Influence of matrix stiffness on microstructure evolution and magnetization of magneto-active elastomers.

Field-induced microstructure evolution can play an important role in defining the coupled magneto-mechanical response of Magneto-Active Elastomers (MAEs). The behavior of these materials is classically modeled using mechanical, magnetic and coupled magneto-mechanical contributions to their free energy function. If the MAE sample is fully clamped so it cannot deform, the mechanical coupling is reduced to the internal microscopic deformations caused by the particles moving and deforming the elastic medium that surrounds them.

Effect of microstructure evolution on the mechanical behavior of magneto-active elastomers with different matrix stiffness.

Evolution of microstructure in magneto-active elastomers (MAEs) which can be caused by an applied magnetic field is a fascinating phenomenon with a significant impact on the mechanical behavior of the composite. To gain insight into the underlying mechanisms of this phenomenon, it is essential to create a model that can appropriately describe the field induced change in the particle distribution and its mechanical implications. The magneto-mechanical coupling is driven by magnetic interactions between the particles in the applied field.

Theoretical analysis of the elastic free energy contributions to polymer brushes in poor solvent: A refined mean-field theory.

We consider polymer brushes in poor solvent that are grafted onto planar substrates and onto the internal and external surfaces of a cylinder using molecular dynamics simulation, self-consistent field (SCF), and mean-field theory. We derive a unified expression for the mean field free energy for the three geometrical classes. While for low grafting densities, the effect of chain elasticity can be neglected in poor solvent conditions, it becomes relevant at higher grafting densities and, in particular, for concave geometries.

Magneto-Mechanical Enhancement of Elastic Moduli in Magnetoactive Elastomers with Anisotropic Microstructures.

Magnetoactive elastomers (MAEs) have gained significant attention in recent years due to their wide range of engineering applications. This paper investigates the important interplay between the particle microstructure and the sample shape of MAEs. A simple analytical expression is derived based on geometrical arguments to describe the particle distribution inside MAEs.

Field-Induced Transversely Isotropic Shear Response of Ellipsoidal Magnetoactive Elastomers.

Magnetoactive elastomers (MAEs) claim a vital place in the class of field-controllable materials due to their tunable stiffness and the ability to change their macroscopic shape in the presence of an external magnetic field. In the present work, three principal geometries of shear deformation were investigated with respect to the applied magnetic field. The physical model that considers dipole-dipole interactions between magnetized particles was used to study the stress-strain behavior of ellipsoidal MAEs.

A Cascading Mean-Field Approach to the Calculation of Magnetization Fields in Magnetoactive Elastomers.

We consider magnetoactive elastomer samples based on the elastic matrix and magnetizable particle inclusions. The application of an external magnetic field to such composite samples causes the magnetization of particles, which start to interact with each other. This interaction is determined by the magnetization field, generated not only by the external magnetic field but also by the magnetic fields arising in the surroundings of interacting particles.

Magneto-Mechanical Coupling in Magneto-Active Elastomers.

In the present work, the magneto-mechanical coupling in magneto-active elastomers is investigated from two different modeling perspectives: a micro-continuum and a particle-interaction approach. Since both strategies differ significantly in their basic assumptions and the resolution of the problem under investigation, they are introduced in a concise manner and their capabilities are illustrated by means of representative examples. To motivate the application of these strategies within a hybrid multiscale framework for magneto-active elastomers, their interchangeability is then examined in a systematic comparison of the model predictions with regard to the magneto-deformation of chain-like helical structures in an elastomer surrounding.

Giant Extensional Strain of Magnetoactive Elastomeric Cylinders in Uniform Magnetic Fields.

Elongations of magnetoactive elastomers (MAEs) under ascending-descending uniform magnetic fields were studied experimentally using a laboratory apparatus specifically designed to measure large extensional strains (up to 20%) in compliant MAEs. In the literature, such a phenomenon is usually denoted as giant magnetostriction. The synthesized cylindrical MAE samples were based on polydimethylsiloxane matrices filled with micrometer-sized particles of carbonyl iron.

Effects of local rearrangement of magnetic particles on deformation in magneto-sensitive elastomers.

Based on the dipole-dipole approach for magnetic interactions we present a comprehensive analysis of spatial rearrangement of magnetic particles under a magnetic field and its effect on the magneto-induced deformation of magneto-sensitive elastomers. The presented formalism allows analyzing non-affine displacements of magnetic particles in a general way and reveals how the local rearrangement of particles under a magnetic field affects the magneto-induced deformation. The formalism includes two contributions: (1) displacements due to elastic coupling with a deformed matrix and (2) rearrangements on the background of the deformed matrix due to magnetic interactions between the particles.

Impact of Bioactive Peptide Motifs on Molecular Structure, Charging, and Nonfouling Properties of Poly(ethylene oxide) Brushes.

Polymer layers capable of suppressing protein adsorption from biological media while presenting extracellular matrix-derived peptide motifs offer valuable new options for biomimetic surface engineering. Herein, we provide detailed insights into physicochemical changes induced in a nonfouling poly(ethylene oxide) (PEO) brush/polydopamine (PDA) system by incorporation of adhesion ligand (RGD) peptides. Brushes with high surface chain densities (σ ≥ 0.

Theoretical models for magneto-sensitive elastomers: A comparison between continuum and dipole approaches.

In the literature, different theoretical models have been proposed to describe the properties of systems which consist of magnetizable particles that are embedded into an elastomer matrix. It is well known that such magneto-sensitive elastomers display a strong magneto-mechanical coupling when subjected to an external magnetic field. Nevertheless, the predictions of available models often vary significantly since they are based on different assumptions and approximations.

Elongated micro-structures in magneto-sensitive elastomers: a dipolar mean field model.

Based on a dipole model for the mutual magnetic interactions among the magnetizable micro-particles in magneto-sensitive elastomers we develop a mean field approach to describe the arrangement of these particles into elongated micro-structures. If these micro-structures are oriented parallel to an external magnetic field the present approach provides an efficient calculation of the behavior of such samples, which is a result of the interplay between micro-structure and shape effects. Accordingly, we are able to draw comprehensive phase diagrams for the resultant deformation and predict for very oblate samples a discontinuous shape change in the presence of a homogeneous external field.

Binary and Bidisperse Polymer Brushes: Coexisting Surface States.

In the present work, we consider polydispersity effects on a mixed polymer brush. Two types of polymer chains with different solvent selectivity being densely grafted together onto an impenetrable surface are forming a binary mixed polymer brush. Using a numerical quasi off-lattice self-consistent field method for heterogeneous chains we study the brush profile upon varying the strength of solvent selectivity (e.

Excluded volume effects in polymer brushes at moderate chain stretching.

We develop a strong stretching approximation for a polymer brush made of self-avoiding polymer chains. The density profile of the brush and the distribution of the end monomer positions in stretching direction are computed and compared with simulation data. We find that our approach leads to a clearly better approximation as compared to previous approaches based upon Gaussian elasticity at low grafting densities (moderate chain stretching), for which corrections due to finite extensibility can be ignored.

Electrokinetics as an alternative to neutron reflectivity for evaluation of segment density distribution in PEO brushes.

Unravelling details of charge, structure and molecular interactions of functional polymer coatings defines an important analytical challenge that requires the extension of current methodologies. In this article we demonstrate how streaming current measurements interpreted with combined self consistent field (SCF) and soft surface electrokinetic theories allow the evaluation of the segment distribution within poly(ethylene oxide) (PEO) brushes beyond the resolution limits of neutron reflectivity technique.