Holger Merlitz

Polymer brushes for surface tuning.

Mixed polymer brushes as functional ultra thin films for surface functionalization have an enormous potential to create a variety of smart, switchable, and multifunctional surfaces and thin films. It is shown how computer simulations can contribute to a better understanding of the switching behavior of brushes. Furthermore, it is described how polymer brushes can be used to create surfaces with switchable ultrahydrophobicity and wettability gradients, as well as functional layers for the immobilization of nanoparticles. Applications of these versatile and multifunctional brush coatings are envisioned in many areas including fluid control, microfluidics, and thin film sensors.

Nanoscale Brushes: How to Build a Smart Surface Coating

Via computer simulations, we demonstrate how a densely grafted layer of polymers, a brush, could be turned into an efficient switch through chemical modification of some of its end monomers. In this way, a surface coating with reversibly switchable properties can be constructed. We analyze the fundamental physical principle behind its function, a recently discovered surface instability, and demonstrate that the combination of a high grafting density, an inflated end-group size, and a high degree of monodispersity is a condition for an optimal functionality of the switch.

Molecular dynamics simulations of polyelectrolyte brushes under poor solvent conditions: Origins of bundle formation

Molecular dynamics simulations are applied to investigate salt-free planar polyelectrolyte brushes under poor solvent conditions. Starting above the Θ-point with a homogeneous brush and then gradually reducing the temperature, the polymers initially display a lateral structure formation, forming vertical bundles of chains. A further reduction of the temperature (or solvent quality) leads to a vertical collapse of the brush. By varying the size and selectivity of the counterions, we show that lateral structure formation persists and therefore demonstrate that the entropy of counterions being the dominant factor for the formation of the bundle phase. By applying an external compression force on the brush we calculate the minimal work done on the polymer phase only and prove that the entropy gain of counterions in the bundle state, as compared to the homogeneously collapsed state at the same temperature, is responsible for the lateral microphase segregation. As a consequence, the observed lateral structure formation has to be regarded universal for osmotic polymer brushes below the Θ-point.

Polymer-Induced Inverse-Temperature Crystallization of Nanoparticles on a Substrate

Using molecular dynamics simulations, we study the properties of liquid state polymer-nanoparticle composites confined between two parallel substrates, with an attractive polymer-substrate interaction. Polymers are in the semidilute regime at concentrations far above the overlap point, and nanoparticles are in good solvent and without enthalpic attraction to the substrates. An increase of temperature then triggers the crystallization of nanoparticles on one of the two substrate surfaces-a surprising phenomenon, which is explained in terms of scaling theory, such as through competing effects of adsorption-and correlation blobs. Moreover, we show that the first, closely packed layer of nanoparticles on the substrate increases the depletion attraction of additional nanoparticles from the bulk, thereby enhancing and stabilizing the formation of a crystalline phase on the substrate. Within the time frame accessible to our numerical simulations, the crystallization of nanoparticles was irreversible; that is, their crystalline phase, once created, remained undamaged after a decrease of the temperature. Our study leads to a class of thermoreactive nanomaterials, in which the transition between a homogeneous state with dissolved nanoparticles and a surface-crystallized state is triggered by a temperature jump.

Accurate Calculations of Interstellar Lines of Mg+ Using the Coupled Cluster Approach

One of the most successful ab initio, highly correlated all-order many-body methods, the relativistic coupled cluster theory, is employed to calculate excitation energies of the doublet states of Mg+ and allowed transitions among them that are of interest in astrophysical problems. We have also calculated oscillator strength for the 3s-4p doublet transitions, which is improved over the existing results. These transition lines have been sought after in astronomical observations because they represent the best column density identifier in the interstellar medium. Our calculated oscillator strength (9.3 × 10-4) and branching ratio (1.80) of these doublet lines matches well with the recent empirical and semiempirical calculations.

Relativistic coupled cluster calculations using hybrid basis functions

In this paper we present a new method of generating a relativistic basis set for atomic Dirac–Fock (DF) orbitals. Here, all the occupied and a few low-lying unoccupied DF orbital wavefunctions of atoms obtained from the finite basis set expansion approach are replaced by orbital wavefunctions obtained from numerical solutions. We compare this with the Gaussian basis set generation by employing orbitals obtained from both the approaches in a coupled cluster method and computing the ionization potential and oscillator strengths for Mg + and Ca + using the above two different bases. The new method is found to be more appropriate for high-precision calculations.

Numerical evidences for a free energy barrier in starlike polymer brushes

The existence of a free energy barrier, which prohibits the upward motion of retracted molecules into the surface region of starlike polymer brushes, is analyzed through molecular dynamics simulations in good solvent. This barrier emerges at moderate and high grafting densities, as a result of a density-discontinuity at the branching points of the highly stretched starlike molecules. The vertical force profiles of brushes of varying densities are taken with the help of a probe-particle that is gradually moved into the brush, and the results are compared with the density profiles and their negative gradients which generate the local osmotic pressures. Chain expulsion simulations, supported by scaling theory, are conducted to understand the dynamics of individual molecules inside the brushes. We prove that the flip-rates between retracted and extended states, being of relevance for the generation of efficiently switchable, environment-responsive brush layers, are determined by the elastic tension of the stretched molecules.

Influence of correlation effects on the magnetic dipole hyperfine interaction in the low-lying states of Ca+

The relativistic coupled cluster theory is employed to calculate the hyperfine structure of the 2S 1/2, 2P 1/2, 2P 3/2, 2D 3/2 and 2D 5/2 states of singly ionized calcium. The importance of correlation effects is highlighted. Our results are compared with other theoretical calculations and experiments.

Relativistic coupled cluster calculations of the energies for rubidium and cesium atoms

Ionization potentials and excitation energies of rubidium and cesium atoms are computed using the relativistic coupled cluster (CC) method. The effect of electron correlations on the ground and excited state properties is investigated using different levels of CC approximations and truncation schemes. The present work demonstrates that the even-parity channel truncation scheme produces results almost as accurate as obtained from the all-parity channel approximation scheme at a reduced computational cost. The present study also indicates that for a given basis the linearized CC method tends to overestimate the ground and excited state properties compared to the full CC method.

Electric dipole transition amplitudes for 207Pb

Electric dipole transition amplitudes of certain low-lying states of Pb + have been calculated using the relativistic coupled-cluster theory and compared with previous calculations. The role of electron correlation is found to be important. Some of the results we have obtained would be useful in estimating the size of the parity non-conserving amplitude for the 6p 2 P1/2 → 6p 2 P3/2 transition.