Functionalized thermoresponsive microgels based on N-isopropylacrylamide: Energetics and mechanism of phase transitions

Polymer International, V.68, Issue 7, July 2019, Pages 1303-1314

Grinberg Valerij Y., Burova Tatiana V., Grinberg Natalia V., Buyanovskaya Anastasiya G., Khokhlov Alexei R., Kozhunova Elena Yu, Vyshivannaya Oxana V., Nasimova Irina R.

Nano- and micro-sized stimuli-responsive polymer containers capable of an effective controlled binding and release of ionic amphiphilic drugs are highly demanded in drug delivery. Thermoresponsive cross-linked microgels based on N-isopropylacrylamide (NIPAM) were synthesized by precipitation polymerization in aqueous media. The microgels were functionalized by the introduction of anionic component (acrylic acid, AA, or vinyl sulfonate, VSA) either as a comonomer (NIPAM-co-AA and NIPAM-co-VSA microgels) or interpenetrating polymer network (PNIPAM-PAA microgel). The thermoresponsive behavior of the microgels was investigated by high-sensitivity differential scanning calorimetry. Thermodynamic parameters of the phase transitions (the transition temperature, enthalpy, and width) for the copolymer and interpenetrating microgels were determined. The copolymer microgels involving weak and strong ionogenic groups differ drastically by the transition energetics. This implies different types of their primary structures provided by either comonomer affinity or segregation in the reaction mixture under the polymerization conditions. The microgel functionalization via interpenetrating networks does not affect notably the transition temperature typical of the reference parent PNIPAM microgel but reduces the transition cooperativity. An analysis of the heat capacity profile of the microgel phase transition reveals some features of the mechanism of the thermoresponsivity in the microgels.

DOI: 10.1016/j.eurpolymj.2020.109722

Simulation of interpenetrating networks microgel synthesis

Soft Matter, 2020,16, 4858-4865

Vladimir Yu. Rudyak, Elena Yu. Kozhunova and  Alexander V. Chertovich

In this paper, we have implemented the sequential template synthesis of interpenetrating network (IPN) microgels in computer simulations and studied the behavior of such particles. We explored the influence of the interaction between the components of primary and secondary networks on the polymerization process and determined the necessary conditions for IPN particle formation. The interconnection between the parameters of synthesis and topological properties of the resulting microgels was investigated. We studied the morphologies of microgels in “good”, “poor” and “selective” solvents. For the first time, we demonstrated the possibility of the formation of shell-corona structures in IPN microgels obtained by in silico synthesis from monomers and exposed to a selective solvent. These results allow for the better understanding of the required experimental conditions and data interpretation such as static structure factors.

DOI: 10.1039/D0SM00287A

Emulsifier-free RAFT emulsion polymerization of alkyl acrylates mediated by symmetrical trithiocarbonates based on poly(acrylic acid)

Polymer International, V.68, Issue 7, July 2019, Pages 1303-1314

Natalia S. Serkhacheva, Nikolay I. Prokopov, Elena V. Chernikova, Elena Y. Kozhunova, Inna O. Lebedeva, Oleg V. Borisov

Emulsifier‐free batch emulsion polymerization of n‐butyl acrylate and its semi‐batch copolymerization with 2,2,3,3,4,4,5,5‐octafluoropentyl acrylate and 2,2,3,4,4,4‐hexafluorobutyl acrylate both mediated by poly(acrylic acid) containing the trithiocarbonate group in the chain was employed to produce amphiphilic triblock copolymers. The polymerization‐induced self‐assembly of these copolymers in aqueous media gave rise to spherical core–shell particles. Irrespective of the experimental conditions, the polymeric product was characterized by a bimodal molecular weight distribution. The apparent violation of the reversible addition–fragmentation chain transfer polymerization mechanism may be attributed to restricted accessibility of the trithiocarbonate group in the self‐assembled block copolymers for propagating radicals that enter into the particle. Mean‐field theoretical arguments were employed to explain the exclusively spherical morphology of the particles observed in the experiment.

DOI: 10.1002/pi.5817

Thermo- and pH-Sensitive Microgels Based on Interpenetrating Networks as Components for Creating Polymeric Materials

Soft Matter, 2019,15, 5554-5561

I. R. Nasimova, O. V. Vyshivannaya, M. O. Gallyamov & E. Yu. Kozhunova

A technique of creating films from thermo- and pH-sensitive microgels based on poly(N-isopropylacrylamide) and poly(acrylic acid) interpenetrating networks is presented. Various methods of crosslinking microgel particles into a single material are investigated. It is shown that, with increasing temperature, all films undergo a thermally induced transition from the swollen to collapsed state. The collapse time is a few seconds, which is much shorter than the collapse time of a macrogel of comparable size and similar composition.

DOI: 10.1134/S0965545X19060063

“Smart” IPN microgels with different network structures: Self-crosslinked vs conventionally crosslinked

Polymer, Volume 176, 2 August 2019, Pages 127-134

Elena Yu.Kozhunova, Oxana V.Vyshivannaya, Irina R.Nasimova

Comparative study of thermo- and pH-sensitive conformational behavior of interpenetrating network (IPN) microgels based on poly(N-isopropylacrylamide) (PNIPA) and poly(acrylic acid) (PAA) with different network structures was conducted. For that purpose, PNIPA microgels were obtained by thermo-induced precipitation polymerization in the presence of the conventional cross-linking agent (N,N′-methylenebis(acrylamide)) and as a result of the self-crosslinking process. Then in situ polymerization of the acrylic acid network within these two types of PNIPA microgel particles was performed resulting in an IPN microgels formation. Dynamic and static light scattering methods were used for in-depth analysis of the system behavior in different conformational states of both networks of the IPN. The studies showed that the self-crosslinking method promoted the increase of pH-response of the system and its swelling ratio. The volume swelling ratio was about 3 times greater for the IPN microgels based on the self-crosslinked PNIPA microgels. Larger swelling coefficient is desirable, as it allows to expand the storage volume of a microgel particle.

DOI: 10.1016/j.polymer.2019.05.037

Orientational structures in cholesteric droplets with homeotropic surface anchoring

Soft Matter, 2019,15, 5554-5561

Mikhail N. Krakhalev, Vladimir Yu. Rudyak, Oxana O. Prishchepa, Anna P. Gardymova, Alexander V. Emelyanenko, Jui-Hsiang Liu  &  Victor Ya. Zyryanov

The dependency of orientational structures in cholesteric droplets with homeotropic surface anchoring on the helicity parameter has been studied by experiment and simulations. We have observed a sequence of structures, in which the director configurations and topological defects were identified by comparison of polarized microscopy pictures with simulated textures. A toron-like and low-symmetry intermediate layer-like structures have been revealed and studied in detail. The ranges of stability of the observed structures have been summarized in a general diagram and explained by the helicity parameter dependence of the free energy terms.

DOI: 10.1039/c9sm00384c

Seeding the UEFA Champions League participants: evaluation of the reforms

Scientific Reports volume 9, Article number: 13052 (2019)

Dmitry Dagaev, Rudyak Vladimir Yu.

We evaluate the sporting effects of the seeding system reforms in the Champions League, the major football club tournament organized by the Union of European Football Associations (UEFA). Before the 2015–2016 season, the teams were seeded in the group stage by their ratings. Starting from the 2015–2016 season, national champions of the Top-7 associations are seeded in the first pot, whereas other teams are seeded by their rating as before. Taking effect from the season 2018–2019, the team’s rating no longer includes 20% of the rating of the association that the team represents. Using the prediction model, we simulate the whole UEFA season and obtain numerical estimates for competitiveness changes in the UEFA tournaments caused by these seeding reforms. We report only marginal changes in tournament metrics that characterize ability of the tournament to select the best teams and competitive balance. Probability of changes in the UEFA national association ranking does not exceed several percent for any association.

DOI: 10.1515/jqas-2017-0130

Toroidal Configuration of a Cholesteric Liquid Crystal in Droplets with Homeotropic

JETP Letters volume 109, pages478–481(2019)

M. N. Krakhalev, V. Yu. Rudyak, A. P. Gardymova & V. Ya. Zyryanov

Orientational structures formed in cholesteric droplets with homeotropic surface anchoring have been studied by means of polarization optical microscopy and computer simulations. It has been found that, when the ratio of the size of droplets to the pitch of a cholesteric helix ranges from 1.4 to 2.9, an axisymmetric toroidal cholesteric structure is formed with a topological linear defect in the form of an equatorially located surface ring. The features of the toroidal structure in cholesteric droplets and their optical textures for various observation schemes are examined in detail.

DOI: 10.1134/S0021364019070075

Towards the realistic computer model of precipitation polymerization microgels

Scientific Reports volume 9, Article number: 13052 (2019)

Vladimir Yu. Rudyak, Elena Yu. Kozhunova & Alexander V. Chertovich

In this paper we propose a new method of coarse-grained computer simulations of the microgel formation in course of free radical precipitation polymerization. For the first time, we simulate the precipitation polymerization process from a dilute solution of initial components to a final microgel particle with coarse grained molecular dynamics, and compare it to the experimental data. We expect that our simulation studies of PNIPA-like microgels will be able to elucidate the subject of nucleation and growth kinetics and to describe in detail the network topology and structure. Performed computer simulations help to determine the characteristic phases of the growth process and show the necessity of prolongated synthesis for the formation of stable microgel particles. We demonstrate the important role of dangling ends in microgels, which occupy as much as 50% of its molecular mass and have previously unattended influence on the swelling behavior. The verification of the model is made by the comparison of collapse curves and structure factors between simulated and experimental systems, and high quality matching is achieved. This work could help to open new horizons in studies that require the knowledge of detailed and realistic structures of the microgel networks.

DOI: 10.1038/s41598-019-49512-3

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.

DOI: 10.1021/acs.macromol.8b02326

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