Porous Anisometric PNIPAM Microgels: Tailored Porous Structure and Thermal Response.

The porous structure of microgels significantly influences their properties and, thus, their suitability for various applications, in particular as building blocks for tissue scaffolds. Porosity is one of the crucial features for microgel-cell interactions and significantly increases the cells' accumulation and proliferation. Consequently, tailoring the porosity of microgels in an effortless way is important but still challenging, especially for nonspherical microgels. This work presents a straightforward procedure to fabricate complex-shaped poly(N-isopropyl acrylamide) (PNIPAM) microgels with tuned porous structures using the so-called cononsolvency effect during microgel polymerization. Therefore, the classical solvent in the reaction solution is exchanged from water to water-methanol mixtures in a stop-flow lithography process. For cylindrical microgels with a higher methanol content during fabrication, a greater degree of collapsing is observed, and their aspect ratio increases. Furthermore, the collapsing and swelling velocities change with the methanol content, indicating a modified porous structure, which is confirmed by electron microscopy micrographs. Furthermore, swelling patterns of the microgel variants occur during cooling, revealing their thermal response as a highly heterogeneous process. These results show a novel procedure to fabricate PNIPAM microgels of any elongated 2D shape with tailored porous structure and thermoresponsiveness by introducing the cononsolvency effect during stop-flow lithography polymerization.

Synthesis of Polymer Single-Chain Nanoparticle with High Compactness in Cosolvent Condition: A Computer Simulation Study.

Polymeric single-chain nanoparticles (SCNPs) are soft nano-objects synthesized by intramolecular crosslinking of isolated single polymer chains. Syntheses of such SCNPs usually need to be performed in a dilute solution. In such a condition, the bonding probability of the two active crosslinking units at a short contour distance along the chain backbone is much higher than those which are far away from each other. Such a reaction condition often results in local spheroidization and, therefore, the formation of loosely packed structures. How to inhibit the local spheroidization and improve the compactness of SCNPs is thus a major challenge for the syntheses of SCNPs. In this study, computer simulations are performed and the fact that a precollapse of the polymer chain conformation in a cosolvent condition can largely improve the probability of the crosslinking reactions at large contour distances is demonstrated, favoring the formations of closely packed globular structures. As a result, the formed SCNPs can be more spherical and have higher compactness than those fabricated in ultradilute good solvent solution in a conventional way. It is believed this simulation work can provide a insight into the effective syntheses of SCNPs with spherical conformations and high compactness.

HCl Gas Sensor Coating Based on Poly(N-isopropylacrylamide) Nanoparticles Prepared from Water-Methanol Binary Solvent.

Poly(N-isopropylacrylamide) (PNIPAM) nanoparticles formed in water-methanol binary solvent were successfully deposited on a resonator surface at room temperature by exploiting the cononsolvency effect on the phase transition of PNIPAM aqueous solutions. Scanning electron microscopic observation revealed that the nanoparticles were secondary and made up of agglomerated primary spherical particles of about 10-nm diameter, buried in the film. The magnitude of the sensor response toward HCl gas was larger than that of the nanoparticle sensor prepared from pure water solvent, and the sensitivity to 1 ppm of HCl of sensor-coated nanoparticles based on the present method was 3.3 Hz/ppm. The recovery of the sensors was less than 90% at first cycle measurement, but had improved to almost 100% at the third cycle.

Quantifying the Interactions in the Aggregation of Thermoresponsive Polymers: The Effect of Cononsolvency.

The aggregation kinetics of thermoresponsive core-shell micelles with a poly(N-isopropyl acrylamide) shell in pure water or in mixtures of water with the cosolvents methanol or ethanol at mole fractions of 5% is investigated during a temperature jump across the respective cloud point. Characteristically, these mixtures give rise to cononsolvency behavior. At the cloud point, aggregates are formed, and their growth is followed with time-resolved small-angle neutron scattering. Using the reversible association model, the interaction potential between the aggregates is determined from their growth rate in dependence on the cosolvents. The effect of the cosolvent is attributed to the interaction potential on the structured layer of hydration water around the aggregates. It is surmised that the latter is perturbed by the cosolvent and thus the residual repulsive hydration force between the aggregates is reduced. The larger the molar volume of the cosolvent, the more pronounced is the effect. This framework provides a molecular-level understanding of solvent-mediated effective interactions in polymer solutions and new opportunities for the rational control of self-assembly in complex soft matter systems.

FLIM nanoscopy resolves the structure and preferential adsorption in the co-nonsolvency of PNIPAM microgels in methanol-water.

Polymer microgels are swollen macromolecular networks with a typical size of hundred of nanometers to several microns that show an extraordinary open and responsive architecture to different external stimuli, being therefore important candidates for nanobiotechnology and nanomedical applications such as biocatalysis, sensing and drug delivery. It is therefore crucial to understand the delicate balance of physical-chemical interactions between the polymer backbone and solvent molecules that to a high extent determine their responsivity. In particular, the co-nonsolvency effect of poly(N-isopropylacrylamide) in aqueous alcohols is highly discussed, and there is a disagreement between molecular dynamics (MD) simulations (from literature) of the preferential adsorption of alcohol on the polymer chains and the values obtained by several empirical methods that mostly probe the bulk solvent properties. It is our contention that the most efficacious method for addressing this problem requires a nanoscopic method that can be combined with spectroscopy and record fluorescence spectra and super-resolved fluorescence lifetime images of microgels labeled covalently with the solvatochromic dye Nile Red. By employing this approach, we could simultaneously resolve the structure of sub-micron size objects in the swollen and in the collapsed state and estimate the solvent composition inside of them in - mixtures for two very different polymer architectures. We found an outstanding agreement between the MD simulations and our results that estimate a co-solvent molar fraction excess of approximately 3 with a very flat profile in the lateral direction of the microgel.

Unexpected cononsolvency behavior of poly (N-isopropylacrylamide)-based microgels.

Poly (N-isopropylacrylamide) (pNIPAm)-based microgels undergo a transition from fully water swollen (solvated) to deswollen (desolvated) as the temperature of the water they are dissolved in is increased >32 °C. In this submission, we examine how the temperature of this transition, i.e., the volume phase transition temperature (VPTT), depends on the concentration of methanol (MeOH) in water the microgels are dissolved in. To accomplish this, pNIPAm-based etalons are utilized, and it is shown that the VPTT for the microgels is much less than that previously observed for linear pNIPAm and pNIPAm-based microgels. Furthermore, and most interestingly, it is determined that the microgels can collapse in solutions containing high MeOH (>∼65% MeOH) concentration. This is in contrast to previous studies, which show that no VPTT is observed for pNIPAm in aqueous solutions containing >∼65% MeOH.

Tough Hydrogel Electrolytes for Anti-Freezing Zinc-Ion Batteries.

As the soaring demand for energy storage continues to grow, batteries that can cope with extreme conditions are highly desired. Yet, existing battery materials are limited by weak mechanical properties and freeze-vulnerability, prohibiting safe energy storage in devices that are exposed to low temperature and unusual mechanical impacts. Herein, a fabrication method harnessing the synergistic effect of co-nonsolvency and "salting-out" that can produce poly(vinyl alcohol) hydrogel electrolytes with unique open-cell porous structures, composed of strongly aggregated polymer chains, and containing disrupted hydrogen bonds among free water molecules, is introduced. The hydrogel electrolyte simultaneously combines high strength (tensile strength 15.6 MPa), freeze-tolerance (< -77 °C), high mass transport (10× lower overpotential), and dendrite and parasitic reactions suppression for stable performance (30 000 cycles). The high generality of this method is further demonstrated with poly(N-isopropylacrylamide) and poly(N-tertbutylacrylamide-co-acrylamide) hydrogels. This work takes a further step toward flexible battery development for harsh environments.

Mechanofluorescent Polymer Brush Surfaces that Spatially Resolve Surface Solvation.

Polymer brushes, consisting of densely end-tethered polymers to a surface, can exhibit rapid and sharp conformational transitions due to specific stimuli, which offer intriguing possibilities for surface-based sensing of the stimuli. The key toward unlocking these possibilities is the development of methods to readily transduce signals from polymer conformational changes. Herein, we report on single-fluorophore integrated ultrathin (<40 nm) polymer brush surfaces that exhibit changing fluorescence properties based on polymer conformation. The basis of our methods is the change in occupied volume as the polymer brush undergoes a collapse transition, which enhances the effective concentration and aggregation of the integrated fluorophores, leading to a self-quenching of the fluorophores' fluorescence and thereby reduced fluorescence lifetimes. By using fluorescence lifetime imaging microscopy, we reveal spatial details on polymer brush conformational transitions across complex interfaces, including at the air-water-solid interface and at the interface of immiscible liquids that solvate the surface. Furthermore, our method identifies the swelling of polymer brushes from outside of a direct droplet (i.e., the polymer phase with vapor above), which is controlled by humidity. These solvation-sensitive surfaces offer a strong potential for surface-based sensing of stimuli-induced phase transitions of polymer brushes with spatially resolved output in high resolution.

Fibrillar gel self-assembly via cononsolvency of amphiphilic polymer.

HYPOTHESIS: Polymer with amphiphilic repeating units bathed in the mixed binary solvent can exhibit fibrillar formation and gelation via cononsolvency effect. Mechanism of gelation will be highly dependent on the solvent's interactions and morphology of the resulting fibrillar gel can be fine-tuned by changing binary solvent composition. EXPERIMENTS: Amphiphilic homopolymers dissolved in a mixture of two solvents with different affinities to monomeric units and to each other were modeled using dissipative particle dynamics. Morphological transitions in dilute and concentrated solutions were investigated depending on the solvent-cosolvent interaction and binary solvent composition. Fibrillar gel structure was characterized via calculation of fibril's and pore's diameters. FINDINGS: Amphiphilic macromolecules can combine into fibrils and form a gel as a result of the cononsolvency caused by microsegregation with one or both components of a binary solvent. In the former, the fibrils are loose, in the latter - dense. For dilute solution, state diagram in terms of solvent/cosolvent miscibility and binary solvent composition is constructed and two regions with different fibrillar gels are distinguished. For concentrated solution, the conditions, governing fibrillar diameter, pore size, number and functionality of branching points, are highlighted. These findings would be useful for the controlling synthetic matrix morphologies in the tissue engineering.

On Going to a New Era of Microgel Exhibiting Volume Phase Transition.

The discovery of phenomena of volume phase transition has had a great impact not only on bulk gels but also on the world of microgels. In particular, research on poly(N-isopropylacrylamide) (PNIPAM) microgels, whose transition temperature is close to body temperature, has made remarkable progress in almost 35 years. This review presents some breakthrough findings in microgels that exhibit volume phase transitions and outlines recent works on the synthesis, structural analysis, and research direction of microgels.

Cononsolvency Transition of Polymer Brushes: A Combined Experimental and Theoretical Study.

In this study, the cononsolvency transition of poly(N-isopropylacrylamide) (PNiPAAm) brushes in aqueous ethanol mixtures was studied by using Vis-spectroscopic ellipsometry (SE) discussed in conjunction with the adsorption-attraction model. We proved that the cononsolvency transition of PNiPAAm brushes showed features of a volume phase transition, such as a sharp collapse, reaching a maximum decrease in thickness for a very narrow ethanol volume composition range of 15% to 17%. These observations are in agreement with the recently published preferential adsorption model of the cononsolvency effect.

Hydrostatic pressure effect on PNIPAM cononsolvency in water-methanol solutions.

When methanol is added to water at room temperature and 1atm, poly (N-isopropylacrylamide), PNIPAM, undergoes a coil-to-globule collapse transition. This intriguing phenomenon is called cononsolvency. Spectroscopic measurements have shown that application of high hydrostatic pressure destroys PNIPAM cononsolvency in water-methanol solutions. We have developed a theoretical approach that identifies the decrease in solvent-excluded volume effect as the driving force of PNIPAM collapse on increasing the temperature. The same approach indicates that cononsolvency, at room temperature and P=1atm, is caused by the inability of PNIPAM to make all the attractive energetic interactions that it could be engaged in, due to competition between water and methanol molecules. The present analysis suggests that high hydrostatic pressure destroys cononsolvency because the coil state becomes more compact, and the quantity measuring PNIPAM-solvent attractions increases in magnitude due to the solution density increase, and the ability of small water molecules to substitute methanol molecules on PNIPAM surface.