Co-nonsolvency of PNiPAM at the transition between solvation mechanisms

Cosolvent incorporation modulates the thermal and structural response of PNIPAM/silyl methacrylate copolymers

Polymers functionalized with inorganic silane groups have been used in wide-ranging applications due to the silane reactivity, which enables formation of covalently-crosslinked polymeric structures. Utilizing stimuli-responsive polymers in these hybrid systems can lead to smart and tunable behavior for sensing, drug delivery, and optical coatings. Previously, the thermoresponsive polymer poly(N-isopropyl acrylamide) (PNIPAM) functionalized with 3-(trimethoxysilyl)propyl methacrylate (TMA) demonstrated unique aqueous self-assembly and optical responses following temperature elevation. Here, we investigate how cosolvent addition, particularly ethanol and N,N-dimethyl formamide (DMF), impacts these transition temperatures, optical clouding, and structure formation in NIPAM/TMA copolymers. Versus purely aqueous systems, these solvent mixtures can introduce additional phase transitions and can alter the two-phase region boundaries based on temperature and solvent composition. Interestingly, TMA incorporation strongly alters phase boundaries in the water-rich regime for DMF-containing systems but not for ethanol-containing systems. Cosolvent species and content also alter the aggregation and assembly of NIPAM/TMA copolymers, but these effects depend on polymer architecture. For example, localizing the TMA towards one chain end in 'blocky' domains leads to formation of uniform micelles with narrow dispersities above the cloud point for certain solvent compositions. In contrast, polydisperse aggregates form in random copolymer and PNIPAM homopolymer solutions - the size of which depends on solvent composition. The resulting optical responses and thermoreversibility also depend strongly on cosolvent content and copolymer architecture. Cosolvent incorporation thus increases the versatility of inorganic-functionalized responsive polymers for diverse applications by providing a simple way to tune the structure size and optical response.

Smart Polymers for Soft Materials: From Solution Processing to Organic Solids

Polymeric materials are ubiquitous in our everyday life, where they find a broad range of uses-spanning across common household items to advanced materials for modern technologies. In the context of the latter, so called "smart polymers" have received a lot of attention. These systems are soluble in water below their lower critical solution temperature Tℓ and often exhibit counterintuitive solvation behavior in mixed solvents. A polymer is known as smart-responsive when a slight change in external stimuli can significantly change its structure, functionm and stability. The interplay of different interactions, especially hydrogen bonds, can also be used for the design of lightweight high-performance organic solids with tunable properties. Here, a general scheme for establishing a structure-property relationship is a challenge using the conventional simulation techniques and also in standard experiments. From the theoretical side, a broad range of all-atom, multiscale, generic, and analytical techniques have been developed linking monomer level interaction details with macroscopic material properties. In this review, we briefly summarize the recent developments in the field of smart polymers, together with complementary experiments. For this purpose, we will specifically discuss the following: (1) the solution processing of responsive polymers and (2) their use in organic solids, with a goal to provide a microscopic understanding that may be used as a guiding tool for future experiments and/or simulations regarding designing advanced functional materials.

Morphogenesis of starfish polymersomes

The enhanced membrane stability and chemical versatility of polymeric vesicles have made them promising tools in micro/nanoreactors, drug delivery, cell mimicking, etc. However, shape control over polymersomes remains a challenge and has restricted their full potential. Here we show that local curvature formation on the polymeric membrane can be controlled by applying poly(N-isopropylacrylamide) as a responsive hydrophobic unit, while adding salt ions to modulate the properties of poly(N-isopropylacrylamide) and its interaction with the polymeric membrane. Polymersomes with multiple arms are fabricated, and the number of arms could be tuned by salt concentration. Furthermore, the salt ions are shown to have a thermodynamic effect on the insertion of poly(N-isopropylacrylamide) into the polymeric membrane. This controlled shape transformation can provide evidence for studying the role of salt ions in curvature formation on polymeric membranes and biomembranes. Moreover, potential stimuli-responsive non-spherical polymersomes can be good candidates for various applications, especially in nanomedicine.

Equilibrium Swelling of Thermo-Responsive Gels in Mixtures of Solvents

Thermo-responsive (TR) gels of the LCST (lower critical solution temperature) type swell in water at temperatures below their volume phase transition temperature Tc and collapse above the critical temperature. When water is partially replaced with an organic liquid, these materials demonstrate three different types of equilibrium solvent uptake diagrams at temperatures below, above, in the close vicinity of Tc. A model is developed for equilibrium swelling of TR gels in binary mixtures of solvents. It takes into account three types of phase transitions in TR gels driven by (i) aggregation of hydrophobic side groups into clusters from which solvent molecules are expelled, (ii) replacement of water with cosolvent molecules in cage-like structures surrounding these groups, and (iii) replacement of water with cosolvent as the main element of hydration shells around backbone chains. The model involves a relatively small number of material constants that are found by matching observations on covalently cross-linked poly(N-isopropylacrylamide) macroscopic gels and microgels. Good agreement is demonstrated between the experimental data and results of numerical analysis. Classification is provided of the phase transition points on equilibrium swelling diagrams.

Poly(sulfobetaine)-Based Diblock Copolymer Thin Films in Water/Acetone Atmosphere: Modulation of Water Hydration and Co-nonsolvency-Triggered Film Contraction

The water swelling and subsequent solvent exchange including co-nonsolvency behavior of thin films of a doubly thermo-responsive diblock copolymer (DBC) are studied via spectral reflectance, time-of-flight neutron reflectometry, and Fourier transform infrared spectroscopy. The DBC consists of a thermo-responsive zwitterionic (poly(4-((3-methacrylamidopropyl) dimethylammonio) butane-1-sulfonate)) (PSBP) block, featuring an upper critical solution temperature transition in aqueous media but being insoluble in acetone, and a nonionic poly(N-isopropylmethacrylamide) (PNIPMAM) block, featuring a lower critical solution temperature transition in water, while being soluble in acetone. Homogeneous DBC films of 50-100 nm thickness are first swollen in saturated water vapor (H₂O or D₂O), before they are subjected to a contraction process by exposure to mixed saturated water/acetone vapor (H₂O or D₂O/acetone-d6 = 9:1 v/v). The affinity of the DBC film toward H₂O is stronger than for D₂O, as inferred from the higher film thickness in the swollen state and the higher absorbed water content, thus revealing a pronounced isotope sensitivity. During the co-solvent-induced switching by mixed water/acetone vapor, a two-step film contraction is observed, which is attributed to the delayed expulsion of water molecules and uptake of acetone molecules. The swelling kinetics are compared for both mixed vapors (H₂O/acetone-d6 and D₂O/acetone-d6) and with those of the related homopolymer films. Moreover, the concomitant variations of the local environment around the hydrophilic groups located in the PSBP and PNIPMAM blocks are followed. The first contraction step turns out to be dominated by the behavior of the PSBP block, whereas the second one is dominated by the PNIPMAM block. The unusual swelling and contraction behavior of the latter block is attributed to its co-nonsolvency behavior. Furthermore, we observe cooperative hydration effects in the DBC films, that is, both polymer blocks influence each other's solvation behavior.

Cononsolvency of thermoresponsive polymers: where we are now and where we are going

Cononsolvency is an intriguing phenomenon where a polymer collapses in a mixture of good solvents. This cosolvent-induced modulation of the polymer solubility has been observed in solutions of several polymers and biomacromolecules, and finds application in areas such as hydrogel actuators, drug delivery, compound detection and catalysis. In the past decade, there has been a renewed interest in understanding the molecular mechanisms which drive cononsolvency with a predominant emphasis on its connection to the preferential adsorption of the cosolvent. Significant efforts have also been made to understand cononsolvency in complex systems such as micelles, block copolymers and thin films. In this review, we will discuss some of the recent developments from the experimental, simulation and theoretical fronts, and provide an outlook on the problems and challenges which are yet to be addressed.

Assessing the perfluoroalkyl acid-induced swelling of Förster resonance energy transfer-capable poly(N-isopropylacrylamide) microgels

As a method to combat the extensive contamination of poly- and perfluoroalkyl substances (PFAS) in water supplies, poly(N-isopropylacrylamide) (PNIPAM) microgels copolymerized with 2,2,2-trifluoroethylacrylate (TFEA) represent a potential sensing tool for recognizing PFAS at dilute aqueous concentrations. The microgels exhibit exceptional temperature responsiveness, transitioning from a swollen z-average diameter of 890.8 ± 19.8 nm to a collapsed diameter of 246.4 ± 10.3 nm below and above their lower critical solution temperature, respectively, for non-fluorinated gels, offering broad size fluctuations that are susceptible to coadded contaminants. Monitoring size perturbations as a function of analyte concentration, the polymers were observed to deswell in the presence of perfluorooctanoic acid, octanoic acid, phenol, and sodium 1-octane sulfonate while tetraethylammonium perfluorooctane sulfonate (TPFOS) augmented swelling. Adding up to 40 mol% TFEA to the networks lowered the concentration at which the microgels' normalized z-average diameter demonstrated a significant deviation from 0.25 mM to 0.1 mM for TPFOS, indicating fluorophilicity as a key contributor to the copolymers' associative capacity. Implanting Förster resonance energy transfer-compatible dyes, cyanine 3 and cyanine 5, into non-fluorinated microgels largely reiterated results from light scattering, as expected for the size-dependent energy transfer mechanism. Including dyes did, however, reinforce the customizability of this system, leaving windows open for functionalization with other signal transduction motifs to lower the detection limits of the polymer further. The swelling changes for PNIPAM microgels stimulated by the acidic constituents of PFAS highlight the polymer as a candidate for detecting the substances following additional development.

Thin Thermoresponsive Polymer Films for Cell Culture: Elucidating an Unexpected Thermal Phase Behavior by Atomic Force Microscopy

Application of poly-N-isopropylacrylamide (PNIPAM) and its more hydrophobic copolymers with N-tert-butylacrylamide (NtBA) as supports for cell sheets has been validated in numerous studies. The binary systems of these polymers with water are characterized by a lower critical solution temperature (LCST) in a physiologically favorable region. Upon lowering the temperature below the LCST, PNIPAM chains undergo a globule-to-coil transition, causing the film dissolution and cell sheet detachment. The character of the PNIPAM-water miscibility behavior is rather complex and not completely understood. Here, we applied atomic force microscopy to track the phase transition in thin films of linear thermoresponsive (co)polymers (PNIPAM and PNIPAM-co-NtBA) prepared by spin-coating. We studied the films' Young's modulus, roughness, and thickness in air and in distilled water in a full thermal cycle. In dry films, in the absence of water, all the measured parameters remained invariant. The swollen films in water above the LCST were softer by 2-3 orders of magnitude and about 10 times rougher than the corresponding dry films. Upon lowering the temperature to the LCST, the films passed through the phase transition observed as a drastic drop of Young's modulus (about an order of magnitude) and decrease in roughness in both polymers in a narrow temperature range. However, the films did not lose their integrity and demonstrated almost fully reversible changes in the mechanical properties and roughness. The thermal dependence of the films' thickness confirmed that they dissolved only partially and required an external force to induce the complete destruction. The reversible thermal behavior which is generally not expected from non-cross-linked polymers is a key finding, especially with respect to their practical application in cell culture. Both the thermodynamic and kinetic factors, as well as the confinement effect, may be responsible for this peculiar film robustness, which requires overcooling and the aid of an external force to destroy the film.

FRET-Integrated Polymer Brushes for Spatially Resolved Sensing of Changes in Polymer Conformation

Polymer brush surfaces that alter their physical properties in response to chemical stimuli have the capacity to be used as new surface-based sensing materials. For such surfaces, detecting the polymer conformation is key to their sensing capabilities. Herein, we report on FRET-integrated ultrathin (<70 nm) polymer brush surfaces that exhibit stimuli-dependent FRET with changing brush conformation. Poly(N-isopropylacrylamide) polymers were chosen due their exceptional sensitivity to liquid mixture compositions and their ability to be assembled into well-defined polymer brushes. The brush transitions were used to optically sense changes in liquid mixture compositions with high spatial resolution (tens of micrometers), where the FRET coupling allowed for noninvasive observation of brush transitions around complex interfaces with real-time sensing of the liquid environment. Our methods have the potential to be leveraged towards greater surface-based sensing capabilities at intricate interfaces.

Leveraging the thermoresponsiveness of fluorinated poly(N-isopropylacrylamide) copolymers as a sensing tool for perfluorooctane sulfonate

Due to mounting evidence of the negative health effects of persistent perfluoroalkyl acids (PFAAs) with long (i.e., >C7) tails, there is a need for convenient systems capable of sensing these contaminants at dilute aqueous concentrations. To address this concern, a thermoresponsive polymeric network composed of poly(N-isopropylacrylamide) copolymerized with fluorinated comonomers was studied to characterize the gel's physical response to fluorosurfactants in solution. Incorporating fluorinated comonomers into the polymer backbone raised their swelling in fluorocontaminant solutions relative to water - gels synthesized with 10.0 mol% 2,2,2-trifluoroethyl acrylate (TFEA) displayed a heightened maximum water-analyte swelling difference of 3761 ± 147% compared to 3201 ± 466% for non-fluorinated gels in the presence of 1 mM tetraethylammonium perfluorooctane sulfonate (TPFOS). The normalized area under the curve for gels with 12.5 mol% TFEA was further raised to 1.77 ± 0.09, indicating a broadened response window for the contaminant, but at the cost of reducing the overall swelling ratio to 3227 ± 166% and elongating the time required to reach swelling equilibrium. Overall, a copolymer fed with 10.7 mol% TFEA was predicted to maximize both the swelling and response window of the polymer toward TPFOS. Equilibration times followed a logarithmic increase as the percentage of comonomer was raised, noting gradual fluorosurfactant penetration into the gels impeded by initial gel compaction caused by the addition of fluorinated comonomers. Comparative study of gels containing 1H,1H,7H-dodecafluoroheptyl acrylate, TFEA, or 1,1,1,3,3,3-hexafluoroisopropyl acrylate identified careful selection of fluorinated comonomers and their feed ratios as useful tools for tailoring the network's swelling response to TPFOS.

Temperature Controlled Loading and Release of the Anti-Inflammatory Drug Cannabidiol by Smart Microgels

CBD is a promising candidate for treatment of many diseases and plays a major role in the growing trend to produce high-end drugs from natural, renewable resources. In the present work, we demonstrate a way to incorporate the anti-inflammatory drug CBD into smart microgel particles. The copolymer microgels that we chose as carrier systems exhibit a volume phase transition temperature of 39 ∘C, which is just above normal body temperature and makes them ideal candidates for hyperthermia treatment. While a simple loading route of CBD was not successful due to the enormous hydrophobicity of CBD, an alternative route was developed by immersing the microgels in ethanol. Despite the expected loss of thermoresponsive behaviour of the microgel matrix due to the solvent exchange, a temperature-dependent release of CBD was detected by the material, creating an interesting question of interactions between CBD and the microgel particles in ethanol. Furthermore, the method developed for loading of the microgel particles with CBD in ethanol was further improved by a subsequent transfer of the loaded particles into water, which proves to be an even more promising approach due to the successful temperature-dependent release of the drug above the collapse temperature of the microgels.

An interplay of excluded-volume and polymer-(co)solvent attractive interactions regulates polymer collapse in mixed solvents

Cosolvent effects on the coil-globule transitions in aqueous polymer solutions are not well understood, especially in the case of amphiphilic cosolvents that preferentially adsorb on the polymer and lead to both polymer swelling and collapse. Although a predominant focus in the literature has been placed on the role of polymer-cosolvent attractive interactions, our recent work has shown that excluded-volume interactions (repulsive interactions) can drive both preferential adsorption of the cosolvent and polymer collapse via a surfactant-like mechanism. Here, we further study the role of polymer-(co)solvent attractive interactions in two kinds of polymer solutions, namely, good solvent (water)-good cosolvent (alcohol) (GSGC) and poor solvent-good cosolvent (PSGC) solutions, both of which exhibit preferential adsorption of the cosolvent and a non-monotonic change in the polymer radius of gyration with the addition of the cosolvent. Interestingly, at low concentrations, the polymer-(co)solvent energetic interactions oppose polymer collapse in the GSGC solutions and contrarily support polymer collapse in the PSGC solutions, indicating the importance of the underlying polymer chemistry. Even though the alcohol molecules are preferentially adsorbed on the polymer, the trends of the energetic interactions at low cosolvent concentrations are dominated by the polymer-water energetic interactions in both the cases. Therefore, polymer-(co)solvent energetic interactions can either reinforce or compensate the surfactant-like mechanism, and it is this interplay that drives coil-to-globule transitions in polymer solutions. These results have implications for rationalizing the cononsolvency transitions in real systems such as polyacrylamides in aqueous alcohol solutions where the understanding of microscopic driving forces is still debatable.

A cosolvent surfactant mechanism affects polymer collapse in miscible good solvents

The coil-globule transition of aqueous polymers is of profound significance in understanding the structure and function of responsive soft matter. In particular, the remarkable effect of amphiphilic cosolvents (e.g., alcohols) that leads to both swelling and collapse of stimuli-responsive polymers has been hotly debated in the literature, often with contradictory mechanisms proposed. Using molecular dynamics simulations, we herein demonstrate that alcohols reduce the free energy cost of creating a repulsive polymer-solvent interface via a surfactant-like mechanism which surprisingly drives polymer collapse at low alcohol concentrations. This hitherto neglected role of interfacial solvation thermodynamics is common to all coil-globule transitions, and rationalizes the experimentally observed effects of higher alcohols and polymer molecular weight on the coil-to-globule transition of thermoresponsive polymers. Polymer-(co)solvent attractive interactions reinforce or compensate this mechanism and it is this interplay which drives polymer swelling or collapse.

Influence of TMAO as co-solvent on the gelation of silica-PNIPAm core-shell nanogels at intermediate volume fractions

We study the structure and dynamics of poly(N-isopropylacrylamide) (PNIPAm) core-shell nanogels dispersed in aqueous trimethylamine N-oxide (TMAO) solutions by means of small-angle X-ray scattering and X-ray photon correlation spectroscopy (XPCS). Upon increasing the temperature above the lower critical solution temperature of PNIPAm at 33 °C, a colloidal gel is formed as identified by an increase of I(q) at small q as well as a slowing down of sample dynamics by various orders of magnitude. With increasing TMAO concentration the gelation transition shifts linearly to lower temperatures. Above a TMAO concentration of approximately 0.40 mol/L corresponding to a 1 : 1 ratio of TMAO and NIPAm groups, collapsed PNIPAm states are found for all temperatures without any gelation transition. This suggests that reduction of PNIPAm-water hydrogen bonds due to the presence of TMAO results in a stabilisation of the collapsed PNIPAm state and suppresses gelation of the nanogel.

Thermoresponsive Polymer Gating System on Mesoporous Shells of Silica Particles Serving as Smart Nanocontainers

Spherical silica nanoparticles with solid cores and mesoporous shells (SCMS) were decorated with thermoresponsive polymer brushes that were shown to serve as macromolecular valves to control loading and unloading of a model dye within the mesopores. Thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) brushes were grafted from the surfaces of both solid core (SC) and SCMS particles of similar size using surface-initiated atom transfer radical polymerization. Both systems based on porous (SCMS-PNIPAM) and nonporous (SC-PNIPAM) particles were characterized using cryo-TEM, thermogravimetry and elemental analysis to determine the structure and composition of the decorated nanoparticles. The grafted PNIPAM brushes were found to be responsive to temperature changes enabling temperature-controlled gating of the pores. The processes of loading and unloading in the obtained systems were examined using a model fluorescent dye-rhodamine 6G. Polymer brushes in SCMS-PNIPAM systems were shown to serve as molecular valves enabling significant adsorption (loading) of the dye inside the pores with respect to the SC-PNIPAM (no pores) and SCMS (no valves) systems. The effective unloading of the fluorescent cargo molecules from the decorated nanoparticles was achieved in a water/methanol solution. The obtained SCMS-PNIPAM particles may be used as smart nanocontainers or nanoreactors offering also facile isolation from the suspension due to the presence of dense cores.

Why Do Elastin-Like Polypeptides Possibly Have Different Solvation Behaviors in Water-Ethanol and Water-Urea Mixtures?

The solvent quality determines the collapsed or the expanded state of a polymer. For example, a polymer dissolved in a poor solvent collapses, whereas in a good solvent it opens up. While this standard understanding is generally valid, there are examples when a polymer collapses even in a mixture of two good solvents. This phenomenon, commonly known as co-non-solvency, is usually associated with a wide range of synthetic (smart) polymers. Moreover, recent experiments have shown that some biopolymers, such as elastin-like polypeptides (ELPs) that exhibit lower critical solution behavior T l in pure water, show co-non-solvency behavior in aqueous ethanol mixtures. In this study, we investigate the phase behavior of elastin-like polypeptides (ELPs) in aqueous binary mixtures using molecular dynamics simulations of all-atom and complementary explicit solvent generic models. The model is parameterized by mapping the solvation free energy obtained from the all-atom simulations onto the generic interaction parameters. For this purpose, we derive segment-based (monomer level) generic parameters for four different peptides, namely proline (P), valine (V), glycine (G), and alanine (A), where the first three constitute the basic building blocks of ELPs. Here, we compare the conformational behavior of two ELP sequences, namely -(VPGGG)- and -(VPGVG)-, in aqueous ethanol and -urea mixtures. Consistent with recent experiments, we find that ELPs show co-non-solvency in aqueous ethanol mixtures. Ethanol molecules have preferential binding with all ELP residues, with an interaction contrast of 6-8 k B T, and thus driving the coil-to-globule transition. On the contrary, ELP conformations show a weak variation in aqueous urea mixtures. Our simulations suggest that the glycine residues dictate the overall behavior of ELPs in aqueous urea, where urea molecules have a rather weak preferential binding with glycine as observed from the all atom simulations, i.e., less than k B T. This weak interaction dilutes the overall effect of other neighboring residues and thus ELPs exhibit a different conformational behavior in aqueous urea in comparison to aqueous ethanol mixtures. While the validation of the latter findings will require a more detailed experimental investigation, the results presented here may provide a new twist to the present understanding of cosolvent interactions with peptides and proteins.

Coil-Globule Transition Thermodynamics of Poly(N-isopropylacrylamide)

Thermosensitive polymers such as poly(N-isopropylacrylamide) (PNIPAM) undergo a phase transition in aqueous solution from a random-coil structural ensemble to a globule structural ensemble at the lower critical solution temperature (LCST). Above this temperature, PNIPAM agglomerates and becomes insoluble, whereas it is soluble below the temperature. Thus, thermosensitive polymers represent essential targets for several applications, e.g., in drug delivery. Although their ability to change structure in response to a temperature alteration is highly relevant for industrial processes, their thermodynamic properties are mostly qualitatively understood, and the quantitative thermodynamic picture is still elusive. In this study, we used a combined atomistic molecular dynamics and well-tempered metadynamics simulation approach to estimate coil-globule transition thermodynamics. An isotactic 30-mer of PNIPAM was investigated over a broad temperature range between 200 and 360 K. The transition from the globule to the random-coil structure was observed with well-tempered metadynamics. For the first time, the free energy surface of PNIPAM was estimated and it is shown that the simulation results are in line with the experimentally observed thermosensitive behavior. Below the LCST, the random-coil ensemble represents the global energy minimum and is thermodynamically favored by 21 ± 9 kJ/mol compared to the globule ensemble; both are separated by a barrier of 49 ± 14 kJ/mol. In contrast, above the LCST, the globule ensemble is thermodynamically favored by 21 ± 8 kJ/mol over the random-coil ensemble. The barrier from random-coil to globule is 17 ± 10 kJ/mol.

Poly(2-propyl-2-oxazoline)s in Aqueous Methanol: To Dissolve or not to Dissolve

At room temperature, poly(N-isopropylacrylamide) (PNIPAM) is soluble in water and methanol, but it is not soluble in certain water/methanol mixtures. This phenomenon, known as cononsolvency, has been explored in great detail experimentally and theoretically in an attempt to understand the complex interactions occurring in the ternary PNIPAM/water/co-nonsolvent system. Yet little is known about the effects of the polymer structure on cononsolvency. To address this point, we investigated the temperature-dependent solution properties in water, methanol, and mixtures of the two solvents of poly(2-cyclopropyl-2-oxazoline) (PcyPOx) and two structural isomers of PNIPAM (M_n ∼ 11 kg/mol): poly(2-isopropyl-2-oxazoline) (PiPOx) and poly(2-n-propyl-2-oxazoline) (PnPOx). The phase diagram of the ternary water/methanol/poly(2-propyl-2-oxazolines) (PPOx) systems, constructed based on cloud point (T_CP) measurements, revealed that PnPOx exhibits cononsolvency in water/methanol mixtures. In contrast, methanol acts as a cosolvent for PiPOx and PcyPOx in water. The enthalpy, ΔH, and temperature, T_max, of the coil-to-globule transition of the three polymers in various water/methanol mixtures were measured by high-sensitivity differential scanning calorimetry. T_max follows the same trends as T_CP, confirming the cononsolvency of PnPOx and the cosolvency of PiPOx and PcyPOx. ΔH decreases linearly as a function of the methanol content for all PPOx systems. Ancillary high-resolution ¹H NMR spectroscopy studies of PPOx solutions in D₂O and methanol-d₄, coupled with DOSY and NOESY experiments revealed that the n-propyl group of PnPOx rotates freely in D₂O, whereas the rotation of the isopropyl and cyclopropyl groups of PiPOx and PcyPOx, respectively, is limited due to steric restriction. This factor appears to play an important role in the case of the PPOxs/water/methanol ternary system.

Atypical catalytic function of embedded gold nanoparticles by controlling structural features of polymer particle in alcohol-rich solvents

This work demonstrates an in situ approach to incorporate multiple gold nanoparticles (NPs) within a functional-group-free poly(N-isopropylacrylamide) particle and examine their catalytic activity in carbon-carbon forming reactions in pure alcohol and alcohol-rich aqueous solvents under ambient aerobic conditions. The alcohol-rich solvent environments eliminated the cononsolvency effect of the polymer particle template to maintain a fully swollen structure while providing great stability to the embedded gold NPs. In addition, the dispersion of the composites in alcohol solvents efficiently reduced the surface adsorbed stabilizing agent around the embedded gold NPs. Given their high stability and readily accessible surfaces with a minimal physical barrier, these macromolecule-derived composite particles as quasi-homogeneous catalysts exhibited unexpectedly high activity in homocoupling reactions to form C-C bonds. The increased mass transfer capability for reactants and products in pure alcohol and alcohol-rich solvents was also responsible for the highly improved yields in the coupling reactions. Furthermore, the composite particles exhibited great selectivity to solely form targeted compounds without any side products and showed the robustness to be recycled multiple times without losing their catalytic activity in pure alcohol solvent environments. By simply controlling the structural feature of the polymer particle matrix with alcohol solvents, the embedded gold NPs exhibited atypical catalytic activity and selectivity as well as recyclability in C-C bond forming reactions.

Nanomechanics and Nanorheology of Microgels at Interfaces

The review addresses nanomechanics and nanorheology of stimuli responsive microgels adsorbed at an interface. In order to measure the mechanical properties on a local scale, an atomic force microscope is used. The tip presents an indenter with a radius of curvature of a few 10 s of nm. Static indentation experiments and dynamic studies with an excited cantilever are presented. The effect of several internal and external parameters on the mechanical properties is reviewed. The focus is on the correlation between the swelling abilities of the gels and their mechanical properties. Several results are surprising and show that the relationship is not as simple as one might expect.

Phase Behavior of Co-Nonsolvent Systems: Poly( N-isopropylacrylamide) in Mixed Solvents of Water and Methanol

Cloud points of poly( N-isopropylacrylamide) in aqueous mixed solvents, with methanol as the cosolvent, are experimentally measured for polymer concentrations varied up to as high as the weight fraction 0.25. They are shown to form closed loops on the ternary phase plane in the temperature region between 5 and 30 °C, and hence co-nonsolvency is complete. Miscibility loops shrink by cooling, or equivalently, they exhibit lower critical solution temperature behavior. For a fixed polymer concentration, there is a composition of the mixed solvent at which the cloud-point temperature takes the lowest value. This minimum cloud-point temperature composition of the mixed solvent turned out to be almost independent of the polymer concentration, at least within the measured dilute region below the weight fraction 0.25. On the basis of the assumption that the phase separation is closely related to the preferential adsorption of the solvents by hydrogen bonding, we employ a model solution of Flory-Huggins type, augmented with direct and cooperative polymer-solvent hydrogen bonds, to construct the ternary phase diagrams. Theoretical calculation of the spinodal curves is performed, and the results are compared with the obtained experimental cloud-point data. The effect of molecular volume of the cosolvent is also studied within the same theoretical framework. Possibility for a upper critical solution temperature co-nonsolvency to appear for cosolvents with larger molecular volume is discussed.

Polyacrylamide "revisited": UCST-type reversible thermoresponsive properties in aqueous alcoholic solutions

Combining experiments and all-atom molecular dynamics simulations, we study the conformational behavior of polyacrylamide (PAM) in aqueous alcohol mixtures over a wide range of temperatures. This study shows that even when the microscopic interaction is dictated by hydrogen bonding, unlike its counterparts that present a lower critical solution temperature (LCST), PAM shows a counterintuitive tunable upper critical solution temperature (UCST)-type phase transition in water/alcohol mixtures that was not reported before. The phase transition temperature was found to be tunable between 4 and 60 °C by the type and concentration of alcohol in the mixture as well as by the solution concentration and molecular weight of the polymer. In addition, molecular dynamics simulations confirmed a UCST-like behaviour of the PAM in aqueous alcoholic solutions. Additionally, it was observed that the PAM is more swollen in pure alcohol solutions than in 80% alcoholic solutions due to θ-like behaviour. Additionally, in the globular state, the size of the aggregates was found to increase with increasing solvent hydrophobicity and polymer concentration of the solutions. Above its phase transition temperature, PAM might be present as individual polymer chains in the coil state (≤10 nm). As PAM is a widespread polymer in many biomedical applications (gel electrophoresis, etc.), this finding could be of high relevance for many more practical applications in high performance pharmaceuticals and/or sensors.

Flory-type theories of polymer chains under different external stimuli

In this Review, we present a critical analysis of various applications of the Flory-type theories to a theoretical description of the conformational behavior of single polymer chains in dilute polymer solutions under a few external stimuli. Different theoretical models of flexible polymer chains in the supercritical fluid are discussed and analysed. Different points of view on the conformational behavior of the polymer chain near the liquid-gas transition critical point of the solvent are presented. A theoretical description of the co-solvent-induced coil-globule transitions within the implicit-solvent-explicit-co-solvent models is discussed. Several explicit-solvent-explicit-co-solvent theoretical models of the coil-to-globule-to-coil transition of the polymer chain in a mixture of good solvents (co-nonsolvency) are analysed and compared with each other. Finally, a new theoretical model of the conformational behavior of the dielectric polymer chain under the external constant electric field in the dilute polymer solution with an explicit account for the many-body dipole correlations is discussed. The polymer chain collapse induced by many-body dipole correlations of monomers in the context of statistical thermodynamics of dielectric polymers is analysed.

Combined Cononsolvency and Temperature Effects on Adsorbed PNIPAM Microgels

The present study addresses the multiresponsive behavior of poly(N-isopropylacrylamide) (PNIPAM) microgels adsorbed to interfaces. The microgels react to changes in temperature by shrinking in aqueous solution above their volume phase transition temperature (VPTT). Additionally, they shrink in mixtures of water and ethanol, although both individual liquids are good solvents for PNIPAM. The combination of this so-called cononsolvency effect and the temperature response of adsorbed microgels is studied by atomic force microscopy (AFM). Adsorbed microgels are of special interest because they are compressed considerably compared to those in bulk solution. It is shown that the impact of adsorption on swelling depends on the specific surface details, as well as the sample preparation. Thereby, the microgels are deposited on two different kinds of surfaces: on gold surface and on polycation (PAH) coating which show different interactions with the microgels in terms of electrostatic interaction and wettability. In addition, the microgels were deposited from different solvent mixtures. This influences the microgel structure and thereby the swelling properties. Nanorheology studies by dynamic AFM measurements lead to surprising results which are explained by the fact that not only polymer density but a subtle interaction between polymer and solvent might dominate the rheological properties. This work supports the view that preferential adsorption of ethanol at PNIPAM drives cononsolvency, while the shrinking at T > VPTT is caused by general breaking of hydrogen bonds between solvents and PNIPAM.

Statistical description of co-nonsolvency suppression at high pressures

We present an application of Flory-type theory of a flexible polymer chain dissolved in a binary mixture of solvents to theoretical description of co-nonsolvency. We show that our theoretical predictions are in good quantitative agreement with the recently published MD simulation results for the conformational behavior of a Lennard-Jones flexible chain in a binary mixture of the Lennard-Jones fluids. We show that our theory is able to describe co-nonsolvency suppression through pressure enhancement to extremely high values recently discovered in experiments and reproduced by full atomistic MD simulations. By analysing the co-solvent concentration in the internal polymer volume at different pressure values, we speculate that this phenomenon is caused by the suppression of the co-solvent preferential solvation of the polymer backbone at the rather high pressure imposed. We show that when the co-solvent-induced coil-globule transition takes place, the entropy and enthalpy contributions to the solvation free energy abruptly decrease, while the solvation free energy remains continuous.

Poly(N-isopropylacrylamide) Microgels under Alcoholic Intoxication: When a LCST Polymer Shows Swelling with Increasing Temperature

Poly(N-isopropylacrylamide) (PNIPAM) microgel is a smart polymer that shows a volume phase transition temperature (VPTT) at around 32 °C in aqueous solutions, above which it collapses. In this work, combining experiments and molecular simulations, it is shown that PNIPAM microgels do not always exhibit a collapsed structure above the VPTT. Instead, PNIPAM in aqueous alcohol mixtures shows a two-step conformational transition, i.e., a collapse at low temperatures (T < 32 °C) and a reswelling when T > 50 °C. The present analysis indicates that delicate microscopic interaction details, together with the bulk solution properties, play a key role in dictating the reswelling behavior. Even when PNIPAM microgels swell with increasing T, this is not a standard upper critical solution behavior.

Tuning the Size of Thermoresponsive Poly(N-Isopropyl Acrylamide) Grafted Silica Microgels

Core-shell microgels were synthesized via a free radical emulsion polymerization of thermoresponsive poly-(N-isopropyl acrylamide), pNipam, on the surface of silica nanoparticles. Pure pNipam microgels have a lower critical solution temperature (LCST) of about 32 °C. The LCST varies slightly with the crosslinker density used to stabilize the gel network. Including a silica core enhances the mechanical robustness. Here we show that by varying the concentration gradient of the crosslinker, the thermoresponsive behaviour of the core-shell microgels can be tuned. Three different temperature scenarios have been detected. First, the usual behaviour with a decrease in microgel size with increasing temperature exhibiting an LCST; second, an increase in microgel size with increasing temperature that resembles an upper critical solution temperature (UCST), and; third, a decrease with a subsequent increase of size reminiscent of the presence of both an LCST, and a UCST. However, since the chemical structure has not been changed, the LCST should only change slightly. Therefore we demonstrate how to tune the particle size independently of the LCST.