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.

Field-induced surface deformation of magnetoactive elastomers with anisometric fillers: a single-particle model.

Surface relief of magnetoactive elastomers (MAEs) based on soft polymer matrices filled with anisometric magnetically hard fillers is studied theoretically in magnetic fields applied perpendicular to the MAE surface. A single-particle 2D cell model describing the rotation of one individual elliptical particle in a near-surface MAE layer is developed. The equilibrium rotation angle of particles is defined by a balance between Zeeman, magnetic anisotropy and elastic (generated in the polymer matrix) energy increments. The Stoner-Wohlfarth model is used to describe magnetic properties of the filler particles while the elastic energy as a function of the particle rotation angle is evaluated numerically using FEM simulations. A representative surface MAE system is constructed via superposition of single-particle cells with field-driven magnetic particles, and surface relief characteristics are derived for various sets of geometric and statistical parameters. Limitations of the proposed approach have been discussed.

An interplay of electrostatic and excluded volume interactions in the conformational behavior of a dipolar chain: theory and computer simulations.

The effect of an interplay between electrostatic and excluded volume interactions on the conformational behavior of a dipolar chain has been studied theoretically and by means of molecular dynamics simulations. Every monomer unit of the dipolar chain comprises a dipole formed by a charged group of the chain and an oppositely charged counterion. The counterion is assumed to freely move around the chain but keeping the distance between oppositely charged ions (the dipole length) fixed. The novelty of the developed mean-field theory is that variations of the dipole parameters (the dipole length and the counterion size) have been accounted for in both electrostatic and excluded volume contributions to the total free energy of the dipolar chain. It has been shown that conformational transitions between swollen and collapsed states of the chain can be induced by fine-tuning the balance between electrostatic and excluded volume interactions. In particular, in low-polar media not only globule but also extended coil conformations can be realized even under strong electrostatic attraction. The results of MD simulations of a dipolar chain with variable dipolar length support theoretical conclusions.

Molecular dynamics simulations of single siloxane dendrimers: Molecular structure and intramolecular mobility of terminal groups.

Molecular dynamics simulations of two types of isolated siloxane dendrimers of various generations (from the 2nd to the 8th) have been performed for temperatures ranging from 150 K to 600 K. The first type of dendrimer molecules has short spacers consisting of a single oxygen atom. In the dendrimers of the second type, spacers are longer and comprised of two oxygen atoms separated by a single silicon atom. A comparative analysis of molecular macroscopic parameters such as the gyration radius and the shape factor as well as atom distributions within dendrimer interior has been performed for varying generation number, temperature, and spacer length. It has been found that the short-spacer dendrimers of the 7th and 8th generations have a stressed central part with elongated bonds and deformed valence angles. Investigation of the time evolution of radial displacements of the terminal Si atoms has shown that a fraction of the Si groups have a reduced mobility. Therefore, rather long time trajectories (of the order of tens of nanoseconds) are required to study dendrimer intramolecular dynamics.

[Mo-dified scleral buckling for additional fixation of the retina with endovitreal magnetic implant (experimental study)]. / Modifikatsiia épiskleral'nogo plombirovaniia dlia dopolnitel'noi mekhanicheskoi fiksatsii setchatki magnitnym implantatom so storony vitreal'noi polosti (éksperimental'noe issledovanie).

Scleral buckling (SB) has been regaining its popularity in the treatment of retinal detachments. On large clinical material, it has been proved to be the technique of choice in phakic patients with retinal detachment of moderate severity. A combined procedure that incorporates features of episcleral as well as intravitreal surgeries has also become widely used. Aim - to investigate the prospects for increasing technical potential of SB, particularly, to investigate the possibility of additional atraumatic mechanical fixation of the retina from within the vitreous cavity. The proposed device consists of an episcleral magnetic buckle and endovitreal magnetic buckles (endobuckles). The episcleral magnetic buckle is made of medical grade silicone and cannot be distinguished from common buckles, except that it contains one or more permanent magnets on the basis of neodymium-iron-boron powders. Endobuckles are small flat elastic elements made of silicone elastomer filled with magnetic particles. Еndobuckles are implanted into the vitreous cavity through a small pars plana incision and placed on the retina at the projection of the scleral magnetic buckle. Thus, in experiments with cadaver eyes, the authors have showed the principle feasibility of retinal fixation by the force of magnetic interaction between the magnetic scleral buckle and endobuckles. The described technique of additional mechanical fixation of the retina provides wider surgical opportunities in the management of retinal detachments.

Dissipative particle dynamics for systems with high density of charges: Implementation of electrostatic interactions.

In this work, we study the question of how to introduce electrostatic interactions in dissipative particle dynamics (DPD) method in order to correctly reproduce the properties of systems with high density of charges, including those with inhomogeneous charge distribution. To this end, we formulate general requirements for the electrostatic force in DPD and propose a new functional form of the force which suits better for satisfying these requirements than the previously used ones. In order to verify the proposed model, we study the problem of a single polyelectrolyte chain collapse and compare the results with molecular dynamics (MD) simulations in which the exact Coulomb force is used. We show that an excellent quantitative agreement between MD and DPD models is observed if the length parameter D of the proposed electrostatic force is chosen properly; the recommendations concerning the choice of this parameter value are given based on the analysis of a polyelectrolyte chain collapse behavior. Finally, we demonstrate the applicability of DPD with the proposed electrostatic force to studying microphase separation phenomenon in polyelectrolyte melts and show that the same values of D as in the case of single chain collapse should be used, thus indicating universality of the model. Due to the charge correlation attraction, a long-range order in such melts can be observed even at zero Flory-Huggins parameter.

Effect of ion pair formation on the structure of polymer micelles with ionic amphiphilic coronae.

We report a theoretical study of micelles from diblock copolymers with an insoluble core-forming block and an amphiphilic ionic corona-forming block. We calculate the micelle structural parameters depending on the composition of the coronal block (ratio between the non-polar and ion-containing groups) as well as solvent quality and polarity for the coronal block. We focus on the effect of ion pair formation in a low polar corona medium and predict the existence of novel micelles with ionomer-type coronae. In these micelles most part of counterions is bound with ions in polymer chains. Two consecutive jump-like first-order phase transitions between different-type micelles can take place in the solution upon change of hydrophobic/polyelectrolyte balance within the micelle corona: large micelles with polyelectrolyte collapsed coronae → large micelles with ionomer-type coronae → small micelles with polyelectrolyte swollen coronae. These transitions are accompanied by non-monotonous change in the micelle aggregation number. New insight into the role of counterions is important for design of stimuli responsive systems.

Conformational behavior of diblock comb copolymers.

The conformational behavior of a single diblock copolymer chain with comb blocks is studied by computer simulation. The comb blocks have equal lengths of the backbones and side chains and differ only by the degree of grafting of the side chains. The collapse behavior is studied for two cases: (i) comb diblock copolymer with attracting backbones of the blocks and swollen side chains and (ii) comb diblock copolymer with attracting monomer units of the side chains of the blocks and swollen backbones. In both cases the conformation of a globule with a tail is observed when one of the block collapses while the other stays swollen. At high attraction micellar-type globules are formed. The core of the micelle consists of attracting monomeric units and the soluble segments (side chains or backbones) comprise the corona. It is shown that separation of monomeric units belonging to different blocks can take place both in the core and in the corona regions of the micelle.

Stoichiometric polyelectrolyte complexes of ionic block copolymers and oppositely charged polyions.

Micellization in dilute solutions of diblock copolymers with a polyelectrolyte and a hydrophilic nonionic blocks and oppositely charged polyions is studied using mean-field theory. In aqueous solutions the micelle core consists of the polyelectrolyte complex (PEC) while the corona is formed by hydrophilic blocks of the block copolymers. Describing PEC as a globule in the framework of the Lifshitz [Zh. Eksp. Teor. Fiz. 55, 2408 (1968)] globule theory we calculate the surface tension of the micellar core/solvent interface as a function of the polyion degree of ionization, solvent quality, and concentration of low-molecular-mass salt. The equilibrium aggregation number of starlike micelles formed by block copolymers and homopolymers of opposite charge at stoichiometric mixture compositions is found as a function of the system parameters. It is shown that micelles disintegrate upon addition of salt.

Stoichiometric polyelectrolyte complexes as comb copolymers.

The collapse behavior of a single comblike copolymer chain has been studied by Monte Carlo simulations. It has been supposed that the solvent is good for the side chains but the solvent quality for the backbone chain changes. It has been shown that depending on the structural parameters of the comb copolymer (the lengths of the backbone and side chains, grafting density of the side chains) various thermodynamically stable morphologies of the collapsed backbone chain can be realized. In addition to ordinary spherical globule we have observed elongated structures as well as necklace-like conformations. The proposed model can be used to describe conformational behavior of stoichiometric complexes between block copolymers with a polyelectrolyte short block and oppositely charged linear homopolymers.