Biaxial potential of surface-stabilized ferroelectric liquid crystals

Phys. Rev. E, vol. 97(4), pp. 042703, 2018.

Kaznacheev A., Pozhidaev E., Vladimir Rudyak V.Yu., Emelyanenko A.V., Khokhlov A.R.

A biaxial surface potential Φs of smectic-C∗ surface-stabilized ferroelectric liquid crystals (SSFLCs) is introduced in this paper to explain the experimentally observed electric-field dependence of polarization ˜Pcell(E), in particular the shape of the static hysteresis loops. Our potential consists of three independent parts. The first nonpolar part Φn describes the deviation of the prime director n (which is the most probable orientation of the long molecular axes) from the easy alignment axis R, which is located in the boundary surface plane. It is introduced in the same manner as the uniaxial Rapini potential. The second part Φp of the potential is a polar term associated with the presence of the polar axis in a FLC. The third part Φm relates to the inherent FLC biaxiality, which has not been taken into consideration previously. The Φm part takes into account the deviations of the secondary director m (which is the most probable orientation of the short molecular axes) from the normal to the boundary surface. The overall surface potential Φs, which is a sum of Φn,Φp, and Φm, allows one to model the conditions when either one, two, or three minima of the SSFLC cell free energy are realized depending on the biaxiality extent. A monodomain or polydomain structure, as well as the bistability or monostability of SSFLC cells, depends on the number of free-energy minima, as confirmed experimentally. In this paper, we analyze the biaxiality impact on the FLC alignment. We also answer the question of whether the bistable or monostable structure can be formed in an SSFLC cell. Our approach is essentially based on a consideration of the biaxial surface potential, while the uniaxial surface potential cannot adequately describe the experimental observations in the FLC.

Photoinduced orientational structures of nematic liquid crystal droplets in contact with polyimide coated surface

Journal of Molecular Liquids, vol. 267, pp. 222-228, 2018.

Shvetsov S.A., Rudyak V.Yu, Emelyanenko A.V., Boiko N.I., Yan-Song Zhang, Jui-Hsiang Liu, Khokhlov A.R.

Orientational structure transitions due to a photoinduced surface anchoring modulation were achieved and investigated in the case of NLC microdroplets placed between glycerol and solid surface. The boundary conditions at NLC-glycerol varied under the light action on the dendromer dopant containing azobenzene terminal fragments. At the same time, the NLC had a strong planar anchoring with the polyimide treated substrate. Numerical simulation of the observed orientational structures in NLC microdroplets were carried out.

Applying the deconvolution approach in order to enhance RRDE time resolution: Experimental noise and imposed limitations

Electrochimica Acta, vol. 298, pp. 858-865, 2019.

Sergeev A.V., Zakharchenko T.K., Chertovich A.V., Itkis D.M.

The ring electrode of an RRDE setup is commonly used to detect redox active species produced at the disk electrode. It is especially useful when some side processes occur at the disk (e.g. passivation film growth) along with a main electrochemical reaction of interest, which produces a soluble redox-active specie. Unfortunately, the detected ring current signal is a delayed and smeared-out representation of the disk faradaic process so that fast changes of its magnitude cannot be studied. The deconvolution approach is a mathematical data processing procedure that enables reconstruction of the disk signal with a hypothetically infinite accuracy. There are, however, practical limitations arising mainly from inevitable presence of noise in the measured ring current used for the reconstruction. In this paper the deconvolution approach is discussed in details and its applicability is investigated basing on a series of experiments with a model system. A procedure to filter out spurious artifacts from the reconstructed disk signal is proposed and tested. 

DOI: 10.1016/j.electacta.2018.12.124

Nuclear lamina integrity is required for proper spatial organization of chromatin in Drosophila

Nature communications, vol. 1176, pp. 1-11, 2019.

Ulianov S.V., Doronin S.A., Khrameeva E.E.,Kos P.I., Luzhin A.V., Starikov S.S., Galitsyna A.A.,Nenasheva V.V., Ilyin A.A., Flyamer I.M., Mikhaleva E.A., Logacheva M.D., Gelfand M.S., Chertovich A.V.,Gavrilov A.A., Razin S.V., Shevelyov Yu.Y.

How the nuclear lamina (NL) impacts on global chromatin architecture is poorly understood. Here, we show that NL disruption in Drosophila S2 cells leads to chromatin compaction and repositioning from the nuclear envelope. This increases the chromatin density in a fraction of topologically-associating domains (TADs) enriched in active chromatin and enhances interactions between active and inactive chromatin. Importantly, upon NL disruption the NL-associated TADs become more acetylated at histone H3 and less compact, while background transcription is derepressed. Two-colour FISH confirms that a TAD becomes less compact following its release from the NL. Finally, polymer simulations show that chromatin binding to the NL can per se compact attached TADs. Collectively, our findings demonstrate a dual function of the NL in shaping the 3D genome. Attachment of TADs to the NL makes them more condensed but decreases the overall chromatin density in the nucleus by stretching interphase chromosomes.

Peculiarities of Polyrotaxanes Collapse: Polymorphism of Globular Structure and Stability of Unimolecular Micelles

Macromolecules, vol. 52(1), pp. 135-142, 2019.

Gavrilov A.A.Potemkin I.I.

In the present paper we investigate the behavior of polyrotaxanes under bad solvent conditions by means of computer simulations. Polyrotaxanes can be viewed as amphiphilic macromolecules with annealed sequences because the rings can freely move along the backbone. We showed that, depending on the ring inclusion ratio and the rings radius, one can observe various structures. If the rings radius is small (i.e., enough to fit only one monomer unit), in bad solvent we observed the formation of a collapsed core surrounded by long loose tails and loops with an increased ring density. Upon increasing of the backbone–solvent incompatibility, the core grows as well as the number of rings located on its surface; the latter leads to a partial screening of the backbone–solvent interactions. When the rings are large enough to fit several monomer units, the backbone packs inside the rings, which allows for complete isolation from the solvent. If the number of rings (i.e., the inclusion ratio) is large enough to fit all the monomer units, unimolecular cylindrical micelles are formed in bad solvent. Such micelles are formed by the backbone packed inside the rings that are stacked on “top” of each other. We showed that the transition from the coil to the unimolecular micelle is sensitive to the solubility of the rings: if the rings tend to distribute along the backbone, higher incompatibilities are required for the unimolecular micelles formation compared to the case when the rings tend to form aggregates. In the state of unimolecular micelles the backbone is completely isolated from the solvent, which makes the micelles soluble even at rather high concentrations.

Adaptive structure of gels and microgels with sliding cross-links: enhanced softness, stretchability and permeability

Soft matter, vol. 14(24), pp. 5098-5105, 2018.

Gavrilov A.A.Potemkin I.I.

We propose an experimentally-inspired model of gels and microgels with sliding cross-links, and use this model to study the mechanical and structural properties with molecular dynamics simulations. In the model, the gels and microgels are made of linear polymer chains with threaded rings, which are capable of sliding along the chains, and bulky end-groups keeping the rings threaded (thus mimicking polyrotaxanes); the chains are covalently linked to each other not through the backbones but through the rings. Both gels and microgels are shown to be much softer in the regime of intermediate and large deformations and also much more stretchable than the topologically equivalent chemical counterparts. The physical reason for that is the mobility of the cross-links which leads to the formation of long, longitudinally oriented “subchains” between cross-linked rings upon uniaxial deformation. The microgels are tested for adsorption on a solid flat surface and for interaction with colloidal particles of different sizes. We demonstrate that the sliding microgel is subjected to stronger flattening on the surface than the chemical one. Enforced penetration of solid particles into the sliding microgel without breaking of covalent bonds is predicted even if the size of the particles is comparable to or larger than the mesh size of the chemical microgel and smaller than the size of polyrotaxane. This penetration is accompanied by the disappearance of the cavity: the microgel is characterized by adaptive porosity tunable to the guest-object.

Effect of counterion excluded volume on the conformational behavior of polyelectrolyte chains

Soft matter, vol. 14, pp. 1474-1481, 2018.

Gordievskaya Yu.D.Gavrilov A.A.Kramarenko E.Yu

Conformational behavior of a single strongly charged polyelectrolyte chain in a dilute solution is studied by molecular dynamics simulations. The novel feature of the model is variation of the excluded volume of counterions for investigating its effect on the chain conformation, especially in low-polar media. It has been confirmed that the chain with conventional counterions collapses into a dense globule with increasing electrostatic interactions. However, if the counterions are bulky enough, they prevent the chain collapse even in media with strong electrostatic interactions. They stay bound in the vicinity of the backbone of the chain that adopts a swollen conformation. In this conformation, the scaling relation for the polymer dimensions with the chain length is the same as for neutral macromolecules in a good solvent, however the polyelectrolyte chain complexed with bulky counterions has a larger gyration radius than its uncharged analogue due to the excluded volume of the counterions contributing to the chain rigidity. Study of the counterion mobility has shown that, similar to the conventional counterions, the bulky counterions do not form stable ion pairs with ions on the polymer chain even in media with strong electrostatic interactions, but rather freely move along the chain backbone. In solutions containing mixtures of counterions with a bimodal size distribution, the conformations of linear polyelectrolytes depend considerably on the fraction of bulky counterions. Furthermore, a kind of intramolecular microphase separation can take place within a polyelectrolyte globule with the formation of a core–shell particle: the smaller counterions concentrate within the globular core while the bulkier counterions form a shell on the globule surface. The stability of the core–shell globule depends on the relative size of the counterions as well as their fractions in the solution. Thus, fine tuning of the balance between the counterion excluded volume and the electrostatic interactions opens new ways for controlling the conformational behavior of polyelectrolytes.

Effect of the Number of Subnetworks on the Topology and Mechanical Properties of Interpenetrating Networks: Computer Simulation

Polymer Science - Series A, vol. 60(1), pp. 110-115, 2018.

Gavrilov A.A.

The mechanical properties of solvent-fks obtained by three different crosslinking mechanisms are investigated by the dissipative particle dynamics method. It is shown that, upon crosslinking without additional conditions (the vulcanization model), the amount of defects increases with an increase in the number of subnetworks, thereby worsening the rigidity of the system. If crosslinking mechanisms excluding formation of a certain kind of defects are used, that is, the network topology approaches the ideal one, interpenetrating networks with a higher number of subnetworks, on the contrary, demonstrate a higher rigidity owing to the fact that subchains in them are more strained. This effect is not observed in networks synthesized upon crosslinking without additional conditions. The reasons for this behavior and probable experimental analogies are discussed.

Crosslinking Mechanisms, Structure and Glass Transition in Phthalonitrile Resins: Insight from Computer Multiscale Simulations and Experiments

Journal of Polymer Science, Part B: Polymer Physics, vol. 58(5), pp. 362-374, 2018.

Guseva D.V., Rudyak V.Y., Komarov P.V., Sulimov A.V., Bulgakov B.A., Chertovich A.V.

The influence of crosslinking process on the resulting structural properties of phthalonitrile matrices is studied through theoretical and experimental investigations. Multiscale procedure for generating fully atomistic phthalonitrile networks with simulation of radical polymerization reactions and specific reactions of triazine formation at the mesoscale level is presented and applied to the case of phthalonitrile resin based on low‐melting monomer bis(3‐(3,4‐dicyanophenoxy)phenyl)phenyl phosphate. The structural properties of the generated networks of various conversions and with various amount of triazine are analyzed using the dissipative particle dynamics and atomistic molecular dynamics. Triazine‐containing networks are much sparser in comparison with triazine‐free ones in terms of simple cycle size. The values of density, coefficients of linear thermal expansion and glass transition temperatures (Tgs) agree with obtained experimental data, and are very similar for different crosslinking mechanisms. The dependence of Tg on conversion correlates well with the sol–gel transition in network structure.

Dynamic and Static Mechanical Properties of Crosslinked Polymer Matrices: Multiscale Simulations and Experiments

Polymers, vol. 10(7), pp. 792, 2018.

Guseva D.V., Rudyak V.Yu, Komarov P.V., Bulgakov B.A., Babkin A.V., Chertovich A.V.

We studied the static and dynamic mechanical properties of crosslinked polymer matrices using multiscale simulations and experiments. We continued to develop the multiscale methodology for generating atomistic polymer networks, and applied it to the case of phthalonitrile resin. The mechanical properties of the resulting networks were analyzed using atomistic molecular dynamics (MD) and dissipative particle dynamics (DPD). The Young’s and storage moduli increased with conversion, due both to the appearance of a network of covalent bonds, and to freezing of degrees of freedom and lowering of the glass transition temperature during crosslinking. The simulations’ data showed good quantitative agreement with experimental dynamic mechanical analysis measurements at temperatures below the glass transition. The data obtained in MD and DPD simulations at elevated temperatures were conformable. This makes it possible to use the suggested approach for the prediction of mechanical properties of a broad range of polymer matrices, including ones with high structural heterogeneity.

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