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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.

Smart polymer gels of microscale size are attracting a lot of attention since their properties, such as fast response to various external stimuli, make them suitable candidates for many potential applications including drug delivery, nanoreactors, or separation techniques. In this research, thermosensitive microgels of poly(N-isopropylacrylamide), poly(N-vinylcaprolactam), poly(N-isopropylacrylamide-co-sodium vinylsulfonate) and poly(N-vinylcaprolactam-co-1-vinylimidazole) were prepared via aqueous free-radical precipitation polymerization. The thermo-induced collapse of the microgels and subchain mobility were thoroughly investigated by means of proton nuclear magnetic resonance (1H NMR) spectroscopy, as well as scanning electron microscopy and dynamic light scattering. It was found that in copolymer microgels a part of the thermosensitive monomer units does not collapse upon heating, and the critical temperature is not affected strongly by the addition of ionogenic groups. That could be explained by non-uniform monomer unit distribution, leading to the appearance of thermosensitive core-charged corona structure of the microgel particle. Сore-corona architecture of microgels results in film formation with the ordering of the self-assembly structure.

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.

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.

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.