Magnetic-field-induced stress in confined magnetoactive elastomers.

We present a theoretical approach for calculating the state of stress induced by a uniform magnetic field in confined magnetoactive elastomers of arbitrary shape. The theory explicitly includes the magnetic field generated by magnetizable spherical inclusions in the sample interior assuming a non-linear magnetization behavior. The initial spatial distribution of particles and its change in an external magnetic field are considered. This is achieved by the introduction of an effective demagnetizing factor where both the sample shape and the material microstructure are taken into account. Theoretical predictions are fitted to the stress data measured using a specifically designed experimental setup. It is shown that the theory enables the quantification of the effect of material microstructure upon introducing a specific microstructural factor and its derivative with respect to the extensional strain in the undeformed state. The experimentally observed differences between isotropic and anisotropic samples, compliant and stiff elastomer matrices are explained.

Conformational switching of modified guest chains in polymer brushes.

Using a numerical quasi off-lattice self-consistent field method which describes heterogeneous chains of spherical monomers we study the case of a densely grafted polymer brush with a fraction of free chain ends being replaced by a modified end-group differing in size and solvent selectivity. We can confirm the observation from molecular dynamics simulations that upon changing the solvent conditions, a switching in location of end-groups which are bigger than monomers from a state "exposed" to the solvent (on the top of the brush) to a "hidden" state (inside the brush) takes place. Our numerical method allows a detailed study of the switching effect as a function of the relevant parameters, such as grafting density, chain length, size of end-groups and their volume fraction. We find that the switching effect is enhanced for long chains, low fractions of modified chains, and big end-groups. We consider the case of low fraction of modified chains in more detail using a test chain method. Here, we explore the optimal grafting density as a function of the size of the end-groups, where the switching is most sensitive. These values can be in the experimental range for end-groups which are at least 3-4 times bigger than the monomers. The end-groups can be realized by attaching nano-particles to the last monomer of a brush-chain.

Surface instabilities of minority chains in dense polymer brushes: a comparison of density functional theory and quasi-off-lattice self-consistent field theory.

This work studies surface instabilities in switchable homopolymer brushes where the minority chain differs in length from the brush chains. Both off-lattice numerical self-consistent field theory and classical density functional theory are employed. It is found that the two methods agree well with each other as long as the same equation of state for the polymer chains is used.

A new numerical approach to dense polymer brushes and surface instabilities.

We present a numerical self-consistent field (SCF) method which describes freely jointed chains of spherical monomers applied to densely grafted polymer brushes. We discuss both the Flory-Huggins model and the Carnahan-Starling equation of state and show the latter being preferable within our model at polymer volume fractions above 10%. We compare the results of our numerical method with data from molecular dynamics (MD) simulations [G.-L. He, H. Merlitz, J.-U. Sommer, and C.-X. Wu, Macromolecules 40, 6721 (2007)] and analytical SCF calculations [P. M. Biesheuvel, W. M. de Vos, and V. M. Amoskov, Macromolecules 41, 6254 (2008)] and obtain close agreement between the density profiles up to high grafting densities. In contrast to prior numerical and analytical studies of densely grafted polymer brushes our method provides detailed information about chain configurations including fluctuation, depletion, and packing effects. Using our model we could study the recently discovered instability of densely grafted polymer brushes with respect to slight variations of individual chain lengths, driven by fluctuation effects [H. Merlitz, G.-L. He, C.-X. Wu, and J.-U. Sommer, Macromolecules 41, 5070 (2008)]. The obtained results are in very close agreement with corresponding MD simulations.

Adsorption of random copolymers from a melt onto a solid surface: Monte Carlo studies.

We study the behavior of random AB-copolymer melts near a selective surface. We consider the case where the copolymers do not display phase segregation behavior in the bulk but the surface is strongly selective for the A-component and the probability of finding an A-monomer along the chain is p<<1. Using self-consistent field theory and scaling arguments, we discuss some aspects of conformational rearrangements and composition selection in the surface layer. For strong selectivity we discuss the formation of a polydisperse brush on the surface. Next, we consider selection mechanisms of chains and sequences of A-species in the surface layer. We used the bond-fluctuation method to simulate copolymer melts at different values of the surface selectivity. Several aspects of the surface layer are analyzed, such as the composition profiles, chemical composition of chains on the surface, chain extension, and dynamics. We find evidence for conformational rearrangements in the surface layer according to the polydisperse brush model, as well as enrichment of A-monomers in the adosorbed chains, stretching of chains in the direction perpendicular to the surface, and selection of multiple A-sequences. Slight but systematic variation of the properties of surface layer at long simulation times indicates that selection processes require very long time scales as expected from theoretical arguments.

Polymer chains in confined geometries: massive field theory approach.

The massive field theory approach in fixed space dimensions d<4 is applied to investigate a dilute solution of long-flexible polymer chains in a good solvent between two parallel repulsive walls, two inert walls, and for the mixed case of one inert and one repulsive wall. The well-known correspondence between the field theoretical phi4 O(n) -vector model in the limit n-->0 and the behavior of long-flexible polymer chains in a good solvent is used to calculate the depletion interaction potential and the depletion force up to one-loop order. In order to make the theory UV finite in renormalization-group sense in 3