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Polymer stimuli-responsive microgels find their use in various applications. The knowledge of its internal structure is of importance for further improvement and expanding the scope. Interpenetrating network (IPN) microgels may possess a remarkable feature of strongly non-uniform inner architecture, even microphase separation, in conditions of a selective solvent. In this research, we, for the first time, use a combination of static light scattering (SLS) and small-angle X-ray scattering (SAXS) techniques to collect the structure factors of aqueous dispersions of poly(N-isopropylacrylamide)-polyacrylic acid IPN microgels on the broad scale ofqvalues. We study the influence of solvent quality on microgel conformations and show that in a selective solvent, such a system undergoes microphase separation: the sub-network in a poor solvent conditions forms dense small aggregates inside the large swollen sub-network in a good solvent. We propose the microstructured sphere model for the IPN microgel structure factor interpretation and perform additional analysis and verification through coarse-grained molecular dynamics computer simulations.

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.

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.

This study investigates the formation of microstructured macromaterials from thermo- and pH-sensitive microgels based on interpenetrating networks of poly-N-isopropylacrylamide (PNIPAM) and polyacrylic acid (PAA). Macromaterials are produced as a result of the deposition of microgel particles and subsequent crosslinking of polyacrylic acid subnetworks to each other due to the formation of the anhydride bonds during annealing. Since both PNIPAM and PAA are environment-sensitive polymers, one can expect that their conformational state during material development will affect its resulting properties. Thus, the influence of conditions of preparation for annealing (pH of the solution, the temperature of preliminary drying) on the swelling behavior, pH- and thermosensitivity, and macromaterial inner structure was investigated. In parallel, the study of the effect of the relative conformations of the IPN microgel subnetworks on the formation of macromaterials was carried out by the computer simulations method. It was shown that the properties of the prepared macromaterials strongly depend both on the temperature and pH of the PNIPAM-PAA IPN microgel dispersions. This opens up new opportunities to obtain materials with pre-chosen characteristics and environmental sensitivity.

Specific features of polyelectrolyte behavior and structure of the sodium polyacrylamido-2-methyl-1-propanesulfonate cryogels were studied. The cryogels were synthesized in frozen aqueous solutions at −15°С, the reference gels – at 25°С. It was shown that the polyelectrolyte swelling of cryogels is much weaker than that of gels synthesized in solutions. The ethanol-induced collapse of both gels and cryogels occurs stepwise at similar ethanol concentrations. The high-porosity structure of the cryogels is preserved in the collapsed state. The small-angle X-ray scattering (SAXS) spectra show maxima corresponding to ordered inhomogeneities of 0.5–3.0 nm scale. The positions of the observed maxima depend on the conditions of the synthesis and on the concentration of the polymer in the gels and cryogels.

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 an ionic 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.

The radical polymerization of n-butyl acrylate in organic, aqueous, and water–alcohol media in the presence of poly(acrylic acid) containing a trithiocarbonate group within the chain is studied for the first time. It is shown that in nonselective solvents (1,4-dioxane and DMF) poly(acrylic acid) serves as a reversible addition–fragmentation chain-transfer agent and the triblock copolymer poly(acrylic acid)–block–poly(n-butyl acrylate)-–block-poly(acrylic acid) is formed. In aqueous and aqueous–organic media (under conditions of emulsion, dispersion, and miniemulsion polymerizations as well as polymerization-induced selfassembly), the block copolymer being formed additionally serves as a stabilizer of polymer–monomer particles. The sizes of these particles and the molecular-mass characteristics of the resulting polymers may be controlled via variation in the concentration ratio of the components. It is found that, during polymerization in aqueous media, there is the formation of spherical polymer particles that preserve their morphology in thin films prepared via precipitation of the synthesized dispersion.

Thermo-responsive gels and cryogels with embedded microdroplets of Vaseline, olive, peanut, and linseed oils and their mixtures with hydrophobic dye Sudan 3 have been synthesized and studied. These composite gel matrices were obtained by the three-dimensional copolymerization of N-isopropylacrylamide and N, N'-bis(acryloyl)cystamine in the presence of oil emulsions stabilized with sodium dodecylsulfate or Span 80. Polymerization was performed at room temperature for conventional gels and at −15°C for cryogels. It was shown that all synthesized systems exhibit heat-induced collapse at temperatures higher then 34°C. For conventional gels prepared at room temperature shrinking lasts within 20 to 80 min in accordance with gel composition. No squeezing of oil droplets was observed. In the case of cryogels, shrinking was accompanied by release of oils and response time was significantly shorter, about tens of seconds. Collapse character and release of lipophilic phase did not depend on the chemical nature of oils, dissolved compounds, and surfactant used for emulsion stabilization. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

We report a high-throughput study of the kinetics of a multicomponent polymerization reaction in a microfluidic reactor integrated with in situ attenuated total reflection Fourier transform infrared spectroscopy. The technique was used to study the kinetics of an exemplary free-radical polymerization reaction of N-isopropylacrylamide, which was initiated by ammonium persulfate in the presence of the accelerator N,N,N′,N′-tetramethylethylenediamine in water. By monitoring the rate of disappearance of the monomer double bonds, we determined the effects of the concentration of the monomer, initiator, and accelerator on the rate of polymerization and the effect of the pH of the reaction system on the reaction kinetics. This work opens the way for the kinetic studies of complex polymer systems in a microfluidic format.

A microfluidic strategy for the encapsulation and stimulus-responsive release of molecules with distinct polarities from the interior of microgels is reported. The approach relies on (i) the generation of a primary O/W emulsion by the ultrasonication method, (ii) MF emulsification of the primary emulsion, and (iii) photopolymerization of the monomer present in the aqueous phase of the droplets, thereby transforming them into microgels. Non-polar molecules are dissolved in oil droplets embedded in the microgels. Polar molecules are physically associated with the hydrogel network. Upon heating, the microgels contract and release polar and non-polar cargo molecules. The approach paves the way for stimuli-responsive vehicles for multiple drug delivery.

The appearance of chemomechanical oscillations in polymer gels during the Belousov-Zhabotinskii reaction is studied. The effect of the size of composite gels based on polyacrylamide and silica, as well as the gels of acrylamide-sodium acrylate copolymer, on the mode of the reaction and the period of oscillations is examined. It is established that there is a limiting gel diameter below which the chemical oscillations of the oxidation state of a catalyst do not result in the appearance of chemical waves: Redox transformations occur uniformly within the sample. The gel-size threshold value below which the mechanical oscillations of the polymer network are not observed is determined. On the basis of the comparison of phase difference between mechanical and chemical oscillations for samples of various sizes and data on the swelling degree of gel in equilibrium states in which the catalyst is in either the oxidized or the reduced state, a conclusion is made about the mechanism of volume oscillations of the polymer network.

Chemical and mechanical oscillations arising when the Belousov-Zhabotinsky reaction is performed in composite gels based on polyacrylamide and silica gel and gels of poly(acrylamide-co-sodium acrylate) have been investigated. The catalyst of the reaction can be incorporated into a gel through electrostatic interactions. Mechanical and absorption properties of polymer matrices are characterized, and the periods and amplitudes of mechanical oscillations arising in them are estimated. The mechanism of the phenomenon under consideration is advanced.

The effect of poly(acrylic acid) and poly(methacrylic acid) on the molecular state of a cationic cyanine dye (3,3′-diethyl-2,2′-thiacarbocyanine iodide) was studied. Gels based on the above polymers were shown to efficiently absorb an oppositely charged dye. The absorption of dye ions by a gel induced their aggregation. It was shown that, in the acrylic acid-based gels, H-aggregates, dimers, and single ions of the dye coexist. In gels containing methacrylic acid units, dye ions mainly formed H-aggregates. A comparison of aggregation of dye ions in the gels with that in aqueous solutions of corresponding polyelectrolytes was performed. When dye ions were immobilized in a gel network, the proportion of the ions forming the H-aggregates was considerably larger than that in solutions. The effect of the gel network charge density on the aggregation of dye ions was investigated. It was shown that an increase in the fraction of charged units in network chains facilitates aggregation.

The relationships between the molecular weight (Mw) and the characteristic decay time distribution function G ( Γ ) of irradiated kappa ( κ -) and iota ( ι -) carrageenan were studied by static and dynamic light scattering (DLS). Mw and the characteristic decay time ( Γ −1 ) are both steep decreasing exponential function with radiation dose. The dynamic behavior of irradiated ι -carrageenan was compared to irradiated κ -carrageenan by DLS. The intensity correlation function of both carrageenans shifted towards shorter relaxation times with increasing radiation dose. Irradiated ι -carrageenan like κ -carrageenan exhibits power law behavior at 0–50 kGy (at 0.05–0.1M KCl) indicating similar gelation behaviors. The temperature at which transition from coil to helix takes place (conformational transition temperature) decreases with increasing irradiation dose. A new faster relaxation mode appears at around 0.1–1 ms for both carrageenans between 100 and 150 kGy. Maximum peak height for this mode is at 100 kGy which corresponds to the optimum biological activity of κ - and ι -carrageenan.

The ionization effects on the pressure-induced phase transition of weakly charged poly(N-isopropylacrylamide-co-acrylic acid) (PNIPA-AAc) gels have been investigated by small-angle neutron scattering. At low temperature, T, and pressure, P, the structure factor of PNIPA-AAc gels was well represented by a Lorentzian (L) function, which was similar to noncharged PNIPA gels. However, at high Ps, the contribution of inhomogeneities became large and a squared-Lorentzian term had to be added in addition to the L term. At high Ts, on the other hand, a scattering maximum appeared, indicating microphase separation. This scattering maximum was suppressed by increasing P up to P approximately 100 MPa and then reincreased at higher Ps. The following facts were disclosed: (1) The peak position and height were very sensitive to P, which is mainly ascribed to strong pressure dependence of hydrophobic interaction, (2) ionization leads to microphase separation at elevated temperatures, (3) the re-entrant phase behavior is commonly observed in the P-T plane due to the parabolic variation of the polymer-solvent interaction with P, and (4) the pressure and temperature dependence of the structure factor was reproduced with the Rabin-Panyukov theory and was interpreted with a convexity of hydrophobic interaction with respect to pressure.

Effects of pressure on the phase separation and structure of weakly charged poly(N-isopropylacrylamide-co-acrylic acid) (PNIPA−AAc) solutions and gels with 7 wt % were investigated by small-angle neutron scattering (SANS). The phase diagrams were determined in the pressure−temperature (P−T) plane for the solution and gel with a light scattering method, which were parabolic functions, having maxima at (P = 38.1 MPa, T = 37.1 °C; for the solution) and (121.6 MPa, 51.6 °C; for the gel). SANS experiments were carried out according to the P−T phase diagrams. At the low temperature and pressure region, where the systems were in one phase, the scattering intensity functions, I(q)s, were well described with an Ornstein−Zernike function. However, a scattering maximum appeared in I(q)s at elevated temperatures and pressures. The regions where a scattering maximum was observed at T = 45 °C were P < 70 MPa and 200 MPa < P for the gel and P < 70 MPa for the solution. This scattering maximum is due to microphase separa...

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...

The complex formation of organogels based on poly(N-vinylcaprolactam) (PVCa) and organic dyes [5-(4-dimethylaminobenzylidene) rhodanine, α-nitroso-β-naftol, and dithizone] and the properties of the complexes in salt aqueous solutions were studied. PVCa gels absorbed these dyes in organic media (ethanol and chloroform). Our main focus was the behavior of the gel/dye complex immersed in a salt aqueous solution if the dye was the chelating ligand for metal ions. The behavior of the PVCa/dithizone (Dz)/chloroform system was studied in AgNO3 aqueous solutions. The absorption spectra of gel/dye and gel/dye/metal ion systems were investigated. The results obtained indicate that Ag+ ions penetrate the gel phase and that the Dz/silver complexes are indeed formed inside the gel. The dependence of the optical density of the gel/dye/metal ion systems on the AgNO3 concentration was observed. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 3238–3243, 2001

The behavior of slightly crosslinked poly(N-vinylcaprolactam) (PVCa) gels in a CuSO4 solution in water and ethanol was studied. The investigation was carried out for PVCa gels with polymer contents of 4% and 7%. The effective absorption of CuSO4 by PVCa hydrogel accompanied by gel shrinking was observed for the CuSO4 aqueous solution. It was shown that dendrite crystals of CuSO45H2O localized within the gel sample are obtained after placement of gels samples into ethanol. The crystal habit depends on gel concentration. It was found that slightly crosslinked PVCa gels are promising as media for the growth of crystals. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2838–2842, 2001

The present work is devoted to the study of the complex formation of polymer gel with organic dye and their properties in the aqueous salt solutions. Two systems were studied: 1) polyelectrolyte gel based on poly(diallyldimethylammonium chloride) and water soluble oppositely charged organic dyes (alizarin red S and catechol violet) and 2) organogel based on poly(N-vinylcaprolactam) and dithizone. The collapse of the polyelectrolyte gels in the presence of oppositely charged dyes together with the effective absorption of dyes was observed. The shrinking degree and the dye absorption by the gel depends on the dye concentration. In the case of PVCa gel in organic media the dye absorption takes place. The main attention has been concentrated on the study of the behaviour of gel/dye complex immersed in the salt solution if dye is the chelating ligand for metal ions. It was shown that polyelectrolyte gels generally form stable complexes with oppositely charged dyes. The behaviour of PVCa-dithizone-chloroform system was studied in AgNO 3 aqueous solution. The release of dithizone to the external aqueous solution of AgNO 3 reservoir is completely suppressed. Absorption spectra of gel/dye and gel/dye/metal ion systems were studied. It was shown that metal ions penetrate inside the gel phase and the dye/metal ion complexes form within the gel. The dependence of the optical density for the systems of gel/dye/metal ion on the salt concentration is observed.

General principles of information processing by chemical lightsensitive reaction—diffusion media are discussed. New modes of image evolution in the process of its transformation by reaction—diffusion medium are proposed. New approach for the design of materials that are capable to exhibit oscillating swelling-deswelling behaviour based on complexes of catalyst of Belousov—Zhabotinsky (BZ) reaction with polymer gels is described.

Investigation of the dynamic behavior of irradiated kappa carrageenan (in KCl) as a function of irradiation dose and temperature was done by dynamic light scattering (DLS). The intensity correlation function (ICF) shifted towards shorter relaxation times with increasing radiation dose as a result of radiolysis. The characteristic decay time distribution function, G(gamma), indicates the presence of fast and slow mode peaks respectively at around 0.1-10 ms and 100-1000 ms. A peak broadening of the fast mode peak in G(gamma) appeared with decreasing temperature, indicating that coil-to-helical conformational transition took place. The conformation transition temperature (CTT) decreased with increasing radiation dose. No transition was observed for kappa-carrageenan irradiated at 200 kGy. A new faster relaxation mode appeared at around 0.1-1 ms at temperatures below the CTT. This peak is found in kappa-carrageenan irradiated at doses exclusively between 75 and 175 kGy. The peak height of this mode is largest at 100 kGy which corresponds to the optimum biologic activity of kappa-carrageenan reported previously.

A strategy for the preparation of smart polymer gels with gradient weak polyacid distribution that exhibit pH-induced bending/straightening behaviour is reported. Gradient distribution of acid units inside the gel sample is obtained by copolymerization of N-isopropylacrylamide with acrylic acid sodium salt between two surfaces with different degrees of hydrophobicity/hydrophilicity. The reversibility of pH-induced bending is demonstrated.