Influence of Side Chain Sizes on Dielectric and Electrorheological Responses of Poly(ionic liquid)s

Preparation of Anisotropic Trimeric Poly(Ionic Liquid) Microspheres via Microwave-Assisted Dual-Crosslinked Seed Emulsion Polymerization

A microwave-assisted dual-crosslinked seed emulsion polymerization method is reported to prepare anisotropic trimeric poly(ionic liquid) (PIL) microspheres. First, ethylene glycol dimethacrylate (EGDMA)-crosslinked PIL (CPIL) seed microspheres are prepared. Then, the CPIL microspheres are swollen with ionic liquid (IL) emulsion containing divinylbenzene (DVB) and polymerized to form dual-crosslinked PIL (D-CPIL) microspheres under microwave irradiation. Finally, the D-CPIL microspheres are swollen with IL monomer emulsion to form trimeric morphology and polymerized to obtain trimeric PIL microspheres under microwave irradiation. The formation process of trimeric PIL microspheres is tracked using an optical microscope and their morphology is observed using scanning electron microscopy. Different from the repeat-swelling seed emulsion polymerization that needs dumbbell-like seed microspheres having gradient crosslinking or gradient surface wettability, this method depends on multiple local contraction forces in D-CPIL microspheres containing lowly crosslinked core and highlycrosslinked shell during swelling to form trimeric PIL microspheres. It is found that microwave polymerization is important because it can well retain trimeric morphology compared to conventional heating polymerization in oil or water baths. The morphology of trimeric PIL microspheres can be adjusted by changing the type and amount of crosslinkers, monomer/seed microsphere ratio, initiator dosage, temperature, etc.

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.

Enhanced Electrorheological Performance of Core-Shell-Structured Polymerized Ionic Liquid@Doubly Polymerized Ionic Liquid Microspheres Prepared via Evaporation-Assisted Dispersion Polymerization

A polymerized ionic liquid (PIL) provides a platform for the development of a high-performance water-free polyelectrolyte-based electrorheological fluid (ERF) because of the presence of large-size hydrophobic ion pairs. However, the large-size hydrophobic ion pairs also easily result in a low glass-transition temperature of an ordinary linear PIL, and consequently, the PIL-based ERF has to be subject to a high leaking current density and a narrow working temperature range. In this paper, we prepared a kind of core-shell-structured polymerized ionic liquid@doubly polymerized ionic liquid (PIL@D-PIL) microsphere with a linear PIL as the core and a physically cross-linked D-PIL as the shell via an evaporation-assisted dispersion polymerization method. The core-shell structure of the sample was observed by scanning electron microscopy and transmission electron microscopy. The thermal properties of the sample were tested by differential scanning calorimetery and thermogravimetric analysis. The ER effect and dielectric polarization of PIL@D-PIL microspheres when dispersed in an insulating nonpolar liquid were studied by a rheometer and dielectric spectroscopy. It shows that the glass-transition temperature and thermal stability of a PIL increased after coating with the D-PIL shell. Under electric fields, the ERF of the PIL@D-PIL microspheres exhibits a significantly reduced leaking current density and an enhanced operating temperature range compared to the ERF of single-PIL microspheres. The PIL@D-PIL microspheres can still maintain good ER effect even if the temperature is higher than the glass-transition point of the PIL core due to the protection of the D-PIL shell.

Recent advances in poly(ionic liquid)s for biomedical application

Poly(ionic liquid)s (PILs) are polymers containing ions in their side-chain or backbone, and the designability and outstanding physicochemical properties of PILs have attracted widespread attention from researchers. PILs have specific characteristics, including negligible vapor pressure, high thermal and chemical stability, non-flammability, and self-assembly capabilities. PILs can be well combined with advanced analytical instruments and technology and have made outstanding contributions to the development of biomedicine aiding in the continuous advancement of science and technology. Here we reviewed the advances of PILs in the biomedical field in the past five years with a focus on applications in proteomics, drug delivery, and development. This paper aims to engage pharmaceutical and biomedical scientists to full understand PILs and accelerate the progress from laboratory research to industrialization.

Preparation of Poly(Ionic Liquid) Microbeads via Cooling-Assisted Phase Separation Method

A simple and large-scale non-chemical preparation of uniform poly(ionic liquid) (PIL) microbeads via a cooling-assisted phase separation (CAPS) method is reported. For this method, PIL bulk is dissolved to form a saturated solution in a mixed solvent composed of good solvent and non-solvent at a relatively high temperature. Then, the uniform PIL microbeads are prepared by cooling the solution to room temperature or a lower temperature in the absence of stabilizer. The size of microbeads can be controlled by adjusting the preparation parameters, including PIL concentration, cooling rate, and agitation state. The scale of preparation can be up to 10 g, and the yield of PIL microbeads is more than 70% or 88% when the solution is cooled to room temperature or 0 °C, respectively. The formation mechanism of PIL microbeads is discussed by tracing the nucleation and growth process by the transmittance of light of the solution during cooling. The application of this CAPS method to other polymer microbeads preparation is finally discussed by choosing different good solvent and non-solvent.

Titanium oxide-coated titanium-loaded metal organic framework (MOF-Ti) nanoparticles show improved electrorheological performance

Uniform small-sized MOF-Ti nanoparticles were prepared by a one-step hydrothermal method, and then a 5-10 nm TiO2 shell was coated onto them by using the sol-gel method, and MOF-Ti/TiO2 with a specific surface area of 50.2 m2 g-1 was successfully prepared. The nanoparticles were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), nitrogen adsorption-desorption isotherms (BET), and X-ray photoelectron spectroscopy (XPS). The above-analyses have elaborated the experimental study of their morphology, elements, and energy of organic functional groups. At the same time, through the use of a high-voltage rotary rheometer to test their rheological properties, the analysis of shear stress, ER efficiency, shear viscosity, etc. was performed and their dielectric constant and dielectric loss were studied by using a broadband dielectric spectrometer. Finally, we found that MOF-Ti/TiO2 is a new core-shell nanocomposite particle with a small particle size and good electrorheological properties.

The Electric Field Responses of Inorganic Ionogels and Poly(ionic liquid)s

Ionic liquids (ILs) are a class of pure ions with melting points lower than 100 °C. They are getting more and more attention because of their high thermal stability, high ionic conductivity and dielectric properties. The unique dielectric properties aroused by the ion motion of ILs makes ILs-contained inorganics or organics responsive to electric field and have great application potential in smart electrorheological (ER) fluids which can be used as the electro-mechanical interface in engineering devices. In this review, we summarized the recent work of various kinds of ILs-contained inorganic ionogels and poly(ionic liquid)s (PILs) as ER materials including their synthesis methods, ER responses and dielectric analysis. The aim of this work is to highlight the advantage of ILs in the synthesis of dielectric materials and their effects in improving ER responses of the materials in a wide temperature range. It is expected to provide valuable suggestions for the development of ILs-contained inorganics and PILs as electric field responsive materials.

Preparation and Morphology Control of Poly(ionic liquid) Particles

Poly(ionic liquid)s (PILs) are prepared by the polymerization of ionic liquid (IL) monomers that have polymerizable groups on their cationic or anionic component. PILs also share many of the characteristic properties of ILs and can be used in various materials such as CO₂ sorbents, polymer electrolytes, dispersants, and microwave-absorbing materials. In this feature article, we survey our research, focusing on the preparation of PILs in the particulate state (PIL particles) and on the morphological control of the PIL particles, including (1) the preparation of PIL particles by dispersion polymerization and emulsion polymerization, (2) control of the morphology of composite particles consisting of a PIL and poly(methyl methacrylate) (PMMA), (3) the preparation of hollow particles with a PIL shell, and (4) the preparation of PIL particles containing reduced graphene oxide (rGO). The size of the obtained PIL particles could be controlled through a modification of the synthesis conditions and the mode of polymerization (i.e., dispersion polymerization or emulsion polymerization). The obtained PIL particles maintained the characteristic properties of the corresponding ILs; moreover, the solubility of the PIL particles could be easily modified by changing their counteranion. Using seeded polymerization, we prepared PMMA/PIL composite polymer particles and subsequently demonstrated that their morphology can be manipulated to yield a core-shell or Janus structure. Hollow particles consisting of a PIL shell were also prepared, and modification of the polarity and penetration behavior of the shell through anion exchange was demonstrated.

Influence of Tethered Ions on Electric Polarization and Electrorheological Property of Polymerized Ionic Liquids

Polymerized ionic liquids (PILs) show potential to be used as new water-free polyelectrolyte-based electrorheological (ER) material. To direct ER material design at the molecular level, unveiling structure-property relationships is essential. While a few studies compare the mobile ions in PILs there is still a limited understanding of how the structure of tethered counterions on backbone influences ER property. In this study, three PILs with same mobile anions but different tethered countercations (e.g., poly(dimethyldiallylammonium) P[DADMA]⁺, poly(benzylethyl) trimethylammonium P[VBTMA]⁺, and poly(1-ethyl-4-vinylimidazolium hexafluorophosphate) P[C₂VIm]⁺) are prepared and the influence of tethered countercations on the ER property of PILs is investigated. It shows that among these PILs, P[DADMA]⁺ PILs have the strongest ER property and P[C₂VIm]⁺ PILs have the weakest one. By combining dielectric spectra analysis with DFT calculation and activation energy measurement, it can clarify that the influence of tethered counterions on ER property is mainly associated with ion-pair interaction energy that is affecting ionic conductivity and interfacial polarization induced by ion motion. P[DADMA]⁺ has the smallest ion-pair interaction energy with mobile ions, which can result in the highest ionic conductivity and the fastest interfacial polarization rate for its strongest ER property.

Nonmonotonic Influence of Size of Quaternary Ammonium Countercations on Micromorphology, Polarization, and Electroresponse of Anionic Poly(ionic liquid)s

The size influence of quaternary ammonium countercations in poly[4-styrenesulfonyl(trifluoromethylsulfonyl)imide][tetraalkylammonium] (P[STFSI][Nnnnn], n = 1, 2, and 3) poly(ionic liquid)s on dielectric polarization and the stimuli-responsive electrorheological effect is investigated by dielectric spectroscopy and rheology, and the microstructure-level understanding behind the influence is analyzed by Raman and X-ray scattering spectra. The size influence of quaternary ammonium cations is found to be nonmonotonic. The largest electrorheological effect accompanied by best polarization properties is demonstrated in P[STFSI][N2222]. Raman spectra and activation energy measurements demonstrate that the nonmonotonic influence originates from the fact that, compared to small N1111⁺ and large N3333⁺, intermediate N2222⁺ as countercations can contribute a higher mobile ion number and lower activation energy barrier of ion dissociation and motion. But the experimental values of activation energy are not consistent with theoretically calculated values by considering the ion pair electrostatic potential and elastic force contribution of the matrix. By X-ray scattering and diffraction characterizations, it is clarified that the nonmonotonic influence and the inconsistency of activation energy originate from the size influence of Nnnnn⁺ on the micromorphology of P[STFSI][Nnnnn]. Compared to the semicrystalline structure of P[STFSI][N1111] and the ionic aggregation structure of P[STFSI][N3333], the relatively uniform amorphous structure of P[STFSI][N2222] may be responsible for its lower activation energy barrier of ion motion. This study further provides insights into the design and preparation of future poly(ionic liquid)-based electrorheological materials by considering not only molecular structure but also micromorphology.

The Effect of Dielectric Polarization Rate Difference of Filler and Matrix on the Electrorheological Responses of Poly(ionic liquid)/Polyaniline Composite Particles

By using different conductivity of polyaniline as filler, a kind of poly(ionic liquid)/polyaniline composite particles was synthesized to investigate the influence of dielectric polarization rate difference between filler and matrix on the electrorheological response and flow stability of composite-based electrorheological fluids under simultaneous effect of shear and electric fields. The composite particles were prepared by a post ion-exchange procedure and then treated by ammonia or hydrazine to obtain different conductivity of polyaniline. Their electrorheological response was measured by dispersing these composite particles in insulating carrier liquid under electric fields. It showed that the composite particles treated by ammonia had the strongest electrorheological response and most stable flow behavior in a broad shear rate region from 0.5 s⁻¹ to 1000 s⁻¹. By using dielectric spectroscopy, it found that the enhanced electrorheological response with stable flow depended on the matching degree of the dielectric polarization rates between poly(ionic liquid) matrix and polyaniline filler. The closer their polarization rates are, the more stable the flow curves are. These results are helpful to design optimal composite-based electrorheological materials with enhanced and stable ER performance.

Preparation of core-shell structured metal-organic framework@PANI nanocomposite and its electrorheological properties

A novel core-shell-type electrorheological (ER) composite material was fabricated via using polyaniline as an insulating layer to the outer surface of the core conductive metal-organic framework (MIL-125) with controlled size and morphology. MIL-125 was firstly synthesized by a solvothermal method, and then polyaniline was synthesized in a polar solvent and a tight coating was successfully achieved to form a MIL-125@PANI core-shell nanocomposite. This core-shell structure greatly enhances the polarization ability of dispersed particles, thereby improving their rheological properties. The morphology of pure MIL-125 and MIL-125@PANI has been characterized by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Their structure was characterized by X-ray powder diffraction. Moreover, the ER activity of MIL-125-based and MIL-125@PANI-based ER fluids by dispersing the particles into silicone oil was studied using a rotational rheometer. The results show that the MIL-125@PANI composite particles have higher ER properties.

Recent development of electro-responsive smart electrorheological fluids

The characteristics of an electrorheological (ER) fluid, as a class of smart soft matter, can be actively and accurately tuned between a liquid- and a solid-like phase by the application of an electric field. ER materials used in ER fluids are electrically polarizable particles, which are attracting considerable attention in addition to further research. This perspective reports the latest ER materials along with their rheological understanding and provides a forward-looking summary of the potential future applications of ER technology.

Enhancing Electroresponsive Electrorheological Effect and Temperature Dependence of Poly(ionic liquid) Particles by Hard Core Confinement

Monodisperse core-shell-structured SiO₂@poly(ionic liquid) (SiO₂@PIL) particles are prepared by the polymerization of ionic liquid monomer on the surface of methacryloxypropyltrimethoxysilane-modified SiO₂ particles. The electroresponsive electrorheological (ER) effect of SiO₂@PIL particles when dispersed in insulating carrier liquid is investigated and compared with that of pure poly(ionic liquid) (PIL) particles based on temperature-modulated rheology under electric fields. It is demonstrated that hard SiO₂ core not only enhances the ER effect of PIL particles but also improves the temperature dependence of ER effect. By dielectric spectroscopy analysis, the mechanism behind the property enhancement was discussed. It indicates that the hard SiO₂ core can not only increase the interfacial polarization strength of SiO₂@PIL particles by core-shell architecture but also restrain the segment relaxation or softening of the PIL shell and influence the ion dynamics above the calorimetric glass transition of PILs by the so called "substrate confinement effect", and this should be responsible for the enhanced electroresponsive ER effect and temperature stability of the SiO₂@PIL particles.

Smart and Functional Conducting Polymers: Application to Electrorheological Fluids

Electro-responsive smart electrorheological (ER) fluids consist of electrically polarizing organic or inorganic particles and insulating oils in general. In this study, we focus on various conducting polymers of polyaniline and its derivatives and copolymers, along with polypyrrole and poly(ionic liquid), which are adopted as smart and functional materials in ER fluids. Their ER characteristics, including viscoelastic behaviors of shear stress, yield stress, and dynamic moduli, and dielectric properties are expounded and appraised using polarizability measurement, flow curve testing, inductance-capacitance-resistance meter testing, and several rheological equations of state. Furthermore, their potential industrial applications are also covered.

The preparation and electrorheological behavior of bowl-like titanium oxide nanoparticles

Bowl-like titanium oxide nanoparticles were successfully prepared by a simple solvothermal method using absolute ethanol and isopropanol as the cosolvent. Ostwald ripening coupled with the inner-stress-induce effect were assumed to play an important role in the formation of this unique bowl-like morphology. The morphological evolution from solid nanosphere to bowl-like nanoparticle was investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Besides, the structural characteristics of the as-synthesized TiO₂ nanoparticles were confirmed by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) and thermogravimetric analysis (TGA). Moreover, a rotational rheometer was operated to examine the electrorheological (ER) effect. Excellent ER properties were achieved when the TiO₂ particles were dispersed in silicone oil under an external electric field.