Novel magnetorheological elastomers (MRE) based on a highly elastic silicone rubber filled with carbonyl iron magnetic particles of 3–5 and 3–50 µm are synthesized. The effect of an external ...homogeneous magnetic field on the viscoelastic properties of these materials is studied by dynamic experiments (shear oscillations on a rheometer). It is shown that the magnetic response of the MRE increases with a decrease of the strain. At 1% deformation both the storage and loss moduli of the new MRE demonstrate a giant response to the magnetic field, namely, an increase of more than two orders of magnitude in both moduli in a field of 300 mT is observed. In addition, these new MREs show a twofold increase of the damping ratio, which is important for their application as tunable vibration absorbers.
Viscoelastic properties of magnetic field sensitive elastomers representing a new type of composites consisting of micron‐size particles dispersed in a highly elastic polymeric matrix are studied by dynamic shear oscillation tests. Both the storage and loss moduli of the new materials increase tremendously (more than two orders of magnitude) in magnetic field of up to 300 mT.
Viscoelastic behavior of highly elastic magnetic elastomers has been studied by three different experimental techniques: elongation, static and dynamic shears. It has been shown that the elastic ...modulus of the materials increases considerably in an external homogeneous magnetic field of up to 0.3
T (100-fold increase of the tangential modulus has been observed at small 1–4% deformations). The appearance of the new effect of pseudo-plasticity induced by the magnetic field has been observed leading to a considerable (up to 100-fold) increase in the shear loss modulus of the composites.
We develop a simple analytical model of a high temperature hydrogen fuel cell with proton exchange membrane. The model is validated against experimental results obtained in our group. The model is ...pseudo two dimensional, steady-state and isothermal, it accounts for the crossover of reactant gases through the membrane and it can be solved analytically. The role of the crossover is considered in detail.
By means of computer modelling, the self-organization of dense planar brushes of macromolecules with amphiphilic monomer units was addressed and their state diagram was constructed. The diagram of ...states includes the following regions: disordered position of monomer units with respect to each other, strands composed of a few polymer chains and lamellae with different domain spacing. The transformation of lamellae structures with different domain spacing occurred within the intermediate region and could proceed through the formation of so-called parking garage structures. The parking garage structure joins the lamellae with large (on the top of the brushes) and small (close to the grafted surface) domain spacing, which appears like a system of inclined locally parallel layers connected with each other by bridges. The parking garage structures were observed for incompatible A and B groups in selective solvents, which result in aggregation of the side B groups and dense packing of amphiphilic macromolecules in the restricted volume of the planar brushes.
The state diagram of dense brushes contains regions of lamellae with different domain spacings and regions of intermediate, parking garage structures combining them at different distances from the grafting surface.
Polymer microgels, micrometer-sized cross-linked polymer particles, are considered to be a promising type of advanced materials for a wide range of applications. To enhance the microgels’ ...applicability, it is essential to incorporate various functional groups into a microparticle polymer network. Yet, the availability of functional groups for the interaction with surroundings depends strongly on the properties of the polymer network and has a great impact on further effective usage. In this theoretical study, we address this question and, with the help of coarse-grained molecular dynamics computer simulations, assess the segmental mobility and accessibility of functional groups bound to polymer network depending on microgel architecture and solvent quality. Additionally, we evaluate the minimum number of functional groups needed to facilitate the hopping mechanism between the functional groups (i.e., charge transfer). As an example of practical implementation of the obtained results, we estimate the optimal network topology for redox-active microgels to provide the maximum charge capacity for the dispersion electrolyte in redox-flow batteries.
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•Chitosan thermoresponsivity is induced by glycerophosphate binding.•Binding stimulates formation of highly ordered hydrate structure of chitosan.•Hydrate structure melting triggers ...phase separation of the system.•Restoration of chitin-like structures after phase separation results in gelation.
Thermoresponsivity of chitosan induced by β-glycerophosphate (GP) in diluted aqueous solutions has been first studied by high-sensitivity differential scanning calorimetry. It has been found that the GP solutions of chitosan undergo a first-order phase transition upon heating. The onset of this transition coincides with the cloud point of the system. This allows one to identify the thermoresponsivity of chitosan as a macroscopic demonstration of the phase separation transition. The transition temperature, enthalpy, heat capacity increment, and width were determined as functions of GP and chitosan concentrations, and the dielectric constant of the solvent. Based on this data, we suggested that GP binds cooperatively to the chitosan matrix at low temperatures. The standard free energy of GP binding (Δbgint= −6 ± 1 kJ mol−1) was estimated from the DSC data. It was shown that the Okada–Tanaka model of cooperative hydration of polymers adequately describes the thermogram of the GP induced phase transition of chitosan.
The aggregation phenomena in aqueous solutions of hydrophobically modified (HM) chitosan, containing 4 mol % of n-dodecyl side chains, were studied by viscometry and fluorescence spectroscopy with ...pyrene as a probe. The results are compared with those for unmodified chitosan. Surprisingly, fluorescence data reveal the appearance of intermolecular hydrophobic aggregates both in chitosan and in HM chitosan. Nevertheless, these polymers exhibit quite different rheological properties: upon the formation of aggregates the viscosity of HM chitosan sharply increases, while that of unmodified chitosan raises only slightly. The aggregation models for both chitosan and its hydrophobic derivative were proposed. It was shown that in solutions of HM chitosan two types of hydrophobic domains exist: hydrophobic domains typical for different associating polymers with hydrophobic side chains and hydrophobic domains inherent to chitosan itself.
Magnetic elastomers (MEs) based on a silicone matrix and magnetically hard NdFeB particles have been synthesized and their magnetic and viscoelastic properties have been studied depending on the size ...and concentration of magnetic particles and the magnetizing field. It has been shown that magnetic particles can rotate in soft polymer matrix under applied magnetic field, this fact leading to some features in both magnetic and viscoelastic properties. In the maximum magnetic field used magnetization of MEs with smaller particles is larger while the coercivity is smaller due to higher mobility of the particles within the polymer matrix. Viscoelastic behavior is characterized by long relaxation times due to restructuring of the magnetic filler under the influence of an applied mechanical force and magnetic interactions. The storage and loss moduli of magnetically hard elastomers grow significantly with magnetizing field. The magnetic response of the magnetized samples depends on the mutual orientation of the external magnetic field and the internal sample magnetization. Due to the particle rotation within the polymer matrix, the loss factor increases abruptly when the magnetic field is turned on in the opposite direction to the sample magnetization, further decreasing with time. Moduli versus field dependences have minimum at non-zero field and are characterized by a high asymmetry with respect to the field direction.
Aim Prospective assessment of the nature of cardiac injury in patients with post-COVID syndrome according to contrast-enhanced MRI in routine clinical practice.Material and methods 106 ...previously unvaccinated patients were evaluated. 62 (58.5%) of them were women with complaints that persisted after COVID-19 (median age, 57.5 49; 64 years). In addition to standard indexes, markers of inflammation and myocardial injury were determined, and cardiac contrast-enhanced MRI was performed in each patient.Results The median time from the onset of COVID-19 to cardiac MRI was 112.5 75; 151 days. The nature of cardiac injury according to MRI in patients with post-COVID syndrome was complex and included a decrease in left ventricular (LV) and right ventricular ejection fraction, pericardial effusion, and pathological foci of late and early contrast enhancement at various locations. In 29 (27.4%) cases, there was a combination of any two signs of heart injury. In 28 (26.4%) patients with focal myocardial injury during the acute phase of COVID-19, hydroxychloroquine and tocilizumab were administered significantly more frequently, but antiviral drugs were administered less frequently. The presence of focal myocardial injury was associated with pathological LV remodeling.Conclusion According to contrast-enhanced cardiac MRI, at least 27.4% of patients with post-COVID syndrome may have signs of cardiac injury in various combinations, and in 26.4% of cases, foci of myocardial injury accompanied by LV remodeling are detected. The nature of heart injury after COVID-19 depends on the premorbid background, characteristics of the course of the infectious process, and the type of prescribed therapy. An algorithm for evaluating patients with post-COVID syndrome is proposed.
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