Swelling Of Microgels Research Articles

Chemomechanical Self-Oscillatory Microgel Motility in Stratified Chemical Media.

The design of chemomechanical self-oscillators, which execute oscillations in the presence of constant stimuli lacking periodicity, is a step toward the development of autonomous and interactive soft robotic systems. This work presents a simple design of prolonged chemomechanical oscillatory movement in a microgel system capable of buoyant motility within stratified chemical media containing spatially localized sinking and floating stimuli. Three design elements are developed: a stimuli-responsive membranized calcium alginate microgel, a Percoll density gradient for providing stratified antagonistic chemical media, and transduction of microgel particle size actuation into buoyant motility via membrane-mediated displacement of the Percoll media. The presence of citrate or calcium ions in different layers of the Percoll media gives rise to swelling (buoyancy) or contraction (geotaxis), respectively, which in turn mediate the shuttling of the microgels between the layers to produce prolonged or damped chemomechanical oscillatory trajectories. The concentration-dependence of the oscillatory behavior in the stratified media, the density gap between the Percoll layers, and the kinetic asymmetry of microgel swelling and deswelling are studied. The illustrated modular design allows for the development of chemomechanical self-oscillators responsive to light, pH, or temperature, which will find applications in interactive soft robotics, autonomous microbots, and intelligent materials.

Soft and Deformable Thermoresponsive Hollow Rod-Shaped Microgels.

Depending on their aspect ratio, rod-shaped particles exhibit a much richer 2D and 3D phase behavior than their spherical counterparts, with additional nematic and smectic phases accompanied by defined orientational ordering. While the phase diagram of colloidal hard rods is extensively explored, little is known about the influence of softness in such systems, partly due to the absence of appropriate model systems. Additionally, investigating higher volume fractions for long rods is usually complicated because non-equilibrium dynamical arrest is likely to precede the formation of more defined states. This has motivated us to develop micrometric rod-like microgels with limited sedimentation that can respond to temperature and reversibly reorganize into defined phases via annealing and seeding procedures. A detailed procedure is presented for synthesizing rod-shaped hollow poly(N-isopropylacrylamide) microgels using micrometric silica rods as sacrificial templates. Their morphological characterization is conducted through a combination of microscopy and light scattering techniques, evidencing the unconstrained swelling of rod-shaped hollow microgels compared to core-shell microgel rods. Different aspects of their assembly in dispersion and at interfaces are further tested to illustrate the opportunities and challenges offered by such systems that combine softness, anisotropy, andthermoresponsivity.

Modeling microgel swelling: Influence of chain finite extensibility.

Microgels exhibit the ability to undergo reversible swelling in response to shifts in environmental factors that include variations in temperature, concentration, and pH. While several models have been put forward to elucidate specific aspects of microgel swelling and its impact on bulk behavior, a consistent theoretical description that chains throughout the microscopic degrees of freedom with suspension properties and deepens into the full implications of swelling remains a challenge yet to be met. In this work, we extend the mean-field swelling model of microgels from Denton and Tang [J. Chem. Phys. 145, 164901 (2016)] to include the finite extensibility of the polymer chains. The elastic contribution to swelling in the original work is formulated for Gaussian chains. By using the Langevin chain model, we modify this elastic contribution in order to account for finite extensibility effects, which become prominent for microgels containing highly charged polyelectrolytes and short polymer chains. We assess the performance of both elastic models, namely for Gaussian and Langevin chains, comparing against coarse-grained bead-spring simulations of ionic microgels with explicit electrostatic interactions. We examine the applicability scope of the models under a variation of parameters, such as ionization degree, microgel concentration, and salt concentration. The models are also tested against experimental results. This work broadens the applicability of the microgel swelling model toward a more realistic description, which brings advantages when describing the suspensions of nanogels and weak-polyelectrolyte micro-/nanogels.

Nonionic Microgels Adapt to Ionic Guest Molecules: Superchaotropic Nanoions.

Microgels are commonly applied as solute carriers, where the size, density, and functionality of the microgels depend on solute binding. As representatives for ionic solutes with high affinity for the microgel, we study here the effect of superchaotropic Keggin polyoxometalates (POMs) PW12O403- (PW) and SiW12O404- (SiW) on the aqueous swelling and internal structure of nonionic poly(N-isopropylacrylamide) (pNiPAM) microgels by light scattering techniques and small-angle X-ray scattering. Due to their weak hydration, these POMs bind spontaneously to the microgels at millimolar concentrations. The microgels thus become charged and swell at low POM concentration, surprisingly without strongly increasing the volume phase transition temperature, and deswell at higher POM concentration. The swelling arises because of the osmotic pressure of dissociated counterions of the POMs, while the deswelling is due to POMs acting as physical cross-links in the microgels under screened electrostatics in NaCl or excess POM solution. This swelling/deswelling transition is sharper for PW than for SiW related to the lower charge density, weaker hydration, and stronger binding of PW. The POMs elicit qualitatively and quantitatively different swelling effects from ionic surfactants and classical salts. Moreover, the network softness and topology govern the swelling response upon POM binding. The softer the microgel, the stronger is the swelling response, while, inside the microgel, regions of high polymer density swell/contract more upon electric charging/cross-linking than regions with low polymer density. POM binding thus enables fine-tuning of microgel properties and highlights the role of network topology in microgel swelling. Because POMs decompose at an alkaline pH, these POM/microgel systems also exhibit pH-responsive swelling in addition to the typical temperature responsiveness of pNiPAM microgels.

Rheology of amino-functionalized graphene oxide suspensions in hydrogels

This work investigates the effects of amino-functionalized graphene oxide (AFGO) suspensions on the rheological behavior of Carbopol® hydrogels at pHs 5, 7, and 9. The AFGO concentration and media pH were evaluated and related to the suspension's microstructure and rheology. Graphene oxide (GO) nanosheets were synthesized using the modified Hummers method and functionalized with triethylenetetramine via microwave-assisted reaction to produce AFGO. The nanosheets were characterized by different techniques, such as scanning electron microscopy (SEM), thermogravimetric analysis, Raman spectroscopy, and x-ray photoelectron spectroscopy. The suspensions were characterized by rheological tests through steady-state and dynamic flow, zeta potential, and cryo-SEM for microstructure analysis. All samples presented a viscoplastic behavior and were modeled by the Herschel–Bulkley equation. Concerning the base hydrogels, the sample prepared at pH 9 showed lower viscosity, yield stress, and elastic modulus. At all pHs, the increase in the nanosheet concentration promotes a drop in the yield stress, viscosity, storage, and loss moduli. The cryomicrographs showed the impact of pH on the base hydrogel structure. It was also possible to observe that increasing nanoadditive concentration affects the Carbopol microgel swelling and weakens the suspension microstructure.

X-ray triggered color-tunable microgel-based interferometers for radiation dose sensing

We report the successful development of novel microgel-based interferometers for X-ray dose sensing, which can meet fast-developing and widespread applications of ionizing radiation. Poly(N-isopropylacrylamide) (pNIPAm)-based microgels are crosslinked by different molecules (i.e., N, N’-methylenebisacrylamide (BIS), N, N’-bis(acryloyl)cystamine (BAC), bis(2-acrylamidoethyl) diselenide (BASe), and bis(2-acrylamidoethyl) ditelluride) (BATe). These color-tunable photonic materials are generated by sandwiching X-ray-responsive microgels between two Au layers. The radiolysis of an aqueous solution by X-ray irradiation yields •OH radicals, leading to the cleavage of crosslinkers of pNIPAm-based microgels. The chemical reaction induces the swelling of microgels, increasing the distance between the two Au layers of the interferometers. Eventually, the optical signal of the device (i.e., reflectance spectra and color) varies with the X-ray dose, showing an ultrahigh sensitivity of 101.63 nm/Gy. For the first time, our work provides novel microgel-based interferometers for spectral and visual X-ray dose detection via the chemical–mechanical-optical signal transduction strategy.

Preparation and Properties of Bimetallic Chitosan Spherical Microgels.

The aim of this work was to prepare bimetallic chitosan microgels with high sphericity and investigate the influences of metal-ion type and content on the size, morphology, swelling, degradation and biological properties of microgels. Amino and hydroxyl groups of chitosan (deacetylation degree, DD, of 83.2% and 96.9%) served as ligands in the Cu2+-Zn2+/chitosan complexes with various contents of cupric and zinc ions. The electrohydrodynamic atomization process was used to produce highly spherical microgels with a narrow size distribution and with surface morphology changing from wrinkled to smooth by increasing Cu2+ ions' quantity in bimetallic systems for both used chitosans. The size of the bimetallic chitosan particles was estimated to be between 60 and 110 µm for both used chitosans, and FTIR spectroscopy indicated the formation of complexes through physical interactions between the chitosans' functional groups and metal ions. The swelling capacity of bimetallic chitosan particles decreases as the DD and copper (II) ion content increase as a result of stronger complexation with respect to zinc (II) ions. Bimetallic chitosan microgels showed good stability during four weeks of enzymatic degradation, and bimetallic systems with smaller amounts of Cu2+ ions showed good cytocompatibility for both used chitosans.

Structure of swollen hollow polyelectrolyte nanogels with inhomogeneous cross-link distribution

Hypothesis: Recently, it has become possible to synthesize hollow polyelectrolyte nano- and microgels. The shell permeability can be controlled by external stimuli, while the cavity can serve as a storage place for guest molecules. However, there is a lack of a detailed understanding at the molecular level regarding the role of the network topology, inhomogeneities of the distribution of cross-links, and the impact of the electrostatics on the structural response of hollow microgel to external stimuli. To bridge these gaps, molecular dynamics (MD) of computer simulations are used.Experiments: Here, we propose a fresh methodology to create realistic hollow microgel particles in silico. The technique involves a gradual change in the average local length of subchains depending on the distance to the center of mass of the microgel particles resulting in the microgels with a non-uniform distribution of cross-linking species. In particular, a series of microgels with (i) a highly cross-linked inner part of the shell and gradually decreased cross-linker concentration towards the periphery, (ii) microgels with loosely cross-linked inner and outer parts, as well as (iii) microgels with a more-or-less homogeneous structure, have been created and validated. Counterions and salt ions are taken into account explicitly, and electrostatic interactions are described by the Coulomb potential.Findings: Our studies reveal a strong dependence of the microgel swelling response on the network topology. Simple redistribution of cross-links plays a significant role in the structure of the microgels, including cavity size, microgel size, fuzziness, and extension of the inner and outer parts of the microgels. Our results indicate the possibilities of qualitative justification of the structure of the hollow microgels in the experiments by measuring the relative change in the size of the sacrificial core to the size of the cavity and by estimation of a power law function, RH∼csα, of the hydrodynamic radius of the hollow microgels as a function of added salt concentration.

Swelling and Collapse of Cylindrical Polyelectrolyte Microgels.

In this study, we propose computer simulations of charged cylindrical microgels. The effects of cross-linking density, aspect ratio, and fraction of charged groups on the microgel swelling and collapse with a variation in the solvent quality were studied. The results were compared with those obtained for equivalent neutral cylindrical microgels. The study demonstrated that microgels' degree of swelling strongly depends on the fraction of charged groups. Polyelectrolyte microgels under adequate solvent conditions are characterized by a larger length and thickness than their neutral analogues: the higher the fraction of charged groups, the longer their length and greater their thickness. Microgels' collapse upon solvent quality decline is characterized by a decrease in length and non-monotonous behavior of its thickness. First, the thickness decreases due to the attraction of monomer units (beads) upon collapse. The further thickness increase is related to the surface tension, which tends to reduce the anisotropy of collapsed objects (the minimum surface energy is known to be achieved for the spherical objects). This reduction is opposed by the network elasticity. The microgels with a low cross-linking density and/or a low enough aspect ratio reveal a cylinder-to-sphere collapse. Otherwise, the cylindrical shape is preserved in the course of the collapse. Aspect ratio as a function of the solvent quality (interaction parameter) demonstrates the maximum, which is solely due to the electrostatics. Finally, we plotted radial concentration profiles for network segments, their charged groups, and counterions.

Photoswitchable Microgels for Dynamic Macrophage Modulation.

Dynamic manipulation of supramolecular self-assembled structures is achieved irreversibly or under non-physiological conditions, thereby limiting their biomedical, environmental, and catalysis applicability. In this study, microgels composed of azobenzene derivatives stacked via π-cation and π-π interactions are developed that are electrostatically stabilized with Arg-Gly-Asp (RGD)-bearing anionic polymers. Lateral swelling of RGD-bearing microgels occurs via cis-azobenzene formation mediated by near-infrared-light-upconverted ultraviolet light, which disrupts intermolecular interactions on the visible-light-absorbing upconversion-nanoparticle-coated materials. Real-time imaging and molecular dynamics simulations demonstrate the deswelling of RGD-bearing microgels via visible-light-mediated trans-azobenzene formation. Near-infrared light can induce in situ swelling of RGD-bearing microgels to increase RGD availability and trigger release of loaded interleukin-4, which facilitates the adhesion structure assembly linked with pro-regenerative polarization of host macrophages. In contrast, visible light can induce deswelling of RGD-bearing microgels to decrease RGD availability that suppresses macrophage adhesion that yields pro-inflammatory polarization. These microgels exhibit high stability and non-toxicity. Versatile use of ligands and protein delivery can offer cytocompatible and photoswitchable manipulability of diverse host cells.

Swelling-Mediated Mechanical Stimulation Regulates Differentiation of Adipose-Derived Mesenchymal Stem Cells for Intervertebral Disc Repair Using Injectable UCST Microgels.

Mechanical stimulation is an effective approach for controlling stem cell differentiation in tissue engineering. However, its realization in in vivo tissue repair remains challenging since this type of stimulation can hardly be applied to injectable seeding systems. Here, it is presented that swelling of injectable microgels can be transformed to in situ mechanical stimulation via stretching the cells adhered on their surface. Poly(acrylamide-co-acrylic acid) microgels with the upper critical solution temperature property are fabricated using inverse emulsion polymerization and further coated with polydopamine to increase cell adhesion. Adipose-derived mesenchymal stem cells (ADSCs) adhered on the microgels can be omnidirectionally stretched along with the responsive swelling of the microgels, which upregulate TRPV4and Piezo1channel proteins and enhance nucleus pulposus (NP)-like differentiation of ADSCs. In vivo experiments reveal that the disc height and extracellular matrix content of NP are promoted after the implantation with the microgels. The findings indicate that swelling-induced mechanical stimulation has great potential for regulating stem cell differentiation during intervertebral disc repair.

Reversible and pH-modulated changes in microgel size triggered by electrochemical stimuli

A novel microgel, the volume of which can be reversibly changed by using an electrochemical trigger, was successfully synthesized. To this end, a monomer based on N-(3-aminopropyl)methacrylamide modified with ferrocenecarboxylic acid was obtained and co-polymerized with acrylic acid and N,N′–methylenebisacrylamide. Using the distillation precipitation polymerization method, spherical, smooth-surfaced microgel particles ca. 500 nm in diameter were obtained. The presence of ferrocene groups in the microgel was shown with nuclear magnetic resonance spectroscopy, energy dispersive X-ray spectroscopy, and cyclic voltammetry methods. Dynamic light scattering experiments showed that the microgel size strongly depends on the oxidation state of the ferrocene groups. Next, cyclic voltammetry and chronoamperometry combined with quartz crystal microbalance measurements, obtained with an Au electrochemical quartz crystal microbalance electrode modified with the microgel, proved that the microgel size could be controlled by an electrochemical trigger. More interestingly, the electro-response was found to strongly depend on pH. In acidic pH, where almost all carboxylic groups are protonated, the oxidation of ferrocene groups led to a decrease in the registered frequency shift, related to the microgel swelling process. In alkaline pH, where many carboxylic groups are ionized, the oxidation of ferrocene led to an increase in frequency, related to the microgel shrinking process. These processes were found to be reversible and relatively fast.

Swelling, collapse and ordering of rod-like microgels in solution: Computer simulation studies

Polymer microgels have proven to be highly promising macromolecular objects for a wide variety of applications. In particular, the soft particles of an anisotropic (rod-like) shape are of special interest because of their potential use in tissue engineering or materials design. However, a little is known about the physical behavior of such microgels in solution, which inspired us to study them using mesoscopic computer simulations. For single networks, depending on the solvent quality, the dimensional characteristics were obtained for microgels of different molecular weight, crosslinking density and aspect ratio. In particular, the conditions for the rod-to-rod (preserving the nonspherical shape) and rod-to-sphere collapse were found. In addition, the effect of the liquid-crystalline (LC) ordering was demonstrated for the ensemble of rod-like microgels at different swelling ratios, and the influence of microgel aspect ratio on the volume fraction of the LC transition was shown.

Bottom, Top, or in Between: Combining Plasmonic Nanohole Arrays and Hydrogel Microgels for Optical Fiber Sensor Applications

AbstractAttractive label‐free plasmonic optical fiber sensors can be developed by cleverly choosing the arrangement of plasmonic nanostructures and other building blocks. Here, the final response depends very much on the alignment and position (stacking) of the individual elements. In this work, three different types of fiber optic sensing geometries fabricated by simple layer‐by‐layer stacking are presented, consisting of stimulus‐sensitive poly‐N‐isopropylacrylamide (polyNIPAM) microgel arrays and plasmonic nanohole arrays (NHAs), namely NHA/polyNIPAM, polyNIPAM/NHA, polyNIPAM/NHA/polyNIPAM. Their optical response to a representative stimulus, namely temperature, is investigated. NHA/polyNIPAM monitors the volume phase transition of polyNIPAM microgels through changes in the spectral position and the amplitude of the reflection minimum of plasmonic NHA. In contrast, polyNIPAM/NHA shows a more complex response to the swelling and collapse of polyNIPAM microgels in their reflectance spectra. The most pronounced changes in optical response are observed by monitoring the amplitude of the reflectance minimum of this sensor during heating/cooling cycles. Finally, the triple stack of polyNIPAM/NHA/polyNIPAM at the end of a optical fiber tip combines the advantages of the NHA/polyNIPAM, polyNIPAM/NHA double stacks for optical sensing. The unique layer‐by‐layer stacking of microgel and nanostructure is customizable and can be easily adopted for other applications.

Fabrication of pH-degradable supramacromolecular microgels with tunable size and shape via droplet-based microfluidics

This study presents a versatile method to synthesize stimuli-responsive microgels with supramolecular cross-links exhibiting tunable size and shape via droplet-based microfluidics. The natural polyphenol tannic acid (TA) is used to cross-link poly(N-vinylcaprolactam) (PVCL) chains in aqueous droplets by the formation of hydrogen bonds and hydrophobic interactions between the phenolic groups of TA and the carbonyl group and the hydrophobic segments of lactam ring of PVCL chains. The obtained microgels exhibit diameters in the range of 130–150µm in swollen state in aqueous solution. Synthesized microgels exhibit pH-responsive behavior: at low pH microgels deswell and shrink due to the protonation of phenolic groups and enhanced hydrophobic interactions; at high pH microgels swell and disintegrate due to the deprotonation of phenolic groups and destruction of hydrogen bonds with PVCL chains. Additionally, we present supramacromolecular microgels in cylindrical shape with different aspect ratios using a new design of microfluidic chip by varying flow rates at high concentration of the prepolymerized precursor combined with rapid pH-triggered on-chip gelation. Furthermore, developed synthesis methodology allows on-chip encapsulation of colloidal objects into large supramacromolecular microgels during the cross-linking step. The complete and fast release of objects by pH-triggered degradation indicates that the pH-responsive supramacromolecular microgels can be used for controlled loading/release of various payloads, like probiotics. Moreover, cell studies of L929 fibroblast clearly show the biocompatibility of the microgels.

Comparison of different approaches to describe the thermotropic volume phase transition of smart microgels

The description of gel swelling by Flory and Rehner using the original Flory–Huggins interaction parameter for the polymer–solvent interaction cannot be applied to most smart microgels. Here, we compare descriptions of the swelling curves of such microgels using series expansions of the Flory–Huggins parameter chi with the results of Hill-like equation for chi. We study N-isopropyl-acrylamide particles at different concentrations of the cross-linker N,N-methylenebisacrylamide. The hydrodynamic radius R_{mathrm {H}} of the microgel particles is determined using photon correlation spectroscopy. The fits with the series expansion of chi nicely follow the experimental data. However, already with the first-order series expansion, the computed Theta temperatures are not physically reasonable. Moreover, the physical meaning of the parameters of the series expansion is not clear. The Hill-like equation, which we recently introduced, yields a good description of all measured microgel swelling curves and provides physically meaningful parameters. For instance, the Hill parameter nu corresponds to the number of water molecules per network chain cooperatively leaving the chain at the volume phase transition.Graphical abstractDifferent approaches to model the Flory-Huggins interaction parameter are explored and compared with respect to the quality of the fit of microgel swelling curves.

Swelling of composite microgels with soft cores and thermo-responsive shells

This study deals with constitutive modeling and numerical simulation of equilibrium swelling of composite microgels with soft temperature-insensitive cores and thermo-responsive (TR) shells. Our analysis takes into account a pronounced increase in the elastic moduli of TR gels of the LCST (lower critical solution temperature) type driven by aggregation of hydrophobic segments into clusters above the volume phase transition temperature. Material parameters are found by fitting observations on poly(acrylamide) and poly(N-isopropylacrylamide) gels with various concentrations of cross-linkers. Good agreement is demonstrated between the experimental data and results of numerical analysis. The effects of mechanical properties of the core and the shell and geometry of composite microgels on distribution of water molecules are studied numerically. Several unexpected phenomena are discovered in simulation.

Equilibrium swelling of multi-stimuli-responsive copolymer gels

Copolymer gels prepared by polymerization of thermo-responsive and anionic monomers demonstrate strong sensitivity to several triggers such as temperature, pH and ionic strength of aqueous solutions. For biomedical applications of these materials (as on–off switches in controlled drug delivery and release), fine tuning of their volume phase transition temperature (VPTT) and a sharp decay in degree of swelling upon transition from the swollen to the collapsed state are needed. These requirements are fulfilled under swelling of copolymer gels and microgels in water under acidic conditions, but are violated when tests are conducted under alkaline conditions or in aqueous solutions of salts with physiological salinity.A model is developed for equilibrium swelling of multi-stimuli-responsive copolymer gels in aqueous solutions with arbitrary pH and molar fractions of a monovalent salt. Unlike conventional approaches, the model accounts for secondary interactions between chains (hydrogen bonding) to describe the kinetics of aggregation of hydrophobic segments above VPTT. Material constants are determined by fitting experimental swelling diagrams on poly(N-isopropylacrylamide–co–sodium acrylate) gels with various molar fractions of ionic monomers. The effects of temperature, pH and molar fraction of salt on the equilibrium degree of swelling below and above VPTT are studied numerically.

Carbomer microgels as model yield-stress fluids

Abstract The review presents current research results for Carbopol-based microgels as yield-stress materials, covering three aspects: chemical, physical and rheological. Such a joint three-aspect study has no analog in the literature. The chemical aspects of Carbopol polymers are presented in terms of a cross-linking polymerization of acrylic acid, their molecular structure, microgel formulation, polyacid dissociation and neutralization, osmotic pressure and associated immense microgel swelling. The physical characterization is focused on models of the shear-induced solid-to-liquid transition of microgels, which are formed of mesoscopic particles typical for soft matter materials. Models that describe interparticle effects are presented to explain the energy states of microgel particles at the mesoscale of scrutiny. Typical representatives of the models utilize attributes of jamming dispersions, micromechanical and polyelectrolyte reactions. Selected relationships that result from the models, such as scaling rules and nondimensional flow characteristics are also presented. The rheological part presents the discussion of problems of yield stress in 2D and 3D deformations, appearance and magnitude of the wall slip. The theory and characteristics of Carbopol microgel deformation in rotational rheometers are presented with graphs for the steady-state measurements, stress-controlled oscillation and two types of transient shear deformation. The review is concluded with suggestions for future research.

Accounting for Cooperativity in the Thermotropic Volume Phase Transition of Smart Microgels.

A full quantitative description of the swelling of smart microgels is still problematic in many cases. The original approach of Flory and Huggins for the monomer–solvent interaction parameter cannot be applied to some microgels. The reason for this obviously is that the cross-linking enhances the cooperativity of the volume phase transitions, since all meshes of the network are mechanically coupled. This was ignored in previous approaches, arguing with distinct transition temperatures for different meshes to describe the continuous character of the transition of microgels. Here, we adjust the swelling curves of a series of smart microgels using the Flory–Rehner description, where the polymer–solvent interaction parameter is modeled by a Hill-like equation for a cooperative thermotropic transition. This leads to a very good description of all measured microgel swelling curves and yields the physically meaningful Hill parameter . A linear decrease of is found with increasing concentration of the cross-linker N,N-methylenebisacrylamide in the microgel particles p(NIPAM), p(NNPAM), and p(NIPMAM). The linearity suggests that the Hill parameter corresponds to the number of water molecules per network chain that cooperatively leave the chain at the volume phase transition. Driven by entropy, water molecules of the solvate become cooperatively “free” and leave the polymer network.