Elena Yu. Kramarenko

Strong magnetodielectric effects in magnetorheological elastomers

The effect of a uniform magnetic field on the permittivity of magnetorheological elastomers (MREs) is studied. MREs were synthesized on the basis of silicone rubber and magnetic fillers of various chemical nature (Fe, NdFeB and Fe3O4) and particle sizes. The value of permittivity was obtained from the measurements of the capacity of a plane capacitor with MRE samples. A strong increase of the permittivity (magnetodielectric effect) was observed when the applied field was perpendicular to the capacitor plates. The value of the magnetodielectric effect was found to be strongly dependent on the type of magnetic filler as well as on the size and concentration of magnetic particles within MRE composites. The highest magnetic response reaching 150% was observed for the MRE based on a magnetically hard NdFeB filler. A simple model explaining physical reasons for the magnetodielectric effect in a MRE is proposed. The developed MRE with a strong magnetodielectric effect is very promising for a wide range of applications, in particular, as magnetic field sensors and actuators.

Formation of Salt Bonds in Polyampholyte Chains

In this paper we study the influence of the formation of intrachain ion pairs (salt bonds) and the distribution of counterions on the behavior of single polyampholyte chains in a dilute solution. It has been shown that neutral polyampholyte chains can undergo jump-like collapse transition from the swollen state to the globular state with the formation of ion pairs between oppositely charged ions of the chain. A polyampholyte chain with an excess charge shows the behavior of a conventional polyelectrolyte chain and counterions play an important role in the chain behavior. We distinguish three possible states of counterions: free counterions inside and outside the macromolecule, and a bound state of counterions forming ion pairs with the corresponding ions of the polymer chain. We found a non-monotonous behavior of the chain upon increasing the excess charge on the chain: the chain swells from a compact state to elongated conformation and shrinks again to the compact state when the excess charge of the chain is increased.

Polymer Micelles with Hydrophobic Core and Ionic Amphiphilic Corona. 1. Statistical Distribution of Charged and Nonpolar Units in Corona

Polymer micelles with hydrophobic polystyrene (PS) core and ionic amphiphilic corona from charged N-ethyl-4-vinylpyridinium bromide (EVP) and uncharged 4-vinylpyridine (4VP) units spontaneously self-assembled from PS-block-poly(4VP-stat-EVP) macromolecules in mixed dimethylformamide/methanol/water solvent. The fraction of statistically distributed EVP units in corona-forming block is β = [EVP]/([EVP]+[4VP]) = 0.3-1. Micelles were transferred into water via dialysis technique, and pH was adjusted to 9, where 4VP is insoluble. Structural characteristics of micelles were investigated both experimentally and theoretically as a function of corona composition β. Methods of dynamic and static light scattering, electrophoretic mobility measurements, sedimentation velocity, transmission electron microscopy, and UV spectrophotometry were applied. All micelles possessed spherical morphology. The aggregation number, structure, and electrophoretic mobility of micelles changed in a jumplike manner near β ~ 0.6-0.75. Below and above this region, micelle characteristics were constant or insignificantly changed upon β. Theoretical dependencies for micelle aggregation number, corona dimensions, and fraction of small counterions outside corona versus β were derived via minimization the micelle free energy, taking into account surface, volume, electrostatic, and elastic contributions of chain units and translational entropy of mobile counterions. Theoretical estimations also point onto a sharp structural transition at a certain corona composition. The abrupt reorganization of micelle structure at β ~ 0.6-0.75 entails dramatic changes in micelle dispersion stability in the presence of NaCl or in the presence of oppositely charged polymeric (sodium polymethacrylate) or amphiphilic (sodium dodecyl sulfate) complexing agents.

Two Types of Block Copolymer Micelles with Ion-Containing Cores

We report a combined experimental and theoretical study of micellization of block copolymer with hydrophilic nonionic corona-forming blocks and weak polyelectrolyte (wPE) core-forming blocks with pH-triggered solubility in aqueous solutions. We demonstrate that in addition to micelles with neutral cores, there exist two other types of micelles with PE- or ionomer-like cores, in which monovalent counterions are released or condensed on core wPE block, respectively. The transition between the two types of micelles occurred upon changes in ionization of the PE core block and resulted in nonmonotonous changes of aggregation number as a function of pH. Such micelles with stimulus responsive cores represent promising nanocarriers for controlled delivery applications.

Intranetwork phase separation in polyelectrolyte gels

Abstract We have investigated the swelling behavior of polyelectrolyte gels in a low-molecular-weight solvent taking into account the possibility of intragel phase separation. We have shown that, as for the case of a neutral gel considered recently by Moerkerke et al., the coexistence of two phases differing by polymer concentration can also take place inside the polyelectrolyte gel if the amount of solvent is not enough to ensure its free swelling. The phase diagrams for polyelectrolyte gels with various degrees of ionization and with various degrees of stiffness of the gel subchains have been calculated. It has been shown that the region of stability of the two-phase gel increases significantly with increasing number of charges on the network chains and chain rigidity.

Two-phase structure of polyelectrolyte gel/surfactant complexes

The behavior of lightly cross-linked polyelectrolyte hydrogel swelling in the solution of oppositely charged surfactants is studied theoretically. It is shown that if there is a lack of surfactant in the solution intragel separation into two phases differing in swelling ratios and surfactant content can take place. The surfactant ions concentrate and form micelles in a part of the gel and this part collapses while the rest of the gel remains swollen. The two-phase region widens with an increase of ionization degree of the gel subchains.

Polymer gels with associating side chains and their interaction with surfactants

Conformational behaviour of hydrophobically modified (HM) polymer gels in solutions of nonionic surfactants is studied theoretically. A HM gel contains hydrophobic side chains (stickers) grafted to its subchains. Hydrophobic stickers are capable to aggregate into joint micelles with surfactant molecules. Micelles containing more than one sticker serve as additional physical cross-links of the network, and their formation causes gel shrinking. In the proposed theoretical model, the interior of the gel/surfactant complex is treated as an array of densely packed spherical polymer brushes consisting of gel subchains tethered to the surface of the spherical sticker/surfactant micelles. Effect of stickers length and grafting density, surfactant concentration and hydrophobicity on gel swelling as well as on hydrophobic association inside it is analyzed. It is shown that increasing surfactant concentration can result in a gel collapse, which is caused by surfactant-induced hydrophobic aggregation of stickers, and a successive gel reswelling. The latter should be attributed to a growing fraction of surfactants in joint aggregates and, hence, increasing number of micelles containing only one sticker and not participating in gel physical cross-linking. In polyelectrolyte (PE) gels hydrophobic aggregation is opposed by osmotic pressure of mobile counterions, so that at some critical ionization degree hydrophobic association is completely suppressed. Hydrophobic modification of polymers is shown to open new ways for controlling gel responsiveness. In particular, it is discussed that incorporation of photosensitive groups into gel subchains and/or surfactant tail could give a possibility to vary the gel volume by light. Since hydrophobic aggregation regularities in gels and solutions are common, we hope our findings will be useful for design of polymer based self-healing materials as well.

Magnetorheological behavior of magnetoactive elastomers filled with bimodal iron and magnetite particles

Magnetoactive elastomers (MAE) based on soft silicone matrices, filled with various proportions of large diameter (approximately 50 μm) iron and small diameter (approximately 0.5 μm) magnetite particles are synthesized. Their rheological behavior in homogeneous magnetic fields up to 600 mT is studied in detail. The addition of small magnetite particles facilitates fabrication of uniformly distributed magnetic elastomer composites by preventing aggregation and sedimentation of large particles during curing. It is shown that using the proposed bimodal filler particles it is possible to tailor various magnetorheological (MR) properties which can be useful for different target applications. In particular, either absolute or relative magnetorheological effects can be tuned. The value of the damping factor as well as the range of deformation amplitudes for the linear viscoelastic regime can be chosen. The interdependencies between different MR properties of bimodal MAEs are considered. The results are discussed in the model framework of particle network formation under the simultaneous influence of external magnetic fields and mechanical deformation.

Counterions in Polyelectrolytes

Polyelectrolytes are macromolecules containing electrically charged monomer units. Such units appear due to the dissociation of ionizable (initially neutral) monomer units. In this process, the monomer unit on the chain becomes charged and a counterion is released to the solution. Therefore, counterions are always present in polyelectrolyte systems. Evidently, the number of charged monomer units is equal to the number of counterions.

Communication: Light driven remote control of microgels’ size in the presence of photosensitive surfactant: Complete phase diagram

Here we report on a light triggered remote control of microgel size in the presence of photosensitive surfactant. The hydrophobic tail of the cationic surfactant contains azobenzene group that undergoes a reversible photo-isomerization reaction from a trans- to a cis-state accompanied by a change in the hydrophobicity of the surfactant. We have investigated light assisted behaviour and the complex formation of the microgels with azobenzene containing surfactant over the broad concentrational range starting far below and exceeding several times of the critical micelle concentration (CMC). At small surfactant concentration in solution (far below CMC), the surfactant in the trans-state accommodates within the microgel causing its compaction, while the cis-isomer desorbs out of microgel resulting in its swelling. The process of the microgel size change can be described as swelling on UV irradiation (trans-cis isomerization) and shrinking on irradiation with blue light (cis-trans isomerization). However, at the surfactant concentrations larger than CMC, the opposite behaviour is observed: the microgel swells on blue irradiation and shrinks during exposure to UV light. We explain this behaviour theoretically taking into account isomer dependent micellization of surfactant within the microgels.