Modeling and Elucidating the Behavior of a Thermoresponsive LCST Ionic Liquid

Desalination of seawater by forward osmosis is a technology potentially able to address the global water scarcity problem. The major challenge limiting its widespread practical application is the design of a draw solute that can be separated from water by an energetically efficient process and then reused for the next cycle. Recent experiments demonstrate that a promising draw solute for forward-osmosis desalination is tetrabutylphosphonium 2,4,6-trimethylbenzenesulfonate ([P4444][TMBS]). When mixed with water, this ionic liquid (IL) is thermoresponsive and exhibits a lower critical solution temperature (LCST), above which it phase-separates into an IL-rich phase and a water-rich phase. Elucidating the physical mechanism of the liquid-liquid phase separation, as well as rationally designing optimized derivatives, necessitates an accurate model to describe this and related ILs. In this paper, we resort to explicit-solvent all-atom molecular dynamics simulations and adopt AMBER-based force-field parameters for the cation whose partial charges were assigned by the RESP fitting procedure. Utilizing the same methodology, we parametrize the anion. The simulations' results indicate the IL/water mixture, at the experimental critical composition, can unambiguously phase-separate only when the partial charges of the ions are scaled down. Nevertheless, the best-performing charge scaling factor is found to be 0.95, a value much milder than those reported for ILs in neat phases. This can be explained by a diminished charge transfer, or induced dipoles, within the ions when the IL is in a mixture with water. With this charge scaling, the simulations reproduce well the LCST composition-temperature phase diagram, albeit overestimation of the critical temperature by 10 K. In particular, very good agreement is obtained for the composition of the two segregated phases. Estimation of viscosity points to IL/water mixture that is almost twice as viscous in simulations than that reported experimentally. Furthermore, we analyze changes in energy between different components in the mixture and find that the driving force for phase separation is, at least, enthalpic. Structural analyses of the ions and their interactions with water molecules corroborate the importance of the latter in mediating structural organizations of the anions, as well as in strengthening the interactions between the cations.

The characteristics of temperature-responsive ionic liquids on the integrated operational effectiveness of water reclamation from semiconductor wastewater using forward osmosis

Large amounts of wastewater are produced from semiconductor manufacturing, and the production energy consumption has skyrocketed with its global demand in recent years. Forward osmosis (FO) provides unique merits in reclaiming the wastewater if suitable draw solutes with high water flux, low leakage, and limited energy requirement in regeneration are available. Two lower critical solution temperature-ionic liquids (LCST-ILs), tetrabutylphosphonium trimethylbenzensulfonate ([P4444][TMBS]) and tetrabutylphosphonium maleate ([P4444][Mal]) were synthesized and systematically assessed as recycled draw solutes in FO for the water reclamation from the wastewater of Si-ingot sawing. The water flux of [P4444][TMBS] and [P4444][Mal] was 4.12 and 2.67 LMH in the FO for the authentic sawing wastewater, respectively. The characteristics of relatively higher hydrophobic and the structure configuration ensure the lower thermal-stimulus separation energy (Ets) of the spent [P4444][Mal] solution and its relative ease in recycling because of the higher regeneration ratio. The more hydrophilic [P4444][TMBS] based draw solution (DS) exhibits higher osmotic pressure, which is beneficial for water filtration but leads to higher Ets. The estimated energy required for the integrated processes, including FO filtration for the sawing wastewater, thermal separation of draw solute, and the reclaimed water polishing, for the system with [P4444][Mal] as draw solutes is 14.22 kWh m-3. The value significantly reduced to 1.33 kWh m-3 if low-grade waste heat (<100 °C) was applied for the thermal separation.

Dependencies between effective parameters in coarse-grained models for phase separation of DNA-based fluids

DNA is now firmly established as a versatile and robust platform for achieving synthetic nanostructures. While the folding of single molecules into complex structures is routinely achieved through engineering basepair sequences, very little is known about the emergence of structure on larger scales in DNA fluids. The fact that polymeric DNA fluids can undergo phase separation into dense fluid and dilute gas opens avenues to design hierachical and multifarious assemblies. Here, we investigate to which extent the phase behavior of single-stranded DNA fluids can be captured by a minimal model of semiflexible charged homopolymers while neglecting specific hybridization interactions. We first characterize the single-polymer behavior and then perform direct coexistence simulations to test the model against experimental data. While low-resolution models show great promise to bridge the gap to relevant length and time scales, obtaining consistent and transferable parameters is challenging. In particular, we conclude that counterions not only determine the effective range of direct electrostatic interactions but also contribute to the effective attractions.

Thermal Responsiveness of 1,2,4-Triazolium-Based Poly(ionic liquid)s and Their Applications in Dye Extraction and Smart Switch

Triazoliums are a family of five-membered heterocyclic cations that contain three nitrogen and two carbon atoms. In contrast to the widely studied imidazolium cations, triazoliums are less explored. In terms of the chemical structure, triazolium replaces a carbon atom in the imidazolium cation ring with an electron-withdrawing nitrogen atom, which makes the triazolium more polarized. Among the many physical properties, the thermal responsiveness of triazoliums is of particular interest to us but has been rarely investigated. In this contribution, we prepared a series of 1,2,4-triazolium-based poly(ionic liquid)s (PILs) with varying alkyl substituents and counteranions and studied their thermal-responsive behavior. We found that 1,2,4-triazolim-based PILs with a polymeric backbone structure similar to that of polyimidazoliums exhibited opposite thermal phase transition processes in solvents. For example, methyl-substituted 1,2,4-triazolium-based PILs exhibited an upper-critical-solution-temperature (UCST)-type phase transition in methanol when the counterion was I- and a lower-critical-solution-temperature (LCST)-type phase transition in acetone when the counterion was PF6 -. The thermal responsiveness was reversible and concentration-dependent. Interestingly, the thermal response of 1,2,4-triazolim-based PILs could be retained in the organogel form, which was applied in the pretreatment of anion-containing organic waste liquids and temperature-controlled "smart" switches.

Ionic Liquid-Accelerated Growth of Covalent Organic Frameworks with Tunable Layer-Stacking

Layer-stacking behaviors are crucial for two-dimensional covalent organic frameworks (2D COFs) to define their pore structure, physicochemical properties, and functional output. So far, fine control over the stacking mode without complex procedures remains a grand challenge. Herein, we proposed a "key-cylinder lock mimic" strategy to synthesize 2D COFs with a tunable layer-stacking mode by taking advantage of ionic liquids (ILs). The staggered (AB) stacking (unlocked) COFs were exclusively obtained by incorporating ILs of symmetric polarity and matching molecular size; otherwise, commonly reported eclipsed (AA) stacking (locked) COFs were observed instead. Mechanistic study revealed that AB stacking was induced by a confined interlocking effect (CIE) brought by anions and bulky cations of the ILs inside pores ("key" and "cylinder", respectively). Excitingly, this strategy can speed up production rate of crystalline powders (e.g., COF-TAPT-Tf@BmimTf2N in merely 30 minutes) under mild reaction conditions. This work highlights the enabling role of ILs to tailor the layer stacking of 2D COFs and promotes further exploration of their stacking mode-dependant applications.

Two Methods Based on Integral Equation Approaches in Analyzing Polyelectrolyte Solutions: Macrophase Separation

To understand the phase behaviors of polyelectrolyte solutions, we provide two analytical methods to formulate a molecular equation of state for a system of fully charged polyanions (PAs) and polycations (PCs) in a monomeric neutral component, based on integral equation theories. The mixture is treated in a primitive and restricted manner. The first method utilizes Blum's approach to charged hard spheres, incorporating the chain connectivity contribution by charged spheres via Stell's cavity function method. The second method employs Wertheim's multi-density Ornstein-Zernike treatment of charged hard spheres with Baxter's adhesive potential. The pressures derived from these methods are compared to available molecular dynamics simulations data for a solution of PAs and monomeric counterions as a limiting case. Two-phase equilibrium for the system is calculated using both methods to evaluate the relative strength of phase segregation that leads to complex coacervation. Additionally, the scaling exponents for a selected solution near its critical point are examined.

Polymerized and Colloidal Ionic Liquids─Syntheses and Applications

The breadth and importance of polymerized ionic liquids (PILs) are steadily expanding, and this review updates advances and trends in syntheses, properties, and applications over the past five to six years. We begin with an historical overview of the genesis and growth of the PIL field as a subset of materials science. The genesis of ionic liquids (ILs) over nano to meso length-scales exhibiting 0D, 1D, 2D, and 3D topologies defines colloidal ionic liquids, CILs, which compose a subclass of PILs and provide a synthetic bridge between IL monomers (ILMs) and micro to macro-scale PIL materials. The second focus of this review addresses design and syntheses of ILMs and their polymerization reactions to yield PILs and PIL-based materials. A burgeoning diversity of ILMs reflects increasing use of nonimidazolium nuclei and an expanding use of step-growth chemistries in synthesizing PIL materials. Radical chain polymerization remains a primary method of making PILs and reflects an increasing use of controlled polymerization methods. Step-growth chemistries used in creating some CILs utilize extensive cross-linking. This cross-linking is enabled by incorporating reactive functionalities in CILs and PILs, and some of these CILs and PILs may be viewed as exotic cross-linking agents. The third part of this update focuses upon some advances in key properties, including molecular weight, thermal properties, rheology, ion transport, self-healing, and stimuli-responsiveness. Glass transitions, critical solution temperatures, and liquidity are key thermal properties that tie to PIL rheology and viscoelasticity. These properties in turn modulate mechanical properties and ion transport, which are foundational in increasing applications of PILs. Cross-linking in gelation and ionogels and reversible step-growth chemistries are essential for self-healing PILs. Stimuli-responsiveness distinguishes PILs from many other classes of polymers, and it emphasizes the importance of segmentally controlling and tuning solvation in CILs and PILs. The fourth part of this review addresses development of applications, and the diverse scope of such applications supports the increasing importance of PILs in materials science. Adhesion applications are supported by ionogel properties, especially cross-linking and solvation tunable interactions with adjacent phases. Antimicrobial and antifouling applications are consequences of the cationic nature of PILs. Similarly, emulsion and dispersion applications rely on tunable solvation of functional groups and on how such groups interact with continuous phases and substrates. Catalysis is another significant application, and this is an historical tie between ILs and PILs. This component also provides a connection to diverse and porous carbon phases templated by PILs that are catalysts or serve as supports for catalysts. Devices, including sensors and actuators, also rely on solvation tuning and stimuli-responsiveness that include photo and electrochemical stimuli. We conclude our view of applications with 3D printing. The largest components of these applications are energy related and include developments for supercapacitors, batteries, fuel cells, and solar cells. We conclude with our vision of how PIL development will evolve over the next decade.

Expeditious Discovery of Small-Molecule Thermoresponsive Ionic Liquid Materials: A Review

Ionic liquids (ILs) are a class of low-melting molten salts (<100 °C) constituted entirely of ions, and their research has gained tremendous attention in line with their remarkably growing applications (>124,000 publications dated 30 August 2023 from the Web of ScienceTM). In this review, we first briefly discussed the recent developments and unique characteristics of ILs and zwitterionic liquids (ZILs). Compared to molecular solvents and other conventional organic compounds, (zwitter) ionic liquids carry negligible volatility and are potentially recyclable and reusable. For structures, both ILs and ZILs can be systematically tailor-designed and engineered and are synthetically fine-tunable. As such, ionic liquids, including chiral, supported, task-specific ILs, have been widely used as powerful ionic solvents as well as valuable additives and catalysts for many chemical reactions. Moreover, ILs have demonstrated their value for use as polymerase chain reaction (PCR) enhancers for DNA amplification, chemoselective artificial olfaction for targeted VOC analysis, and recognition-based affinity extraction. As the major focus of this review, we extensively discussed that small-molecule thermoresponsive ILs (TILs) and ZILs (zwitterionic TILs) are new types of smart materials and can be expeditiously discovered through the structure and phase separation (SPS) relationship study by the combinatorial approach. Using this SPS platform developed in our laboratory, we first depicted the rapid discovery of N,N-dialkylcycloammonium and 1,3,4-trialkyl-1,2,3-triazolium TILs that concomitantly exhibited LCST (lower critical solution temperature) phase transition in water and displayed biochemically attractive Tc values. Both smart IL materials were suited for applications to proteins and other biomolecules. Zwitterionic TILs are ZILs whose cations and anions are tethered together covalently and are thermoresponsive to temperature changes. These zwitterionic TIL materials can serve as excellent extraction solvents, through temperature change, for biomolecules such as proteins since they differ from the common TIL problems often associated with unwanted ion exchanges during extractions. These unique structural characteristics of zwitterionic TIL materials greatly reduce and may avoid the denaturation of proteins under physiological conditions. Lastly, we argued that both rational structural design and combinatorial library synthesis of small-molecule TIL materials should take into consideration the important issues of their cytotoxicity and biosafety to the ecosystem, potentially causing harm to the environment and directly endangering human health. Finally, we would concur that before precise prediction and quantitative simulation of TIL structures can be realized, combinatorial chemistry may be the most convenient and effective technology platform to discover TIL expeditiously. Through our rational TIL design and combinatorial library synthesis and screening, we have demonstrated its power to discover novel chemical structures of both TILs and zwitterionic TILs. Undoubtedly, we will continue developing new small-molecule TIL structures and studying their applications related to other thermoresponsive materials.

Adaptive Ion-Gel: Stimuli-Responsive, and Self-Healing Ion Gels

Ion gels are an emerging class of polymer gels in which a three-dimensional polymer network swells with an ionic liquid. Ion gels have drawn considerable attention in various fields such as energy and biotechnology owing to their excellent properties including nonvolatility, nonflammability, high ionic conductivity, and high thermal and electrochemical stability. Since the first report on ion gels (published ∼30 years ago), diverse functional ion gels exhibiting impressive physicochemical properties have been reported. In this review, recent developments in functional ion gels that can modulate their physical properties in response to environmental conditions are outlined. Stimuli-responsive ion gels that can adaptively undergo phase transitions in response to thermal and light stimuli are initially discussed, followed by an evaluation of diverse self-healing ion gels that can spontaneously mend mechanical damage through judiciously designed ion-gel networks.

Anion Effect on Forward Osmosis Performance of Tetrabutylphosphonium-Based Draw Solute Having a Lower Critical Solution Temperature

The applicability of ionic liquids (ILs) as the draw solute in a forward osmosis (FO) system was investigated through a study on the effect of the structural change of the anion on the FO performance. This study evaluated ILs composed of tetrabutylphosphonium cation ([P₄₄₄₄]⁺) and benzenesulfonate anion ([BS]^-), para-position alkyl-substituted benzenesulfonate anions (p-methylbenzenesulfonate ([MBS]^-) and p-ethylbenzenesulfonate ([EBS^-]), and methanesulfonate anion ([MS]^-). The analysis of the thermo-responsive properties suggested that the [P₄₄₄₄][MBS] and [P₄₄₄₄][EBS] ILs have lower critical solution temperatures (LCSTs), which play a beneficial role in terms of the reusability of the draw solute from the diluted draw solutions after the water permeation process. At 20 wt% of an aqueous solution, the LCSTs of [P₄₄₄₄][MBS] and [P₄₄₄₄][EBS] were approximately 36 °C and 25 °C, respectively. The water flux and reverse solute flux of the [P₄₄₄₄][MBS] aqueous solution with higher osmolality than [P₄₄₄₄][EBS] were 7.36 LMH and 5.89 gMH in the active-layer facing the draw solution (AL-DS) mode at osmotic pressure of 25 atm (20 wt% solution), respectively. These results indicate that the [P₄₄₄₄]⁺-based ionic structured materials with LCST are practically advantageous for application as draw solutes.

Thermoresponsive Ionic Liquid with Different Cation-Anion Pairs as Draw Solutes in Forward Osmosis

We synthesized various phosphonium- and ammonium-based ionic liquids (ILs), using benzenesulfonate (BS) and 4-methylbenzenesulfonate (MBS) to establish the criteria for designing an ideal draw solute in a forward osmosis (FO) system. Additionally, the effects of monocationic, dicationic, and anionic species on FO performance were studied. Monocationic compounds ([P₄₄₄₄][BS], [P₄₄₄₄][MBS], [N₄₄₄₄][BS], and [N₄₄₄₄][MBS]) were obtained in one step via anion exchange. Dicationic compounds ([(P₄₄₄₄)₂][BS], [(P₄₄₄₄)₂][MBS], [(N₄₄₄₄)₂][BS], and [(N₄₄₄₄)₂][MBS]) were prepared in two steps via a Menshutkin SN₂ reaction and anion exchange. We also investigated the suitability of ILs as draw solutes for FO systems. The aqueous [P₄₄₄₄][BS], [N₄₄₄₄][BS], [N₄₄₄₄][MBS], and [(N₄₄₄₄)₂][BS] solutions did not exhibit thermoresponsive behavior. However, 20 wt% [P₄₄₄₄][MBS], [(P₄₄₄₄)₂][BS], [(P₄₄₄₄)₂][MBS], and [(N₄₄₄₄)₂][MBS] had critical temperatures of approximately 43, 33, 22, and 60 °C, respectively, enabling their recovery using temperature. An increase in IL hydrophobicity and bulkiness reduces its miscibility with water, demonstrating that it can be used to tune its thermoresponsive properties. Moreover, the FO performance of 20 wt% aqueous [(P₄₄₄₄)₂][MBS] solution was tested for water flux and found to be approximately 10.58 LMH with the active layer facing the draw solution mode and 9.40 LMH with the active layer facing the feed solution.

Synthesis and Physicochemical Properties of Acrylate Anion Based Ionic Liquids

Two polymerizable ionic liquids (or monomeric ionic liquids, mILs) namely 1-butyl-3-methylimidazolium and choline acrylates ([C4mim]A and ChA, respectively) were synthesized using the modified Fukumoto method from corresponding chlorides. The chemical structure of the prepared mILs was confirmed with FTIR and NMR study. Investigation of the thermal properties with DSC demonstrates that both mILs have a Tg temperature of about 180 K and a melting point around 310 K. It was shown that the temperature dependence of FTIR confirm the Tg to be below 200. Both mILs exhibited non-Newtonian shear thinning rheological behavior at shear rates >4 s−1. It was shown that [C4mim]A is able to dissolve bacterial cellulose (BC) leading to a decrease in its degree of polymerization and recrystallisation upon regeneration with water; although in the ChA, the crystalline structure and nanofibrous morphology of BC was preserved. It was demonstrated that the thixotropic and rheological properties of cellulose dispersion in ChA at room temperature makes this system a prospective ink for 3D printing with subsequent UV-curing. The 3D printed filaments based on ChA, containing 2 wt% of BC, and 1% of N,N′-methylenebisacrylamide after radical polymerization induced with 1% 2-hydroxy-2-methylpropiophenone, demonstrated Young’s modulus 7.1 ± 1.0 MPa with 1.2 ± 0.1 MPa and 40 ± 5% of strength and ultimate elongation, respectively.

Effect of the Alkyl Chain Length on Assessment as Thermo-Responsive Draw Solutes for Forward Osmosis

A series of thermo-responsive tetrabutylphosphonium 1-alkanesulfonates (abbreviated as [P₄₄₄₄][C _n S], n = 6, 8, 10, and 12), where n is the number of carbon atoms in the alkyl group on the 1-alkanesulfonate anion, were prepared by an ion-exchange reaction to investigate their potential ability toward the application of draw solutes in forward osmosis (FO). We systematically studied the recovery properties and FO performance of [P₄₄₄₄][C _n S]. This series exhibited lower critical solution temperature (LCST) characteristics, which offer a clear advantage of being energy-efficient for recovering draw solutes; however, [P₄₄₄₄][C₆S] was only observed at 20 wt %. The LCSTs of the 20 wt % [P₄₄₄₄][C₆S], [P₄₄₄₄][C₈S], [P₄₄₄₄][C₁₀S], and [P₄₄₄₄][C₁₂S] draw solutions were approximately 83, 54, 49, and 56 °C, respectively. Moreover, when the orientation of the active layer was heading toward the draw solution (AL-DS mode), the water and reverse solute flux of [P₄₄₄₄][C₁₀S] were about 1.58 LMH and 0.81 gMH, respectively, at 20 wt % aqueous solutions. When the membrane was used in the active layer facing the feed solution (AL-FS) system, the water and reverse solute flux of [P₄₄₄₄][C₁₀S] were approximately 0.71 LMH and 0.38 gMH, respectively, at 20 wt % aqueous solutions. Thus, this study is the first to examine the structural transformations of the bulkier alkyl group on the sulfonate anion moiety and its feasibility as the new draw solute for the FO system.

Reconfiguration of Langmuir Monolayers of Thermo-Responsive Branched Ionic Polymers with LCST Transition

Thermo-responsive ionic polymers have the ability to form adaptive and switchable morphologies, which may offer enhanced control in energy storage and catalytic applications. Current thermo-responsive polymers are composed of covalently attached thermo-responsive moieties, restricting their mobility and global dynamic response. Here, we report the synthesis and assembly at the water-air interface of symmetric and asymmetric amphiphilic thermo-responsive branched polymers with weakly ionically bound arms of amine-terminated poly(N-isopropylacrylamide) (PNIPAM) macro-cations. As we observed, symmetric branched polymers formed multimolecular nanosized micellar assemblies, whereas corresponding asymmetric polymers formed large, interconnected worm-like aggregates. Dramatic changes in localized and large-scale chemical composition confirmed the reversible adsorption and desorption of the mobile PNIPAM macro-cations below and above the low critical solution temperature (LCST) and their non-uniform redistribution within polymer monolayer. Increasing the temperature above LCST led to the formation of large interconnected micellar aggregates because of the micelle-centered aggregation of the hydrophobized PNIPAM macro-cationic terminal chains in the aqueous subphase. Overall, this work provides insights into the dynamic nature of the chemical composition of branched ionic polymers with weakly ionically bound thermo-responsive terminal chains and its effect on both morphology and local/surface chemistry of monolayers at LCST transition.

Sulfonates as Versatile Structural Counterions of Epoxidized Salts

Ionic liquids have recently emerged as monomers to synthesize multifunctional polymeric materials. Among such species, ionic epoxy-based networks represent promising but underdeveloped materials that are hindered by tricky access to the functionalized ionic liquid monomers. To date, the reported epoxidized imidazolium salts have focused on highly toxic epichlorohydrin. This study concerns flexible and efficient methods to synthesize versatile building blocks with sulfonates as valuable anions. The judicious combination of an aliphatic or aromatic sulfonate with an imidazolium leads to new epoxidized salts with high structural variability and good chemical and thermal stability (>300 °C).

Morphological transitions of cationic PISA particles by salt, triflate ions and temperature; comparison of three polycations

Three strong polycation stabilizers, poly((vinylbenzyl) trimethylammonium chloride), PVBTMAC, poly((2-(methacryloyloxy)ethyl)trimethylammonium chloride), PMOTAC, and poly((3-acrylamidopropyl) trimethylammonium chloride), PAMPTMAC have been synthesized with reversible addition-fragmentation chain transfer, RAFT, reactions. Solubilities of the polycations...

Structural Engineering and Optimization of Zwitterionic Salts for Expeditious Discovery of Thermoresponsive Materials

This work reported the discovery of N-triflimide (NTf)-based zwitter-ionic liquids (ZILs) that exhibit UCST-type phase transitions in water, and their further structural optimization in fine-tuning polarity to ultimately afford newfangled thermosensitive materials carrying attractive and biocompatible T_c values that clearly demonstrated the true value of the tunability of ZIL structure. This research established that with non-aromatic, acyclic ZILs as small-molecule thermoresponsive materials, their mixing and de-mixing with water triggered by temperatures are entirely reversible.

Electrochemistry meets polymer physics: polymerized ionic liquids on an electrified electrode

Polymeric ionic liquids are emerging polyelectrolyte materials for modern electrochemical applications. In this paper, we propose a self-consistent field theory of a polymeric ionic liquid on a charged conductive electrode. Taking into account the conformational entropy of rather long polymerized cations within the Lifshitz theory and electrostatic and excluded volume interactions of ionic species within the mean-field approximation, we obtain a system of self-consistent field equations for the local electrostatic potential and average concentrations of monomeric units and counterions. We solve these equations in the linear approximation for the cases of a point-like charge and a flat infinite uniformly charged electrode immersed in a polymeric ionic liquid and derive analytical expressions for local ionic concentrations and electrostatic potential, and derive an analytical expression for the linear differential capacitance of the electric double layer. We also find a numerical solution to the self-consistent field equations for two types of boundary conditions for the local polymer concentration on the electrode, corresponding to the cases of the specific adsorption absence (indifferent surface) and strong short-range repulsion of the monomeric units near the charged surface (hard wall case). For both cases, we investigate the behavior of differential capacitance as a function of applied voltage for a pure polymeric ionic liquid and a polymeric ionic liquid dissolved in a polar organic solvent. We observe that the differential capacitance profile shape is strongly sensitive to the adopted boundary condition for the local polymer concentration on the electrode.

Constrained thermoresponsive polymers - new insights into fundamentals and applications

In the last decades, numerous stimuli-responsive polymers have been developed and investigated regarding their switching properties. In particular, thermoresponsive polymers, which form a miscibility gap with the ambient solvent with a lower or upper critical demixing point depending on the temperature, have been intensively studied in solution. For the application of such polymers in novel sensors, drug delivery systems or as multifunctional coatings, they typically have to be transferred into specific arrangements, such as micelles, polymer films or grafted nanoparticles. However, it turns out that the thermodynamic concept for the phase transition of free polymer chains fails, when thermoresponsive polymers are assembled into such sterically confined architectures. Whereas many published studies focus on synthetic aspects as well as individual applications of thermoresponsive polymers, the underlying structure-property relationships governing the thermoresponse of sterically constrained assemblies, are still poorly understood. Furthermore, the clear majority of publications deals with polymers that exhibit a lower critical solution temperature (LCST) behavior, with PNIPAAM as their main representative. In contrast, for polymer arrangements with an upper critical solution temperature (UCST), there is only limited knowledge about preparation, application and precise physical understanding of the phase transition. This review article provides an overview about the current knowledge of thermoresponsive polymers with limited mobility focusing on UCST behavior and the possibilities for influencing their thermoresponsive switching characteristics. It comprises star polymers, micelles as well as polymer chains grafted to flat substrates and particulate inorganic surfaces. The elaboration of the physicochemical interplay between the architecture of the polymer assembly and the resulting thermoresponsive switching behavior will be in the foreground of this consideration.

Photo cleavable thioacetal block copolymers for controlled release

We present a new light cleavable polymer containing o-nitrobenzene thioacetal groups in the main chain. By conjugation to a PEG block, we synthesized block copolymers capable of forming nanoparticles in aqueous solution. We studied drug encapsulation and release using the model drug Nile Red. Irradiation with UV-A light (365 nm) leads to efficient degradation of the polymers and associated burst release of the payload. Unlike other thioacetal and thioketal polymers, these polymers are stable to reactive oxygen species (ROS), preventing non-triggered release. Moreover, the nanocarriers showed low cytotoxicity in cell viability experiments.

The o-nitrobenzene thioacetal group selectively cleaves upon UV-A irradiation. When incorporated in a block-copolymer, these photoactive groups can be used for controlled release of molecular cargo from polymer nanoparticles.

Pressure Effects on Self-Assembly in Mixtures Containing Zwitterionic Amphiphiles

To understand the responses of self-assembly in mixtures containing zwitterionic amphiphilic chains to high pressure, we introduce a self-consistent field theory in combination with a molecular equation-of-state model for them in a primitive way. The free energy density for those in the bulk state is first formulated. Its locally equilibrated excess part is then incorporated into Edwards Hamiltonian along with the electrostatic energy contributions to elicit the saddle point approximation to the partition function with proper self-consistent field equations. It is shown that charge-charge correlations enhance self-assembling tendency for the amphiphiles with the opposite charges on one component side, as the medium dielectric constant ε_r decreases. Those with the opposite charges at the two chain ends respond in a more complicated way to ε_r. Densification by applied pressure strengthens the self-assembly for both at a moderate ε_r, similar to typical phospholipids, but pressure effects are strongly dependent on the position of charges along the chains at a lower ε_r. It is argued that the manipulation of the dielectric environment and disparity in component dispersion interactions can yield useful materials exhibiting various types of baroresponsivity or thermoresponsivity with re-entrant self-assembly.

Ionic liquid-derived polyelectrolyte promoting the biphasic condensation of immiscible reactants at moderate temperature

Both an ionic liquid catalyst (ILC1) and a related cross-linked polyelectrolyte effectively promote the biphasic condensation of glycerol with oleic acid at moderate temperature.

Molecular theory of the electrostatic collapse of dipolar polymer gels

We develop a new quantitative molecular theory of liquid-phase dipolar polymer gels. We model monomer units of the polymer network as a couple of charged sites separated by a fluctuating distance. For the first time, within the random phase approximation, we have obtained an analytical expression for the electrostatic free energy of the dipolar gel. Depending on the coupling parameter of dipole-dipole interactions and the ratio of the dipole length to the subchain Kuhn length, we describe the gel collapse induced by electrostatic interactions in the good solvent regime as a first-order phase transition. This transition can be realized at reasonable physical parameters of the system (temperature, solvent dielectric constant, and dipole moment of monomer units). The obtained results could be potentially used in modern applications of stimuli-responsive polymer gels and microgels, such as drug delivery, nanoreactors, molecular uptake, coatings, superabsorbents, etc.

Combinatorial discovery of small-molecule 1,2,3-triazolium ionic liquids exhibiting lower critical solution temperature phase transition

Both lower and upper critical solution temperature (LCST and UCST) systems are two typical phase behaviors of thermoresponsive materials with solvents, in which LCST is far less common than UCST. Recent studies on ionic liquids carrying LCST phase transitions have predominantly focused on quaternary ammonium- and phosphonium-based ionic salts. Based on the 1,2,3-triazole core structure assemblable by azide-alkyne cycloaddition click reaction, this work reports the combinatorial synthesis of 1,3,4-trialkylated 1,2,3-triazolium ionic liquids in three libraries with a total of 160 ionic liquids and demonstrates, for the first time, their values in temperature-switchable phase transition with water. In this work, the successful discovery of a new thermoresponsive ionic liquid b26, based on the structure-and-phase separation study of b8 and b9, perfectly exemplified the true value of the tunability of ionic liquid fine structures. For all 160 ionic liquids synthesized, 155 are liquid at room temperature and 22 room-temperature ionic liquids were found to exhibit thermoresponsive phase transitions having low T_c values in water. To the best of our knowledge, this comprehensive study is the first report of small-molecule 1,2,3-triazolium ionic liquids that exhibit LCST property in water.

Combinatorial discovery of thermoresponsive cycloammonium ionic liquids

This work demonstrated, for the first time, the combinatorial discovery and rational identification of small-molecule cycloammonium-based thermoresponsive ionic liquids that exhibit LCST phase transition and carry attractive T_c values in water.

Ionic effects on synthetic polymers: from solutions to brushes and gels

The ionic effects on synthetic polymers have attracted extensive attention due to the crucial role of ions in the determination of the properties of synthetic polymers. This review places the focus on specific ion effects, multivalent ion effects, and ionic hydrophilicity/hydrophobicity effects in synthetic polymer systems from solutions to brushes and gels. The specific ion effects on neutral polymers are determined by both the direct and indirect specific ion-polymer interactions, whereas the ion specificities of charged polymers are mainly dominated by the specific ion-pairing interactions. The ionic cross-linking effect exerted by the multivalent ions is widely used to tune the properties of polyelectrolytes, while the reentrant behavior of polyelectrolytes in the presence of multivalent ions still remains poorly understood. The ionic hydrophilicity/hydrophobicity effects not only can be applied to make strong polyelectrolytes thermosensitive, but also can be used to prepare polymeric nano-objects and to control the wettability of polyelectrolyte brush-modified surfaces. The not well-studied ionic hydrogen bond effects are also discussed in the last section of this review.

Preparation of tough, thermally stable, and water-resistant double-network ion gels consisting of silica nanoparticles/poly(ionic liquid)s through photopolymerisation of an ionic monomer and subsequent solvent removal

We report the preparation of tough, thermally stable, and water-resistant double-network (DN) ion gels, which consist of a partially-clustered silica nanoparticle network and poly(ionic liquid) (PIL) network holding an ionic liquid. Silica nanoparticles/poly([Evim][Tf₂N]) DN ion gels are prepared by photo-induced radical polymerisation of [Evim][Tf₂N] in a mixture containing silica nanoparticles, [Bmim][Tf₂N], ionic liquid based cross-linker [(VIM)₂C₄][Tf₂N]₂, and ethyl acetate, followed by subsequent solvent evaporation. Tensile strength measurements show that the mechanical properties of the PIL DN ion gels were higher than those of the PIL single-network (SN) ion gel. A rheological study indicates that an enhancement in mechanical strength of the PIL DN ion gels can be achieved when silica nanoparticles form partial clusters in [Bmim][Tf₂N]. The cyclic stress-strain measurement of the PIL DN ion gels shows hysteresis loops, suggesting that the silica nanoparticle clusters rupture and dissipate the loading energy when the PIL DN ion gels undergo a large deformation. The fracture strength and Young's modulus of the PIL DN ion gels increase as the diameter of the silica nanoparticles is decreased. Thermogravimetric analysis measurement shows that the PIL DN ion gel has a high decomposition temperature of approximately 400 °C. Moreover, the swelling test shows that the PIL DN ion gel possesses an excellent water-resistant property because of the hydrophobic nature of the PIL backbone. We believe that such tough, thermally stable, and water-resistant PIL DN ion gels can be used as carbon dioxide separation membranes, sensors, and actuators for soft robotics.

Enzyme-responsive Fluorescent Ionic Liquid

Herein, we describe the development of a novel material, "enzyme-responsive fluorescent ionic liquid", which enabled a highly sensitive detection of alkaline phosphatase (ALP). We prepared a plasticized poly(vinyl chloride) (PVC) membrane using this new material as a plasticizer and quantified ALP in aqueous solutions. Preliminary results suggested that the PVC membrane responded to ALP at an interface between the membrane and the sample solution with anion extraction to maintain electroneutrality in the membrane phase. The developed PVC membrane showed an approximately six-times higher sensitivity than the conventional membrane, thereby demonstrating highly sensitive ALP detection. These results suggested the potential applicability of the proposed membrane for highly sensitive protein detection by using ALP-labeled antibodies.

Comparison of poly(ethylene glycol)-based networks obtained by cationic ring opening polymerization of neutral and 1,2,3-triazolium diepoxy monomers

The properties of two cross-linked epoxy networks obtained by ring opening polymerization of a synthetic diepoxy 1,2,3-triazolium and a commercial poly(ethylene glycol)diglycidyl ether using benzylamine trifluoroborate as cationic initiator are compared.

Thermoresponsive behavior of poly[trialkyl-(4-vinylbenzyl)ammonium] based polyelectrolytes in aqueous salt solutions

A systematic method to induce thermoresponsive behavior for polycations with salts from the reversed Hofmeister series is introduced.

Counterion-Tunable Thermosensitivity of Strong Polyelectrolyte Brushes

In this work, poly(sodium styrene sulfonate) brushes have been employed as a precursor to prepare thermosensitive strong polyelectrolyte brushes (SPBs) through a counterion exchange strategy. The substitution of hydrophilic Na⁺ counterions by hydrophobic tetraalkylphosphonium counterions leads to a thermoresponsivity of the SPBs. The thermosensitive properties including hydration, stiffness, and surface water wettability of the SPBs can be modulated by the type of the tetraalkylphosphonium counterions. Nevertheless, the wet thickness of the SPBs with tetraalkylphosphonium counterions does not exhibit an obvious temperature dependency due to the high steric barrier in the crowded environment of SPBs generated by the large tetraalkylphosphonium counterions. The mixtures of small Na⁺ counterions and large tetraalkylphosphonium counterions are employed to realize the thermosensitive wet thickness without sacrificing other thermoresponsive properties of the SPBs because the mixed counterions can bring both a certain hydrophobicity and some free space to the brushes. This work opens up the opportunities available for the use of counterions to tune the thermosensitivity of SPBs.

Biodegradable zwitterionic nanoparticles with tunable UCST-type phase separation under physiological conditions

Thermo-responsive polymeric nanoparticles (NPs) are emerging as a powerful tool in nanomedicine for the fabrication of advanced drug delivery systems. In addition to their size and biodegradation rate, phase separation of NPs upon application of a thermal stimulus provides an additional switch to control the rate of release of active components. Among the materials currently developed for biomedical applications, NPs stabilized by zwitterionic polymers are gaining increasing interest due to their high stability and ability to escape the body immune response. Yet, biodegradable zwitterionic NPs with temperature response under physiological conditions are currently not available. Here, we develop a new class of biodegradable zwitterionic NPs that exhibit UCST phase transition in the biological temperature range (T = 30-45 °C) and in physiological solution (i.e. 0.9% w/w NaCl). We design a strategy that relies on the self-assembly of block copolymers produced via reversible addition-fragmentation chain transfer (RAFT) emulsion polymerization. These copolymers comprise a zwitterionic portion exhibiting an upper critical solution temperature (UCST) and a biodegradable hydrophobic block consisting of oligoesters functionalized with a vinyl group. This modular macromolecular architecture allows us to independently control a variety of NP properties by modifying the individual components of the copolymer. In particular, the zwitterionic block of the copolymers controls the UCST-type phase separation behavior, while the number of the oligoester repeating units governs the size of the NPs and the length of the oligoester dictates the degradation rate. After demonstrating the synthesis of highly controlled degradable NPs, we show the potential of this new class of materials in the context of drug delivery by controlling the release of a drug-mimic molecule upon temperature variations in a broad time range from few minutes to 20 hours.

Electron Beam Patterning of Polymerizable Ionic Liquid Films for Application in Photonics

Planar photonic components can be fabricated with high resolution by electron beam patterning of polymer thin films on solid substrates such as silicon and glass. However, polymer films are normally formed by spin-coating lithographic resists containing not only polymers but also volatile solvents, which is a serious environmental and health issue. Therefore, we investigate a new type of material for planar structure fabrication (i.e., room-temperature ionic liquids (RTILs) with a polymerizable allyl group) that is electron-beam-curable, solvent-free, and thus potentially interesting for processing materials with weak resistance to solvents. We fabricate planar polymer microstructures by electron beam patterning of RTIL thin films in vacuum, which is possible because of the negligible volatility of ionic liquids. Three different polymerizable ionic liquids {i.e., [Allmim][Cl] (1-allyl-3-methylimidazolium chloride), [Allmim][NTf₂] (1-allyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide), and [Allmmim][NTf₂] (1-allyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide)} are compared in terms of the quality of the fabricated microstructures. We demonstrate that the shape of the more viscous RTIL with the Cl^- anion is less distorted during electron-beam-activated polymerization than the shape of the less viscous RTILs with a large NTf₂^- anion. Furthermore, the surface tension of the NTf₂-based ionic liquid decreases significantly with temperature as compared to that of the Cl-based ionic liquid. Thus, we suggest that the thermocapillary effect, that is, the Marangoni flow caused by a temperature gradient, might be responsible for the differences in the shape of the RTIL-derived microstructures. Also, we analyze the chemistry of the electron-beam-activated polymerization of RTIL by the use of Fourier-transform infrared spectroscopy (FTIR) and conclude that because of the disappearance of C═C bonds the free radical polymerization is a probable reaction mechanism. Finally, we show that polymerized microstructures are potentially attractive as planar photonic components because of good optical properties such as a high refractive index.

Synthesis of New Magnetic Crosslinked Poly (Ionic Liquid) Nanocomposites for Fast Congo Red Removal from Industrial Wastewater

Advanced materials reliant on cross-linked magnetic poly (ionic liquids) (PILs) have been widely utilized in environmental applications for water purification. The present work demonstrates our preparation of a new magnetic cross-linked PIL based on quaternized 4-vinyl-pyridine-co-acrylamide (QVP/AAm). The chemical composition, thermal stability, magnetic properties, morphology, particle sizes, and zeta potential of the magnetic QVP/AAm composites were investigated. Fast adsorption and desorption kinetics, high adsorption capacity, rapid magnetic separation, and the absence of secondary pollution in the adsorption process make QVP/AAm-Fe₃O₄ a highly effective adsorbent for the elimination of anionic acidic Congo red contaminants from industrial wastewater.

Characterization of Ionic Liquid Aqueous Two-Phase Systems: Phase Separation Behaviors and the Hydrophobicity Index between the Two Phases

1-Allyl-3-methylimidazolium chloride [Amim][Cl] and 1-butyl-3-methylimidazolium chloride [Bmim][Cl] are water-soluble ionic liquids (ILs) that can from an aqueous two-phase system (ATPS) when mixed with specific salts. Herein, we prepared [Amim][Cl]- and [Bmim][Cl]-ATPSs by adding the salts (K₂CO₃, K₂HPO₄). To investigate the phase separation behavior of the IL-ATPSs, binodal curves were drawn at different temperatures and the length and slope of the tie lines were analyzed. The [Bmim][Cl]/K₂HPO₄ system underwent two-phase separation at lower temperature conditions, suggesting that the phase separation might depend on the salting-out effect in the bottom phase. Using the IL-ATPS, the distribution coefficients, K_aa, of amino acids were determined and used to characterize the hydrophobicity index (HF) between the top and bottom phases, which is a good indicator to understand the molecular partitioning behaviors in conventional ATPSs. The HF values of the IL-ATPSs were in the range 0.13-0.41 mol/kJ; these values were almost the same as the HF values reported for an ATPS composed of poly(ethylene glycol) and salt.