measurements. Based on these comparisons, the ability of the multimode Giesekus model to capture the flow characteristics with reasonable accuracy in the experimentally available Wi range of 21 to 29 has been established. However, it should be noted that the model predictions

New Journal of Physics, 2014

We present a method to map the full equilibrium distribution of the primitivepath (PP) length, ob... more We present a method to map the full equilibrium distribution of the primitivepath (PP) length, obtained from multi-chain simulations of polymer melts, onto a single-chain mean-field 'target' model. Most previous works used the Doi-Edwards tube model as a target. However, the average number of monomers per PP segment, obtained from multi-chain PP networks, has consistently shown a discrepancy of a factor of two with respect to tube-model estimates. Part of the problem is that the tube model neglects fluctuations in the lengths of PP segments, the number of entanglements per chain and the distribution of monomers among PP segments, while all these fluctuations are observed in multi-chain simulations. Here we use a recently proposed slip-link model, which

Soft Matter, 2021

Conformations, entanglements, and dynamics in attractive polymer nanocomposites are investigated ... more Conformations, entanglements, and dynamics in attractive polymer nanocomposites are investigated by means of coarse-grained molecular dynamics simulation, for both weak and strong confinements, at nanoparticle volume fractions ϕ up to 60%.

The Journal of Chemical Physics, 2016

The complete kinetic theory model for concentrated polymer solutions and melts proposed by Curtis... more The complete kinetic theory model for concentrated polymer solutions and melts proposed by Curtiss and Bird is solved for shear flow: (a) analytically by providing a solution for the single-link (or configurational) distribution function as a real basis spherical harmonics expansion and then calculating the materials functions in shear flow up to second order in the dimensionless shear rate and, (b) numerically via the execution of Brownian dynamics simulations. These two methods are actually complementary to each other as the former is accurate only for small dimensionless shear rates where the latter produces results with increasingly large uncertainties. The analytical expansions of the material functions with respect to the dimensionless shear rate reduce to those of the extensively studied, simplified Curtiss-Bird model when ε ′ = 0, and to the rigid rod when ε ′ = 1. It is known that the power-law behavior at high shear rates is very different for these two extremal cases. We employ Brownian dynamics simulation to not only recover the limiting cases but to find a gradual variation of the power-law behaviors at large dimensionless shear rates upon varying ε ′. The fact that experimental data are usually located between these two extremes strongly advocates the significance of studying the solution of the Curtiss-Bird model. This is exemplified in this work by comparing the solution of this model with available rheological data for semiflexible biological systems that are clearly not captured by the original Doi-Edwards or simplified Curtiss-Bird models.

Acta Crystallographica Section A Foundations and Advances, 2016

NaLaF 4 is an efficient up-conversion phosphor [1] belonging to the family of rare earth-doped so... more NaLaF 4 is an efficient up-conversion phosphor [1] belonging to the family of rare earth-doped sodium lanthanide tetrafluorides where the nature of the photoactive sites can unambiguously be explained only in terms of a microscopic model of disorder [2]. X-ray and Neutron Diffuse Scattering (XDS, NDS), as k-space probes, provide statistically reliable data on atomic pair correlation functions and thus on deviations from the average periodic structure. Using high quality XDS and NDS datasets from the distinctive, planar diffuse scattering of NaLaF 4 , we calculated the 3D-Difference Pair Distribution Function (3D-ΔPDF), identified the various types of disorder and derived an abstract model of interatomic vectors [3]. It was then possible by means of Monte Carlo (MC) simulations to build and optimize large model crystals providing a specific atomistic disorder model for NaLaF 4 , whose Fourier transform gives an excellent fit to the experimental diffuse scattering. The distinct contrast displayed by the various atomic species with the two radiation types provides a better sensitivity to the different pair-wise correlations ultimately yielding more robust results.

Physical chemistry chemical physics : PCCP, Jan 11, 2017

A core-polyethylene glycol-lipid shell (CPLS) nanoparticle consists of an inorganic core coated w... more A core-polyethylene glycol-lipid shell (CPLS) nanoparticle consists of an inorganic core coated with polyethylene glycol (PEG) polymers, surrounded by a lipid bilayer shell. It can be self-assembled from a PEGylated core with surface-tethered PEG chains, where all the distal ends are covalently bonded to lipid molecules. Upon adding free lipids, a complete lipid bilayer shell can be formed on the surface driven by the hydrophobic nature of lipid tails, leading to the formation of a CPLS nanoparticle. The stability of CPLS nanoparticles in shear flow has been systematically studied through large scale dissipative particle dynamics simulations. CPLS nanoparticles demonstrate higher stability and less deformation in shear flow, compared with lipid vesicles. Burst leakage of drug molecules inside lipid vesicles and CPLS NPs can be induced by the large pores at their tips. These pores are initiated by the maximum stress in the waist region. It further grows along with the tank-treading m...

AIP Conference Proceedings, 2008

... well in predicting the steady flow kinematics and stress field of moderately concentrated pol... more ... well in predicting the steady flow kinematics and stress field of moderately concentrated polymeric solutions in complex kinematics flows up to moderate Weissenberg numbers [2-4]. However, in most of these studies the presence of end effects close to the walls has led to ...

Physica A: Statistical Mechanics and its Applications, 2002

The quasi-equilibrium or maximum entropy approximation is applied in order to derive constitutive... more The quasi-equilibrium or maximum entropy approximation is applied in order to derive constitutive equations from kinetic models of polymer dynamics. It is shown in general and illustrated for an example how canonical distribution functions are obtained from the maximum entropy principle, how macroscopic and constitutive equations are derived therefrom and how these constitutive equations can be implemented numerically. In addition, a measure for the accuracy of the quasi-equilibrium approximation is proposed that can be evaluated while integrating the constitutive equations. In the example considered, it is confirmed that the accuracy of the approximation is increased by including more macroscopic variables. In steady elongational flow, it is found that more macroscopic variables need to be included above the coil-stretch transition to achieve the same accuracy as below.

Langmuir, 2009

Attractive interactions between proteins and polyethylene glycol (PEG) give rise to ternary adsor... more Attractive interactions between proteins and polyethylene glycol (PEG) give rise to ternary adsorption within PEG brushes. Experimental evidence suggests two ternary adsorption modes: (i) weak, due to nonspecific weak attraction between PEG monomers and the surface of the protein, as exemplified by serum albumin and (ii) strong, due to strong binding of PEG segments to specific protein sites as it occurs for PEG antibodies, which can involve the terminal adsorption of free chain ends or backbone adsorption due to binding to interior chain segments. Ternary adsorption affects the capacity of brushes to repress protein adsorption. The strong adsorption of antibodies can trigger an immune response that may affect the biocompatibility of the surface. Theoretical adsorption isotherms and protein concentration profiles of the three cases are compared for "parabolic" brushes, allowing for the grafting density, 1 / Σ , and degree of polymerization of the PEG chains, N, as well as the volume and surface area of the proteins. The amount of adsorbed protein per unit area, Γ, exhibits a mode-specific maximum in all three cases. For backbone and weak adsorption, Γ ∼ N, whereas for terminal adsorption, Γ ∼ N 0. In every case, the concentration profile of adsorbed proteins, c tern (z), exhibits a maximum at z max >0 that shifts outward as Σ decreases; z max =0 occurs only for weak and backbone adsorption at a high Σ value.