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The phase transitions of poly(N-isopropylacrylamide) (PNIPAM) in water–methanol mixed solutions were studied in detail by high-sensitivity differential scanning calorimetry. From this study, the de...

Poly(ethylaminophosphazene) (PEAP) in salt-free aqueous solution does not reveal thermoresponsive properties but acquires thermoresponsivity upon addition of various poly(protic acid)s. Energetics of the salt-induced phase separation transition of the PEAP solutions upon heating was investigated by high-sensitivity differential scanning calorimetry at pH 3.5. At low concentrations of poly(carboxylic acid)s (10–100 mM), the transition temperature and enthalpy vary from 90 to 25 °C and from 1.5 to 35 J g–1, respectively, depending on the pKa1 and the apparent hydrophobicity of the acids. Dependences of the transition parameters (Tt, Δth, and Δtcp) on the citrate buffer concentration were obtained. The binding curve of citrate anions to PEAP was derived. High concentrations of sulfate anions (0.1–0.8 M) induced a marginal phase transition at high temperatures. A mechanism of the PEAP thermoresponsivity is proposed. The Okado–Tanaka model for cooperative hydration of macromolecules is used to quantitatively d...

Effect of induced self-organization and liquid-crystal ordering of disordered globules of flexible amphiphilic macromolecules was observed in 1:1 mixtures of flexible and stiff helical macromolecules of similar composition. Mixtures of fully flexible macromolecules and helical macromolecules with various stiffness of backbone were studied by means of molecular dynamics simulation. It was found that if the local helical structure of the backbone is stiff enough to induce formation of aggregates with helical packing of backbone in poor solvent, the helical and flexible macromolecules segregate upon worsening of solvent quality. In dilute solution the flexible macromolecules are compacted into single globules with hydrophobic core and hydrophilic shell. The macromolecules with stiff local helical structure form aggregates of few helical chains. The aggregates have a core composed by intertwined backbones which is surrounded by a hydrophilic outer shell consisting of side groups. Increase of volume fraction o...

The properties of new hydrophilic pressure-sensitive adhesives (PSA) obtained by blending poly(N-vinyl caprolactam) (PVCL) with short-molecular weight poly(ethylene glycol) (PEG) were studied in aqueous media by a combination of several calorimetric and adhesion testing techniques. We found that the adhesive properties of the blends are the result of an extensive hydrogen bonding network formed between PVCL and PEG similar to the poly(N-vinylpyrrolidone) (PVP)/PEG blends, except the extent of cross-linking is nearly 3 times higher in PVCL–PEG networks. Accordingly, we observed substantially higher peel adhesion in PVCL–PEG blends, which depends strongly on the amount of adsorbed water and the temperature. The adhesive properties of PVCL–PEG gels are considerably diminished when the amount of absorbed water exceeds 30% or at elevated temperature but can be easily recovered by drying or cooling the sample. The observed responsiveness of PVCL–PEG hydrogels in physiologically relevant temperature range makes ...

We performed a large-scale dissipative dynamics simulation to study the structural changes in unfilled and filled elastomers during uniaxial deformation, which helped to shed some light on the underlying reasons of filler reinforcement in rubber nanocomposites. Equilibrium stress–stain curves for different cross-linker concentrations and filler content were obtained, and their features were compared to the experimental data. Dependences of segmental orientation on deformation and true stress were studied; these dependences are discussed in the light of theoretical predictions and available experimental data. The structural changes in the deformed state were studied as well, namely, the dependences of mean end-to-end distance on the subchain length. For the filled elastomers it was found that in the matrix there are several sets of subchains with distinct properties. Part of the subchains which are not connected to the filler particles are deformed slightly more than in the unfilled matrix; the subchains c...

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)...

Understanding the driving forces for the collapse of a polymer chain from a random coil to a globule would be invaluable in enabling scientists to predict the folding of polypeptide sequences into defined tertiary structures. The HP model considers hydrophobic collapse to be the major driving force for protein folding. However, due to the inherent presence of chirality and hydrogen bonding in polypeptides, it has been difficult to experimentally test the ability of hydrophobic forces to independently drive structural transitions. In this work, we use polypeptoids, which lack backbone hydrogen bonding and chirality, to probe the exclusive effect of hydrophobicity on the coil-to-globule collapse. Two sequences containing the same composition of only hydrophobic "H" N-methylglycine and polar "P" N-(2-carboxyethyl)glycine monomers are shown to have very different globule collapse behaviors due only to the difference in their monomer sequence. As compared to a repeating sequence with an even distribution of H and P monomers, a designed protein-like sequence collapses into a more compact globule in aqueous solution as evidenced by small-angle X-ray scattering, dynamic light scattering, and probing with environmentally sensitive fluorophores. The free energy change for the coil-to-globule transition was determined by equilibrium denaturant titration with acetonitrile. Using a two-state model, the protein-like sequence is shown to have a much greater driving force for globule formation, as well as a higher m value, indicating increased cooperativity for the collapse transition. This difference in globule collapse behavior validates the ability of the HP model to describe structural transitions based solely on hydrophobic forces.

The influence of salt concentration on the size and on the thermodynamic stability of interpolymer complexes composed of oppositely charged macroions having different affinity to solvent was studied from a theoretical viewpoint. It was shown that increasing salt concentration causes changes in the structure of complex particles. At low salt concentration, the particles preserve their structure and size. At a critical salt concentration, nScr, the particle size rises sharply to a slightly larger dimension. From this concentration, the macroions forming the interpolymer complex start to separate, and the complex is fully destroyed at a salt concentration nS*. After separation, the macroions coexist in solution and with further increase in salt concentration reduce their sizes according to the screening of polyion charges by salt ions. nScr and nS* depend on physical parameters such as the degree of polymerization of macroions, their degree of ionization, and macroion−solvent interaction parameters. Experime...

We study the effect of comonomer sequence distributions in random copolymers (RCPs) on RCP adsorption on flat impenetrable surfaces. RCP poly(styrene-co-4-bromostyrene) (PBrxS), where x denotes the mole fraction of 4-bromostryrene (4-BrS), is prepared by bromination of parent homopolystyrene. By varying the solvent quality during the bromination, either “truly random” (good solvent) or “random-blocky” (poor solvent) PBrxS RCPs are prepared. Adsorption studies of PBrxS from various solvents at silica surfaces reveal that the adsorption of PBrxS increases with (1) increasing blockiness of the macromolecule, (2) increasing content of 4-BrS in PBrxS, and (3) decreasing solvent quality. Additionally, the effect of comonomer sequence distribution on RCP adsorption is modeled in detail using a coarse-grained statistical mechanical model and fully atomistic simulations based on configurational-biased grand-canonical Monte Carlo (CB-GCMC) technique. The main result from the simulations can be summarized as follows...

Conformational properties of amphiphilic stiff-chain macromolecules in concentrated solutions in poor solvent have been Studied via computer modeling. We have found that the conformational state of macromolecules ill such systems depends oil the macromolecular Stiffness and oil the way the Solution hits been prepared. Thus, if the concentration of globules increased from a very diluted Solution. the globules remain stable, independent of the macromolecular stiffness, and do not aggregate even in concentrated Solutions. Oil the other hand, if the solvent quality is gradually decreased in I solution with a concentration much larger than that of a semidilute solution, then relatively flexible chains form separate globules. whereas semirigid macromolecules tend to aggregate and form braid-like conformations. The results obtained agree with the published experimental data and call he used for directed synthesis of macromolecules modelin, the behavior of biopolymers.

Copolymers of N-isopropylacrylamide (NIPA) with sodium styrene sulfonate (SSS) synthesized in aqueous medium at 70 °C undergo reversible cooperative conformational transitions without phase separation of their aqueous solution. Data obtained by high-sensitivity differential scanning calorimetry and velocity sedimentation for the NIPA-SSS copolymers have shown that depending on the SSS content either intermolecular association (micellization) or intramolecular chain folding take place upon heating.

Peculiarities of chemical reactions inside polymer globules with sizes falling in the nanometric range have been theoretically studied. The simplest example of a polymer-catalyzed bimolecular reaction between two low-molecular-weight compounds has been considered to illustrate the advantages of conducting chemical transformations in globular solution of macromolecules. Under theoretical examination of polymer-analogous transformations of polymer molecules, the possibility has been shown of reaction-induced conformational transitions between their coil and globular states. Besides, a pronounced effect has been revealed of the preferential sorption of low-molecular-weight reagents on composition of copolymers formed.

International audience; We consider the new type of water-soluble stoichiometric interpolymer polyelectrolyte complexes (IPEC) consisting of macromolecules having different affinities to solvent and different densities of ionic groups along the chain. It was proposed that complexes of such macromolecules have a core-shell structure. The inner part of complex with a radius r contains monomer units of both macromolecules, while exterior part with a radius R consists exclusively of monomer units of hydrophilic macromolecules. With such complex organization, the external charged hydrophilic shell effectively protects the complex from precipitation and fusion with other complex assemblies. These theoretical conclusions allow to explain the experimental data on the stability of IPECs containing poly(L-lysine), poly(L-lysine citramide), poly(L-lysine citramide imide), and poly(acrylic acid), and can be of interest in the field of DNA-polycation complexes aimed at transfecting genes to cell nuclei.

A statistical model is developed for radical graft copolymerization in a solution of monomers A and B in the vicinity of a surface selectively absorbing the monomers of type A and corresponding copolymer sections. The influence of the monomer concentrations and the short-range monomer A−surface attraction on the copolymer sequence is investigated. It is shown that under certain conditions, the adsorption copolymerization can yield gradient copolymers. We find three copolymerization regimes corresponding to different values of dimensionless adsorption energy u. When the growing macroradical is weakly or nonadsorbed, u < uc (uc is the critical adsorption energy), the statistical properties of graft copolymers approach asymptotically, in the long-chain limit, to those of a random copolymer. If u ≳ uc, the statistics of designed and random copolymers is very different. In the vicinity of uc, the adsorption copolymerization leads to copolymers with the largest compositional nonuniformity and well-pronounced gr...

A coil−globule transition in individual double-stranded DNA triggered by chitosan polycations was directly observed by fluorescence microscopy. It was shown that the character of the conformational transition depends essentially on the amount of added low-molecular-weight salt. At low salt content the collapse proceeds via the formation of an intermediate necklace structure, which is stabilized by Coulomb repulsion between uncompensated charges of DNA chains. By contrast, at high salt content, when the electrostatic repulsion is sufficiently screened, the collapse proceeds directly between coil and globule states without intermediate structures. Also, salt affects the collapsed state of chitosan/DNA complexes: it decreases the size of single globules and favors the interglobular aggregation.

A physicochemical model of the interphase free-radical copolymerization of hydrophilic and hydrophobic monomers at the oil−water interface is put forward for the first time. Such a mode of copolymer synthesis makes possible a conformation-dependent design of the sequences of monomeric units in macromolecules since their chemical structure here is governed along with the kinetic and stoichiometric parameters of a reaction system also by the conformational statistics of polymer chains. In the framework of the model proposed the expressions are derived for the rate of copolymerization as well as for main statistical characteristics of the chemical structure of this process products. Theoretical dependencies of these characteristics on the initial composition of monomers and their conversion are presented for different values of the thermodynamic parameter characterizing the degree of incompatibility of hydrophilic and hydrophobic units in a macromolecule.

A new approach to synthesis of copolymers with long-range correlations is proposed. Using Monte Carlo simulations and the lattice bond-fluctuation model, we perform the computer-aided sequence design of a two-letter (AB) copolymer with quenched primary structure near a chemically homogeneous impenetrable surface. We simulate an irreversible radical copolymerization of selectively adsorbed A and B monomers with different affinity to the surface, allowing for a strong short-range monomer- (A-) surface attraction. To describe the chain growth analytically, we introduce and investigate a simple theoretical model based on stochastic processes and probabilistic statistics. We find that this model provides a close approximation to the simulation data and explains a number of statistical properties of copolymer sequences. It is shown that, under certain conditions, the chain propagation near the adsorbing surface proceeds as a randomly alternating growth, leading to a copolymer with a specific quasi-gradient primary structure and power-law long-range correlations in distribution of different monomer units along the chain. The gradient extends along the entire chain for any chain length. We find that the statistical properties of the copolymer sequences correspond to those of a one-dimensional fractal object with scale- invariant correlations. Thus, just by radical copolymerization of two monomers with different affinity to a certain plane surface, it is possible to obtain copolymers with a gradient primary structure.

Hydrogen-bonded interpolymer complexes between poly(N-isopropylacrylamide) (pNIPA) and poly(methacrylic acid) (pMAA) were studied in aqueous solutions by means of high-sensitivity differential scanning calorimetry (HS-DSC), analytical ultracentrifugation, and light scattering. Composition of the complexes was varied by changing the pMAA/pNIPA molar ratio r and pH. Solubility of the pMAA/pNIPA complexes increased with increasing the content of pMAA in the complex and decreased with decreasing pH. According to HS-DSC, the phase transition behavior of pNIPA upon heating is significantly modified in complexes with pMAA. Thus, decreasing pH from pH 7 to 3 or increasing the content of pMAA decreases the phase transition temperature of the complexes by 4−5 °C with respect to the lower critical solution temperature (LCST) of pure pNIPA. At the same time, the enthalpy of the transition decreases down to zero and the transition width increases by more than 1 order of magnitude. Thermograms of the soluble complexes ...

Poly(N-vinylcaprolactam-co-1-vinylimidazole) (PVCL−Vim) and poly(N-isopropylacrylamide-co-1-vinylimidazole) (PNIPA−Vim) are thermosensitive in water and water−2-propanol solutions. These copolymers are soluble at room temperature; however, they undergo phase transition at higher temperatures (Ttransition > 35 °C). Catalytic properties of PVCL−Vim and PNIPA−Vim as well as of 1-methylimidazole and poly(1-vinylimidazole) (PVim) in hydrolysis of p-nitrophenyl acetate were studied. Variation of the reaction rate with temperature was investigated at constant concentrations of substrate and catalyst (imidazole groups). It was found that the reaction catalyzed by the copolymers does not follow Arrhenius-type behavior: as temperature is increased over the aggregation temperature, the reaction rate increases faster than it could be predicted from the Arrhenius law. This was explained via the assumption that the substrate having an affinity to polymer/solvent interface can be adsorbed at the surfaces of polymer agg...

For simulating hydrophobic−amphiphilic (HA) copolymers, we have developed a “side-chain” HA model in which hydrophilic (P) interaction sites are attached to hydrophobic (H) main chain, thereby forming amphiphilic (A) monomer units, each with dualistic (hydrophobic/hydrophilic) properties. Using this coarse-grained model, we performed molecular dynamics simulations of the hydrophobically driven self-assembly in a selective solvent, for both single-chain and multichain systems. The focus is on the regime in which H and P interaction sites are strongly segregated. Single-chain simulations are performed for copolymers with the same HA composition but with different distribution of H and A monomer units along the hydrophobic backbone, including regular copolymers comprising H and A units in alternating sequence, (HA)x, regular multiblock copolymers (HLAL)x composed of H and A blocks of equal lengths L = 3, and quasi-random proteinlike copolymers having quenched primary structure. In a solvent selectively poor ...

Spontaneous curvature of molecular brushes adsorbed on a flat substrate was observed by scanning force microscopy for comblike polymers of poly(butyl acrylate). This phenomenon was analyzed theoretically for a 2D model of a brush molecule in the regime of strong adsorption. Although all side chains were confined to the substrate plane, they were allowed to flip over and change their position with respect to the backbone. Instability of the straight backbone was caused solely by the entropic elasticity of the side chains: smaller extension of the side chains was attained upon their uneven distribution relative to the backbone. An equilibrium, i.e., spontaneous, curvature resulted from competition between elasticity of the side chains and stabilizing factors such as inherent stiffness of the backbone, mixing entropy of the side chains and excluded volume interactions of distant parts of the brush. The curvature radius is found to be proportional to the degree of polymerization of the side chains. The effect of polydispersity of the side chains is discussed.

To simulate amphiphilic polymers, we introduce an extended hydrophobic/hydrophilic (HP) model which, in contrast to the standard “beads-on-a-string” HP model, incorporates the dualistic nature of m...

Laser light scattering and differential scanning calorimetry measurements have been performed for aqueous solutions of thermosensitive linear copolymers of N-vinylcaprolactam and methacrylic acid o...

Proteinlike copolymers were first predicted by the computer-aided biomimetic design (Physica A 1998, 249, 253-261). These copolymers consist of comonomer units of differing hydrophilicity/hydrophobicity. Heterogeneous blockiness, characteristic for such copolymers, facilitates chain folding with the formation of specific spatial packing: a dense core consisting of hydrophobic units and a polar shell formed by hydrophilic units. This paper describes the synthesis of N-vinylcaprolactam/N-vinylimidazole copolymers via the redox-initiated radical copolymerization in the medium of 10% aqueous DMSO at the temperatures both below and above the phase separation threshold. The synthesized macromolecular products were separated into thermally precipitating and nonprecipitating fractions. Their molecular weight characteristics were evaluated using size-exclusion chromatography; their comonomer composition was determined from 1H NMR spectra of copolymers dissolved in DMSO-d6. The temperature-dependent behavior of copolymer macromolecules in water was investigated by thermonephelometry, high-sensitivity differential scanning calorimetry, and 1H NMR spectroscopy of the copolymers dissolved in D2O. It was shown that thermally nonprecipitating copolymer fractions obtained at initial comonomer molar ratios of 85:15 and 90:10 can be identified as proteinlike copolymers. (Less)

The swelling behavior and the structure of composite gels of poly(acrylamide) with incorporated bentonite clay and the products of their reactions with a cationic surfactant, cetylpyridinium chlori...

Dynamic light scattering measurements have been performed for aqueous solutions of linear copolymer of N-vinylcaprolactam (VCa) and methacrylic acid (MA) with variable composition (i.e., charge degree of polymer chain). The swelling behavior of macromolecules was investigated as a function of both the solution pH value and fraction of methacrylic acid groups. It was shown that these copolymers are thermosensitive, and intermacromolecular aggregation takes place at the temperature increase. They exhibit compositionally dependent swelling behavior as a function of both pH and temperature. The behavior of studied copolymer was explained by the partial formation of ion pairs. The dependence of hydrodynamic diameter on pH of the solution is nonmonotonic. In acidic media at room temperatures we observed the intermolecular aggregation of copolymers accompanied by a sharp increase of the light scattering intensity. In alkaline media intermacromolecular aggregation is observed at higher temperatures. The effect of...

Swelling of poly(methyl methacrylate) and poly(butyl methacrylate) samples in supercritical carbon dioxide and propagation of diffusion fronts in the samples were studied in situ at various temperatures and pressures using direct optical observation. The difference in the appearance of diffusion fronts in these polymers with different glass transition temperatures was observed. The diffusion coefficients of carbon dioxide molecules in the polymers were determined both from swelling kinetics and from motion of diffusion fronts. A good agreement between these two values was found. The dependences of the diffusion coefficients and equilibrium swelling degrees of the polymers on pressure and temperature of supercritical carbon dioxide during sample exposure were determined and discussed.

The coexistence of two different conformational states in one molecule has been visualized by scanning force microscopy for a rod−globule transition of brush molecules adsorbed on a water surface. The transition, which occurred upon lateral compression of monolayers, was also examined theoretically by scaling analysis that proved its first-order character. The transition becomes less distinct with decreasing length of the side chains and finally vanishes below a certain critical value.

Conditions for the crossover from the strong to the superstrong segregation regime are analyzed for the case of comb−coil diblock copolymers. It is shown that the critical interaction energy between the components required to induce the crossover to the superstrong segregation regime is inversely proportional to mb = 1 + n/m, where n is the degree of polymerization of the side chain and m is the distance between successive grafting points. As a result, the superstrong segregation regime, being rather rare in the case of ordinary block copolymers, has a much better chance to be realized in the case of diblock copolymers with combs grafted to one of the blocks.

The influence of the migration of charges bound to polymer chains on the solubility of the charged polymer is studied theoretically and experimentally. It is found that, due to the migration of charges, the solubility of such a polymer is lower than the solubility of a polyelectrolyte having the same fraction of nonmigrating charged groups. It is shown that the solubility of a copolymer with an ionizable component depends on the ratio of the degree of ionization i to the fraction x of monomer units B able to be ionized. If this ratio equals unity (i = x), the solubility of the copolymer increases with an increase in the fraction of monomer units B. When the fraction x of monomer units B is much higher than the actual degree of ionization i, the ions bound to the polymer chains can be easily transferred from one chain to another in an optimum way so that to minimize the loss in translational entropy of the counterions. As a result, demixing of the polymer solution for i ≠ x occurs at lower values of the po...

Micellization of diblock copolymers with one long and weakly charged and one short uncharged block in an aqueous solution is considered on the basis of a lattice mean-field theory. The critical micellization concentration (cmc) is examined at different fractional charge and length of the hydrophilic block as well as at different solvent quality and salt concentration. Above the cmc, the concentration of free copolymers and the micellar aggregation number are nearly constant. Volume fraction profiles show that the core-corona interface is sharp and that the hydrophilic segments are highly diluted in the corona. The dependences of the micellar aggregation number, the core radius, and corona thickness on several parameters are presented, and they agree qualitatively with scaling predictions. The predictions of the scaling relations are however restricted by the fact of limited scope of micellar stability when a single parameter is varied.

Association-caused microphase separation in weakly charged polyelectrolytes dissolved in a good solvent is theoretically predicted. To this end, a theoretical treatment of the concentration fluctuations in solutions of homopolymer chains, containing small fractions of both charged groups and reversible stickers (groups A capable of association via thermoreversible reaction A + A ⇆ A2), is presented. Following Lifshitz, thermoreversible cross-linking is considered as an effective interaction leading to the appearance of an additional association-caused term in the expression for the free energy. The density−density correlation functions are calculated and the thermodynamic stability of the spatially homogeneous state toward both micro- and macrophase separation is analyzed within the random phase approximation (RPA). All possible spinodal curves are classified in the limiting case of the annealed gel (solution with fully associated reversible stickers). For partially associated stickers, the interplay of e...

Polymer micelles composed of block copolymers with one charged and one neutral block in aqueous salt free solution are analyzed by scaling and mean-field lattice theory. We study the micellar structure as a function of the length of the both blocks and the fractional charge of the hydrophilic block. The diagram of the different regimes of the behavior of the equilibrium micellar sizes is constructed. The distribution of the counterions in the system is examined using the scaling and the mean-field approaches. The numerical mean-field results are found to be consistent with three scaling regimes: a quasineutral at small fractional charge, the “Pincus” regime where the Coulomb interaction in the corona is unscreened since the counterions are located outside the corona region, and an osmotic regime where the Coulomb interactions are screened by counterions located in the corona region.

Lght-scattering and viscometric results are presented from micellar solutions of (PI) 2 PS (I 2 S) and (PS) 2 PI (S 2 I) three-miktoarm stars and a PSPI (SI) diblock copolymer in n-decane. The influence of architecture on the micellization properties of simple graft copolymers is investigated by keeping the overall molecular weights and compositions of the samples constant. All samples formed spherical micelles in n-decane, a selective solvent for polyisoprene. Aggregation numbers were found to increase in the order I 2 S < S 2 I < SI. Hydrodynamic radii of the micelles increased in the same order. The thickness of the corona is determined by the length of the soluble blocks, which they were found to be stretched to almost the same extent in all cases. The area of the core-corona interface per copolymer chain, Ac, depends on the architecture of the molecule, and it is larger in the case of I 2 S micelles. In the case of S 2 I, larger A c values were found compared to the SI reference sample, indicating that the PS arms are arranged in different ways in the two kinds of micelles. The presence of only one grafted chain per molecule can change considerably the micellar characteristics of complex block copolymers. A simple scaling theory is developed taking into account the free energy contributions from the core, the corona, and the interfacial region of the micelle in the different cases. Theoretical predictions agree qualitatively with the experimental results.

In this paper, we investigate theoretically the structure of an ultrathin layer of diblock copolymers. It is assumed that one of the blocks is strongly adsorbed on a planar surface while the nonadsorbed block aggregates with the formation of surface micelles. We adopt a simple model in which the adsorbed blocks of the copolymers are assumed to form a two-dimensional melt while the surface micelles of the second blocks are approximated by parts of a sphere. The equilibrium structure of the micelles, which is governed by the interplay of the surface energy which favours the aggregation process and the energy of stretching of the adsorbed blocks is analyzed. It is shown that the number of the chains in one micelle, the radius and the height of the micelles scale with the length NB of a nonadsorbed block as NB1/2.

The concepts of osmotically passive and active counterions are introduced for spatially inhomogeneous polyelectrolyte systems. Osmotically passive counterions appear as a result of the trapping of counterions in wells of inhomogeneously distributed electric potential. This effect is additional to the well-known Onsager-Manning condensation (i.e. condensation on the line of charges), which also produces counterions that are osmotically passive. The simplest theory of the swelling of inhomogeneous polyelectrolyte gels taking into account the effect of osmotically passive counterions is developed. It is shown that such gels swell less than the corresponding counterparts with homogeneously distributed charges on the gel chains because osmotically passive counterions are no longer contributing to the exerting of osmotic pressure. Some experimental confirmations of the effect of osmotically passive counterions are discussed.

The swelling and mechanical behavior of polyelectrolyte gels with the degree of ionization varying from 7 to 100 mol % were studied in mixtures of a good solvent and a poor solvent. In a good solvent region for weakly charged gels the shear modulus decreases with the gel swelling, while for strongly charged gels swelling leads to the increase of the shear modulus. Also for strongly charged gels the elastic modulus can remain unchanged during the volume phase transition (for a weakly charged gel, a jump in the volume at the collapse transition is usually accompanied by a discontinuous upturn of the elastic modulus). Both of these features of strongly charged gels are attributed to a significant extension of the highly ionized subchains. At the same degree of swelling the modulus of the fully ionized gel was higher in the medium of lower polarity. This may be due to the formation of new effective cross-links between the subchains as a result of dipole-dipole attraction of ion pairs formed between charged groups of network and condensed counterions.

We have developed a theory for the micelle formation in dilute solution mixtures of two block-copolymers (A−B and C−B) with very long soluble B-blocks and chemically identical insoluble blocks A and C differing in lengths. In this case spherical micelles with A, C, or A/C-core and B-corona are formed. The conditions for the formation of pure micelles and comicelles are determined by using the results of the calculation of the free energy of the core of the mixed micelles made previously by us and the theory of the micelle formation describing this process as a set of association−dissociation reactions with certain chemical constants. It is shown that the block-copolymers with longer blocks are normally the first to make aggregates (micelles). These micelles are enriched with shorter chains at the concentrations much smaller than cmc for these chains. There is a wide region on the phase diagram where the distribution function for the aggregation of block-copolymers in the micelles is bimodal, corresponding...

The effect sodium chloride on the composition and structure of network polymer−surfactant complexes (NPSC) formed by slightly cross-linked networks of poly(diallyldimethylammonium chloride) (PDADMA...

Dynamic light scattering measurements have been performed for aqueous solutions of linear poly(vinylcaprolactam) (PVCa) at several polymer concentrations and over a wide range of ionic surfactant concentration. The intermolecular aggregation of PVCa accompanied by a sharp increase of the light scattering intensity is observed in pure water at 33 °C. The effect of ionic surfactants (cetylpyridinium chloride (CPC) and sodium dodecyl sulfate (SDS)) on the behavior of solutions of linear PVCa was studied as a function of temperature. For polymer concentrations below the overlap concentration (2.5 mg/mL) a decrease of the macromolecular hydrodynamic diameter is observed upon the addition of ionic surfactant (SDS, CPC) at low surfactant concentrations. This effect is in contrast to the behavior of the complexes of surfactant with another thermosensitive polymer, poly(N-isopropylacrylamide) (PNIPA). A further increase of the surfactant concentration leads to the reentrant swelling of macromolecules. In the prese...

A new type of intermacromolecular system consisting of a flexible polymer gel with embedded synthetic rigid-rod linear polyelectrolyte has been investigated for polyacrylamide hydrogel and linear poly(4,4‘‘-(disodium 2,5-dimethyl-1,1‘:4‘,1‘‘-terphenyl-3‘,2‘‘-disulfonate)). It has been shown that the incorporation of stiff-chain polyelectrolyte inside the uncharged network improves both the ability to absorb water and the mechanical strength of the gel. This makes such systems potentially promising as superabsorbent materials. Study of the kinetics of the release of polyelectrolyte rods has shown that the rods are effectively retained by the gel although they are not covalently attached to the network chains. This is apparently due to the formation of aggregates of rods including some of the network chains. The release can be completely suppressed by the addition of low-molecular-weight salt, which favors the self-aggregation of similarly charged rods. An interpenetrating network consisting of a physical n...

Luminescent properties of chemically cross-linked gels of europium, terbium, and neodymium salts of poly(methacrylic acid) swollen in methanol were investigated by steady state and time-resolved fluorescence. The obtained results demonstrate the aggregation of rare earth metal ions bound to the network with the formation of multiplets. The energy transfer Eu 3+ f Nd 3+ in the gel shows that multiplets consist of three to four rare earth metal ions (together with the gel countercharges). This is one of the first observations of multiplet formation in ion-containing polymer gels.

Microphase separation in a blend of A-B and C-B block copolymers with strong A-B and C-B incompability is considered in the strong segregation regime. It is assumed that the length of the B block is always much larger than the lengths of A and C blocks, so the complex spherical micelles containing shorter A and longer C chains are formed. The inner structure of the complex A-C micelle is considered, and the distribution of volume fractions of A and C monomer units is calculated. It is shown that shorter A chains are distributed mainly in the periphery of the micelle, while C chains are coiled in the center of this micelle. Macroscopic separation in this blend is studied. It is shown that, in the mixtures with a small fraction of C-B block copolymer, a two-phase structure should be observed: one phase consists of practically pure A micelles, while the other phase the complex A-C micelles, with enriched contents of longer C chains, are formed. For chemically different A and C blocks, even slight A-C repulsion leads to phase separation between the phases with pure A and pure C micelles.

pH-responsive gels of hydrophobically modified (HM) weak polyacid were prepared from acrylic acid and n-alkyl acrylates (n = 8, 12, 18). The HM gels obtained bear up to 20 mol % of n-alkyl acrylate units randomly distributed along the network chains. The pH-driven swelling of these gels upon ionization in an aqueous medium was studied. The effect of the fraction and of the side chain length of n-alkyl acrylate groups on the equilibrium degree of swelling was examined. It was shown that the swelling transition shifts to alkaline pH with increasing hydrophobicity of the gel. This was explained by the stabilization of the collapsed state of the gel by hydrophobic aggregation of n-alkyl side chains. The formation of such aggregates, which break down in the course of gel ionization, was confirmed by the fluorescent probe method with pyrene as a probe and by NMR spectroscopy. Potentiometric titration data of HM poly(acrylic acid) (PAA) gels and of the corresponding linear copolymers evidence that the introducti...

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.

Structural cooperative transitions in thermosensitive nonionic hydrogels of poly(N-vinylcaprolactam) (PVCa) were studied using high-sensitivity differential scanning calorimetry (HS-DSC). The thermoshrinking of the PVCa gel is endothermic, much like first-order phase transitions. According to the HS-DSC data, the gel system undergoes two successive cooperative transitions between 25 and 50 °C. The low-temperature transition (31.5 °C) is proposed to be associated with the microsegregation resulting in formation of hydrophobic domains (micromicelles) in the gel whereas the high-temperature transition (37.6 °C) is due to the gel volume collapse. The transition temperatures decrease in the presence of NaCl and increase with increasing sodium dodecyl sulfate (SDS) concentration. Dependences of the transition enthalpies and entropies on NaCl and SDS concentrations display a more complex character. The influence of sodium chloride and SDS on the transition temperatures is in agreement with the general expectatio...

Spherical micelles with complex inner structure in A−B−C-triblock copolymer melts with NA + NB ≪ NC are considered in strong and superstrong segregation limits. It is assumed that blocks of polymer chain are different in length and strongly incompatible. One of the particular systems which we have in mind is a melt of B−C-block copolymer (NB ≪ NC) with strongly associative A-groups at the end of B-block. In this case the formation of A-aggregates occurs inside the B-micelle, which is assumed in this paper to be spherical in shape. The inner structure of the B-micelle can be complicated. It is possible to distinguish between two principally different cases: (i) all A-blocks form a single aggregate at the center of the B-micelle and (ii) the A-aggregates form a certain structure inside the core of the B-micelle. It has been shown that, depending on the external parameters (such as temperature, monomer interaction parameters, block length, and monomer unit length), the characteristics of the B-micelle (aggr...