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The nanostructures of polyelectrolyte complexes (PECs) fabricated from a series of rationally designed oppositely charged amphiphilic random copolymers with variable comonomer compositions were stu...

A series of copolymers with controlled monomer sequences are prepared via the reversible addition–fragmentation chain transfer polymerization of tert-butyl carbamate (Boc)-l-alanine (VBA) and Boc-l...

A possibility of microphase separation in a blend of oppositely charged polyelectrolytes, being immiscible when uncharged, is studied theoretically and by means of dissipative particle dynamics (DP...

International audience; The instability of a homogeneous complex coacervate phase induced by incompatibility of oppositely charged polyelectrolytes is studied by means of the random phase approximation (RPA) and the scaling approach. It is found that in a poor solvent for both polyions their short-range mutual repulsions may result in the formation of a microphase separated structure at low salt concentrations cs. At high cs, a decomposition of the homogeneous coacervate into two macroscopic globular phases enriched either with polyanions or with polycations takes place. Both the RPA and scaling estimations predict the period of the microphase separated structure on the order of a few nanometers in the vicinity of the spinodal. The binodal of macroscopic phase separation of the coacervate and the spinodal with respect to the microphase separation are calculated, and the phase diagram of the coacervate is constructed in terms of salt concentration and incompatibility of polyions. The possibility of experimental observation of a microsegregated coacervate is discussed.

Rheological melt behavior of carbosilane dendrimers from the 3rd up to the 8th generation has been studied experimentally under steady shear, creep and dynamic oscillatory shear. Six orders of magnitude jump in viscosity was found between the 5th and the 6th generations. While the low-generation dendrimer melts demonstrate Newtonian behavior in a wide temperature range of 20÷80 °C, starting from the 6 t h generation, the dendrimer melts lose their ability to freely flow and acquire properties of a material with a well-pronounced yield stress. The storage modulus of high-generation dendrimer melts by far exceeds the loss modulus, furthermore, its value increases with temperature. It is demonstrated that the high-generation carbosilane dendrimers in bulk can be considered a polymer material with unique properties and as that, could find new applications in high-tech areas, in particular, as matrices for sensors.

Magnetodielectric properties of magnetoactive elastomers comprising micrometer-sized iron particles dispersed in compliant elastomer matrices are experimentally studied in stepwise time-varying dc magnetic fields. It is found that imposition of magnetic field significantly increases both the effective lossless permittivity of these composite materials as well as their effective conductivity. These magnetodielectric effects are more pronounced for larger concentrations of soft-magnetic filler particles and softer elastomer matrices. The largest observed relative change of the effective dielectric constant in the maximum magnetic field of 0.57 T is of the order of 1000%. The largest observed absolute change of the loss tangent is approximately 0.8. The transient response of the magnetodielectric effect to a step magnetic-field excitation can be rather complex. It changes from a simple monotonic growth with time for small magnetic-field steps ( 0.3 T). The settling time to the magnetic-field step excitation can reach roughly 1000 s and it depends on the applied magnetic field and sample composition. There is also significant hysteresis of the magnetodielectric effect on the externally applied magnetic field. These findings are attributed to the rearrangement of ferromagnetic filler particles in external magnetic fields. The results will be useful for understanding and predicting the transient behavior of magnetoactive elastomers in applications where the control magnetic field is time dependent.

A joint theoretical and experimental study is devoted to the swelling and electrical conductivity of highly charged polyelectrolyte (PE) gels in media of different polarity. Deprotected poly(tert-butyl carbamate l-alanine) gels provided with fluorine, bromine, chlorine, sulfate, and trifluoroacetic counteranions demonstrated different conductivity in aqueous medium as well as solvent uptake in water/methanol mixtures. Counterion specificity in PE gel properties was theoretically explained in the framework of the model which treats ion association within the network as a two-stage process. Manning-type condensation, being the first condensation stage not affected by counterion type, provides understanding of a gel maximal swelling at intermediate degrees of gel ionization f and a slight gel contraction at f tending to unity, which were earlier observed in experimental investigations. The second ion association stage is an ion pairing influenced by the kind of counterion. Since a considerable fraction of io...

Hysteresis in dynamic modulus, loss factor and normal forces of magnetoactive elastomers (MAEs) comprising various proportions of small (3–5 μm) and large (50–60 μm) ferromagnetic particles are experimentally studied using dynamic torsion performed at a fixed oscillation frequency in varying DC magnetic fields. It is shown that hysteresis is a characteristic feature of MAEs observed both under increasing/decreasing magnetic field strength and increasing/decreasing strain amplitude. This hysteresis is attributed to the specific rearrangement of the magnetic filler network under simultaneously applied magnetic field and shear deformation. Rheological properties of the magnetic filler network formed in the magnetic field and, therefore, the rheological properties of MAEs depend strongly on the filler composition and the magnetic field magnitude. Larger magnetic particles and higher magnetic fields provide stronger magnetic networks. Both factors result in the extension of the linear viscoelastic regime to larger strain amplitudes and lead to higher values of shear storage and loss moduli. It is found that the hysteresis width maximises at an intermediate magnetic field where it is attributed to the balance between elastic and magnetic particle interactions. This is apparently where the most significant restructuring of the magnetic network occurs. The hysteresis width decreases with increasing fraction of large particles in the magnetic filler. The loss factor grows significantly when the magnetic network is physically broken by large strains γ > 1%. A huge (more than one order of magnitude) increase of normal force at maximum magnetic field strengths is observed. It is predicted that any physical quantity depending on the internal structuring of the magnetic filler should demonstrate hysteresis either with a changing magnetic field and constant deformation amplitude or under variable deformation in a constant magnetic field.

Magneto-active elastomers (MAEs) based on a silicon polymer matrix filled with magnetic carbonyl iron microparticles have been synthesized. Both isotropic samples and samples micro-structured by an external magnetic field have been obtained. The radio-absorbing properties of the MAEs have experimentally been studied in the frequency range 2–10 GHz and at the central work frequency 30 GHz of the range 26–38 GHz in the absence and in the presence of an external magnetic field of 0.3 T. It has been shown that both the reflection and transmission coefficients of the MAEs change considerably when the magnetic field is applied, besides, their values strongly depend on the direction of the magnetic field as well as on the internal filler structure of MAEs. The developed MAEs could be used as controllable by an external magnetic field radio-shielding or polarizing covers.

We report a combined experimental and theoretical study on the swelling behavior of polyelectrolyte gels with various types of counterions. Experimental research was focused on poly(methacrylic acid) and poly(acrylic acid) gels in methanol neutralized with different bases providing sodium, cesium, tetramethyl-, tetraethyl- or tetrabutylammonium counterions. The novelty of the theoretical treatment is that the counterion size is explicitly taken into account as well as the dependence of the ion association constant on the volume fraction of polymer within the gel. We demonstrate that, depending on the counterion size, three different regimes of the gel behavior are realized. In case of bulky tetrabutylammonium counterions the gel swells upon ionization. For small counterions (Na+, Cs+) the gel swelling at low ionization degrees is succeeded by its collapse. New type of behavior was observed and theoretically described for the gels with counterions of intermediate sizes (tetramethyl- and tetraethylammonium)...

The conformational behavior of a single AB block copolymer is studied by Monte Carlo simulation. The A-A and A-B interactions have the character of excluded volume interactions while the B units attract each other; the attractive B blocks can move along the chain. The collapse transition of the chain with increasing attraction between the B units is analyzed. Intrachain separation of the A and B units takes place in the course of the chain collapse with the formation of "globule with a tail" conformations. The globule is formed by the attractive moving B blocks while the tail consists of the swollen A segments. The model of AB block copolymer with moving B blocks can describe the behavior of interpolymer complexes between a long macromolecule and shorter polymer chains.

We develop a theory of microphase separation in a solution precipitate of flexible AB block copolymers consisting of charged (A) and neutral (B) blocks which are complexed with oppositely charged linear chains (C) via electrostatic interactions. We analyze regimes of selective solvents: the solvent is Θ for the A blocks and the C chains, whereas it is poor for the B blocks. Despite the selectivity, stoichiometric complexes precipitate because of fluctuations induced electrostatic attraction. Depending on composition of the diblock copolymer, selectivity of the solvent and fraction of charged groups, direct and inverse spherical, cylindrical and lamellar structures can be stable in the precipitant. Phase diagrams are constructed in the strong segregation approximation. Morphological transitions induced by changes of solvent quality and fraction of charged groups are predicted.

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.

Magnetorheological elastomers (MRE) have been synthesized on the basis of a silicon compound and a mixture of carbonyl iron particles of sizes 3–5 and 40–80 μm. Their viscoelastic properties have been studied by dynamic shear oscillations of various amplitudes on a stress controlled rheometer. The magnetic response of the obtained materials has been examined in a magnetic field applied perpendicular to the shear plane. It has been shown that under applied magnetic field both the storage G′ and loss G″ moduli became strain-dependent. The values of G′ and G″ decrease with strain, while their ratio (the loss factor), G″/G′, growths with strain. The higher magnetic field is the more pronounced the strain dependence is. At small strain (up to 1%) MRE demonstrate a giant (more than 10 times) increase of the moduli. Some features of hysteretic behavior of MRE under simultaneously applied magnetic field and external mechanical force have been elucidated. Temperature has a negligible effect on viscoelastic properties and stability of the developed MRE. A damper on the basis of MRE has been designed and its properties have been examined.

We report on the theoretical study of interaction of ionic surfactants with oppositely charged microgel particles in dilute solutions. Two approaches are proposed. Within the first approach, the micellization of the surfactants inside the microgel is taken into account while the second model focuses on the hydrophobic interactions of the surfactant tails with the hydrophobic parts of microgel subchains. It has been shown that microgels effectively absorb surfactant ions. At low surfactant concentration this absorption is realized due to an ion exchange between microgel counterions and surfactant ions. The ion exchange is significantly affected by the amount of the microgel counterions initially trapped within the microgel particles which depends on the size of the microgel, its ionization degree, cross-linking density as well as polymer concentration in the solution. Increase of the surfactant concentration causes contraction of the microgels, which can be realized as either a continuous shrinking or a jump-like collapse transition depending on the system parameters. In the collapsed state additional absorption of surfactants by microgels takes place due to an energy gain from micellization or hydrophobic interactions. This leads to microgel precipitation and successive microgel overcharging at an excess of the surfactant in the solution. The theoretical results are compared with the existing experimental data, in particular, on photosensitive surfactant/microgel complexes.

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.

Full Paper: In this paper we consider the influence of counterion distribution on the behavior of polyelectrolyte systems. We propose the unified model to describe and to compare the swelling and collapse properties of single polyelectrolyte chains in dilute solutions, microgel particles of various molecular masses, and (as a limiting case) macroscopic gels. A novel feature of the new approach is that we distinguish three possible states of counterions: free countertions inside and outside the poly ther macromolecule and a bound state of counterions forming ion pairs with corresponding ions of polymer chains. The latter possibility becomes progressively important when macromolecules or gels shrink. In this case the formation of a supercollapsed state is possible, when all couterions are trapped and form ion pairs. On the other hand, the fact that counterions can float in the outer solution affects essentially the conformation of polyelectrolyte chains in dilute solutions of good where practically all counter ions can escape the space inside polymer coils and the repulsion between uncompensated charges plays an important role in the chain behavior.

The simplest theory of the collapse transition of single polyelectrolyte chains in dilute solutions is reconsidered. A novel feature of the new treatment is that the counterions can either remain within the coil or float in the outer solution. It is shown that the latter possibility is realized in many cases; thus, for these situations, the electrostatic repulsion between the uncompensated charges plays an important role in the chain behavior. These uncompensated charges lead to a significant difference between the collapse behavior of single chains and macroscopic gels where electrostatic repulsion is normally negligible and the swelling is mainly due to the osmotic pressure of counterions kept inside the gel sample. In addition the intermediate case of microgel particles of different molecular masses is considered, and the role of counterion redistribution between polymer and solution is investigated.

Several experimental facts indicate that the collapse transition in weakly charged polyelectrolyte gels or single macromolecules can take place with an increase in the degree of ionization (e.g., during titration of a weak polyacid). This unusual behavior can be explained by the energy gain from the formation of ion pairs in the collapsed state of low polarity, which competes with the swollen state where most of the counterions are dissociated. An increasing degree of ionization increases the thermodynamic advantages of the collapsed state with an ionomeric multiplet structure over the swollen polyelectrolyte state. In some regimes, this effect can lead to reentrant phase transition behavior, i.e., to an initial decollapse of the gel upon charging with subsequent jumplike collapse at a higher fraction of charged monomer units.

The theory of collapse of weakly charged polyelectrolyte gels is generalized by taking into account the possibility of counter ion trapping with the formation of ion pairs, which becomes progressively important as the gel shrinks and the dielectric constant of the gel medium decreases. A phenomenon well known in the theory of ionomers, namely the aggregation of ion pairs due to dipole-dipole interactions with the formation of multiplets, is also taken into account. These multiplets act as additional physical cross-links. It is shown that accounting for the two effects mentioned above generally leads to an increase of the region of stability of the collapsed phase and to an increase of the jump in volume at the transition point. The most important, qualitatively new effect is the possibility of existence of a new supercollapsed state of a polymer gel which is very close to the densely packed dry gel. The reason for the thermodynamic stability of the supercollapsed state is a loop of positive feedback: the decrease of the volume of the gel leads to a decrease of the dielectric constant and hence to progressive formation of ion pairs, thus the concentration of mobile counter ions and the corresponding osmotic pressure decrease, the gel shrinks further etc. It is possible to realize the phase transitions between all three states of a polymer gel: swollen, ordinary collapsed and supercollapsed.

Formation of surface-induced nanopattern (SINPAT) in ultrathin diblock copolymer films is studied by scanning force microscopy and Monte Carlo simulation. The pattern is caused by strong adsorption of one of the two blocks forming a quasi-two-dimensional coil while the other block dewets this adsorption layer. Scanning force microscopy allowed to observe an order−disorder transition for a SINPAT film of polystyrene-block-poly(2-vinylpyridine) on mica when the length of the dewetting polystyrene block was varied. The experimental data are compared with the Monte Carlo simulations which demonstrate how the pattern formation depends on the degree of polymerization of the dewetting block and the unfavorable interaction potential between the different components.