• New Type of Swelling Behavior upon Gel Ionization
  

      Swelling of the gels with incorporation of ionic groups in their subchains can be considered as a standard PE behavior. More complex situations appear if a gel swells in media with not very high polarity. In this case some unusual effects originating from counterion association with ions on the gel subchains can take place. In particular, a new so-called supercollapsed state of the gel can exist [1,2].
      When the polarity of solvent allows an effective competition between polyelectrolyte and ionomer behavior, the size of counterions is of crucial importance as it determines the gel swelling [3]. Depending on the size of counterions three different types of swelling behavior upon gel ionization can be observed.
  

  
  Dependence of gel swelling behavior on the type of counterion

      For bulky counterions (TBA) PMAA gel swells at ionization (pure polyelectrolyte behavior). For small counterions (Na, Cs) the initial gel swelling at low values of degree of ionization is followed by its collapse, after which the gel stays in the collapsed state up to the complete gel ionization (polyelectrolyte - ionomer switching). For counterions of intermediate size (TMA, TEA) charging of the gel causes first its swelling, then collapse and finally reswelling, when some degree of ionization is reached (polyelectrolyte - ionomer - polyelectrolyte switching).
      The theoretical analysis shows that it is due to weakness of ionic associations (ion pairs and multiplets) for rather big counterions, which results in their disruption upon some increase of the local polarity of the medium taking place at gel ionization. According to the theoretical calculations the region of ionomer state stability becomes narrower with an increase of the counterion size. This result has rather lucid explanation. The energy gain from ion association decreases with an increase of the size of ion pairs (i.e. the size of counterion), and the supercollapsed ionomer state becomes less favorable.
  

      [1] A.R. Khokhlov, E.Yu. Kramarenko. Macromolecular Theory and Simulations 1994, 3, 45. link
      [2] A.R. Khokhlov, E.Yu. Kramarenko. Macromolecules 1996, 29, 681. link
      [3] O.E. Philippova, A.M. Rumyantsev, E.Yu. Kramarenko, A.R. Khokhlov. Macromolecules 2013, 46, 9359. link

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• Self-Assembly of Ionic Block Copolymers in Solutions
  

      We develop a mean-field theory to study the structure of micelles formed by diblock copolymers with insoluble/amphiphilic ionic blocks in a salt-free dilute solution [4,5]. The core of the micelles comprises the insoluble blocks while the corona has an amphiphilic ionic nature containing non-polar and ionizable groups. We suppose that the solvent is slightly poor for the non-polar monomer units, thus, there is an interplay of the short-range attractive interactions and long-range Coulomb interactions within the corona. We study conformational transitions in the micellar aggregates with the change of the balance between these two types of interactions caused by changes in solvent quality, solvent polarity, and fraction of ionizable groups.
  

  
  Influence of the fraction of charged groups in corona block
  on the structure of polymer micelles

      We focus on the role of counterions in the system behavior. Owing to the micelle corona charge, the distribution of counterions in the solution is strongly inhomogeneous. Translational entropy of counterions completes with their attraction to the charged micellar aggregates resulting in accommodation of a large fraction of counterions in the micelles. The novelty of our approach is that we account for counterion binding with ion pair formation within the micellar corona [6]. While in a swollen hydrophilic corona, the fraction of ion pairs is negligible, in a polymer dense hydrophobic corona ion pairing is expected to be significant. Taking into account a progressive counterion binding within collapsing in a poor solvent micellar corona, we predict a new type of micelles in the solution. The corona of these micelles has an ionomer-type structure, i.e., it contains a large fraction of ion pairs. The region of stability of ionomer micelles increases with a decreasing solvent quality and polarity.
      Another important result is the prediction of two firstorder phase transitions between different-type micelles upon increase of the fraction of ion-containing groups in the micellar corona: large polyelectrolyte micelles with quasineutral coronae - large ionomer micelles - small charged micelles.
  

      [4] E.A. Lysenko, A.I. Kulebyakina, P.S. Chelushkin, A.M. Rumyantsev, E.Yu. Kramarenko, A.B. Zezin. Langmuir 2012, 28, 17108. link
      [5] E.A. Lysenko, A.I. Kulebyakina, P.S. Chelushkin, A.M. Rumyantsev, E.Yu. Kramarenko, A.B. Zezin. Langmuir 2012, 28, 12663. link
      [6] A.M. Rumyantsev, E.Yu. Kramarenko. J. Chem. Phys. 2013, 138, 204904. link

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• Interaction of Polyelectrolyte Gels with Oppositely Charged Surfactants
  

      The first theoretical approach describing interaction of macroscopic PE network with an oppositely charged surfactant was proposed in [7]. According to this work, in the course of ion exchange network counterions are substituted by surfactant molecules capable of micellization. Micelle formation inside the gel occurs to be more favourable than in the outer solution. The aggregation of ionic surfactants in solvent media out of the gel results in the immobilization of oppositely charged surfactant counterions in the vicinity of charged micelles accompanied by the loss in its translational entropy because of electrostatic reasons. In the gel interior micelle excess charge is neutralized by network subchains while counterions are still able to move freely, resulting in the decrease of CMC inside the PE gel by several orders of magnitude [7]. Thus, when the concentration of surfactant inside the gel exceeds new reduced CMC, intence formation of micellar aggregates in the gel interior results in diminution of exerting osmotic pressure and induces gel continious shrinking or abrupt collapse [7]. These theoretical predictions were confimed by experimental studies [8].
      The next step in theoretical comprehension of PE network behavior in the solution of oppositely charged sufractant was the description of two phase core-shell gel structure, observed earlier in a set of experimental studies. The possibility of inhomogeneous distribution of surfactant molecules inside the gel resulting in macroscopic phase separation and formation of swollen surfactant-free core and collapsed shell enriched with surfactant molecules was shown in several publications [9, 10].
  

      [7] A.R. Khokhlov, E.Yu. Kramarenko, E.E. Makhaeva, S.G.Starodubtzev. Makromol. Chem., Theory Simul. 1992, 1, 105. link
      [8] A.R. Khokhlov, E.Yu. Kramarenko, E.E. Makhaeva, S.G.Starodubtzev. Macromolecules 1992, 25, 4779. link
      [9] D.V. Tararyshkin, E.Yu. Kramarenko, A.R. Khokhlov. Polym. Sci., Ser. A 2007, 49, 1129. link
      [10] D. Tararyshkin, E. Kramarenko, A. Khokhlov. J. Chem. Phys. 2007, 127, 164905. link

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• Block Ionomer Complexes
  

      Solutions of linear polyions/charged surfactants and oppositely charged block copolymers consisting of ionogenic and neutral hydrophilic blocks are a new type of self-assembling systems. Cooperative electrostatic attraction between oppositely charged polyions resutls in a formation of polyelectrolyte complexes. Though neutralized monomer units of the complex are insoluble, its colloidal stability is provided by hydrophilic blocks of block copolymer. The structure of aggregates resembles one of micelles of amphiphilic block copolymers, i.e. micelle core consists of neutralized units of interpolyelectrolyte complex, while hydrophilic blocks of block copolymer form micelle corona. These systems are promising for practical applications due to unique ability to self-organize. In particular, solubility of these complexes is one of the main advantages in comparison with other cationic gene delivery systems which tend to precipitate under the same conditions. Moreover, these systems imitate self-assembly processes in numerous biological systems.
      Theoretical model of polyelectrolyte complex taking into account both possibility of ion pair formation and correlation-induced attration was proposed in [11, 12]. Structure and properties of these complexes have been investigated, and a diagram of complex states depending on concentrations of constituents, solvent quality and polarity have been constructed.
  

      [11] E.Yu. Kramarenko, A.R. Khokhlov, P. Reineker. J. Chem. Phys. 2006, 125, 194902. link
      [12] E.Yu. Kramarenko, A.R. Khokhlov. Polym. Sci., Ser. A 2007 , 49, 1053. link

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II.   Magnetically Responsive Polymer Systems

Magnetorheological elastomers are referred to the class of so-called smart materials. They consist of polymer matrix filled with magnetic microparticles. These materials are called as smart elastomers because of high sensitivity to external stimuli, such as temperature, pressure, electric and magnetic fields and ability to alter their physical properties, shape and dimensions in response to changes of external conditions. Observed effects reach considerable values providing successful application in a number of practical tasks. These effects are known to be reversible, i.e. sample restores the initial state under return towards initial conditions.

The following effects have been found and investigated by our group:

• a huge magnetorheological effect. Up to three orders of magnitude increase of the elastic modulus as well as the loss modulus was observed in the magnetic feld of 3 kOe. This effect has been used in tunable dampers and vibration absorbers;

  
  Dependence of the elastomer storage modulus on the value of external magnetic field.

• an enormous magnetodeformational effect in uniform as well as in gradient magnetic fields. This phenomenon seems to be promising for development of various micromovements. An electromagnetic valve based on this effect has been recently patented;

• a high responsiveness to an alternating magnetic field at the frequency of up to 40 Hz can be used for design of various actuators;

• a magnetic field induced plasticity, or memory effect - material keeps its deformed shape in magnetic field and restore its initial shape when the magnetic field is switched off;

• giant magnetodielectric effect, i.e. giant changes in the efficient dielectric permeability under external magnetic field. In elastomers filled with NdFeB powder, which is widely used as permanent magnets, magnitude of effect reaches 150%, a record value for now.

• magnetosensitive acoustic properties;

III.   Hyperbranched Polymer Systems

Dendrimer melts, amphiphilic dendrimer solutions.