Flexoelectric Ionic Liquid-Grafted Triblock Copolymers for Energy Harvesting under Flexural Deformation.

The goal of the present article is to develop flexoelectric polyelectrolyte elastomers for energy harvesting based on a poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) dimethacrylate (PEG-b-PPG-b-PEG-DMA) triblock grafted with an ionic liquid (IL) such as allylmethylimidazolium bis(trifluoromethane sulfonyl) imide (AMIMTFSI). The IL-grafted triblock copolymer network possesses a balance of reasonably good ionic conductivity and high ion polarization during cantilever bending. Of particular importance is the achievement of high flexoelectric coefficients in some flexoelectric polyelectrolyte elastomer (FPE) compositions reaching 1368 µC/m at ambient temperature during mechanical deformation under intermittent square-wave bending mode. With the addition of a 10 wt % lithium bis(trifluoromethane sulfonyl) imide (LiTFSI) salt, the flexoelectric coefficient further improved to 1737 µC/m, which is the highest among all piezoelectric and flexoelectric materials hitherto reported, and thus it opens a new opportunity for clean energy harvesting from a vibrating natural environment.

Enhanced Energy Storage in Lithium-Metal Batteries via Polymer Electrolyte Polysulfide-Polyoxide Conetworks.

The present article entails a novel concept of storing extra energy in a multifunctional polymer electrolyte membrane (PEM) beyond the storage capacity of a cathode, which is achieved by so-called "prelithiation" upon simply deep discharging to a low potential range of a lithium-metal electrode (i.e., -0.5 to 0.5 V). This unique extra energy-storage capacity has been realized recently in the PEM consisting of polysulfide-co-polyoxide conetworks in conjunction with succinonitrile and LiTFSI salt that facilitate complexation via ion-dipole interaction of dissociated lithium ions with thiols, disulfide, or ether oxygen of the conetwork. Although ion-dipole complexation may increase the cell resistance, the prelithiated PEM provides excess lithium ions during oxidation (or Li+ stripping) at the Li-metal electrode. Once the PEM network is fully saturated with Li ions, the remaining excess ions can move through the complexation sites at ease, thereby affording not only facile ion transport but also extra ion-storage capacity within the PEM conetwork. Of particular interest is that the lithiated polysulfide-co-polyoxide polymer network-based PEM exhibits a high conductivity of 1.18 × 10-3 S/cm at ambient, which can also store extra energy with a specific capacity of about 150 mAh/g at a 0.1C rate in the PEM voltage range of 0.01-3.5 V in addition to 165 mAh/g at 0.2C of an NMC622 (nickel manganese cobalt oxide) cathode (i.e., 2.5-4.6 V) with a Coulombic efficiency of approximate unity. Moreover, its Li-metal battery assembly with an NMC622 cathode exhibits a very high specific capacity of ∼260 mAh/g at 0.2C in the full battery range of 0.01-5 V, having a higher Li+ transference number of 0.74, suggestive of domination by the lithium cation transport relative to those (0.22-0.35) of organic liquid electrolyte lithium-ion batteries.

Hierarchical Organization of Single Crystal Polymorphs of Azobenzene Chromophore in Anisotropic Media Subjected to Local Thermal Gradients upon Cold Spraying.

The present article entails the emergence of diverse crystal polymorphs following thermal quenching into various coexistence regions of binary azobenzene chromophore (ACh)/diacrylate (DA) solution and of azobenzene/nematic liquid crystal (E7) mixture. Development of various crystal topologies encompassing rhomboidal and hexagonal shapes can be witnessed in a manner dependent on thermal quenched depths into the crystal + liquid coexistence region of ACh/DA system. Upon spraying with compressed carbon dioxide (CO2 ) fluid, the local temperature gradient is generated resulting in spherulitic morphology showing discrete lamellae undergoing twisting locally in some regions and branched dendrites or seaweeds in another. When ACh/E7 blend is sprayed using compressed CO2 fluid, hierarchical organization of various discrete faceted single crystals including needle, rectangular, rhombus, and truncated hexagonal crystals radiating from the spherulite core can be discerned in a brighter region (off cross-polarization) polarized optical microscopy (POM) and nematic disclination in a darker cross-polarized region. Of particular interest is that the observed faceted single-crystal polymorphs in ACh/E7 may be contrasted to the lamellar twisting and branching observed in the ACh/DA system and plausible mechanisms of polymer spherulitic growth are discussed.

Polymer Electrolytes as Energy-Harvesting Materials to Capture Electrical Energy from Dynamic Mechanical Deformations.

Ionic electroactive polymers (iEAPs) can generate electrical energy under bending deformations exhibiting great potential for fabricating energy harvesters from dynamic vibrating environments. According to a previous study, this flexoelectric energy-harvesting potential is explored in polymer electrolyte membrane (PEM) assemblies subjected to intermittent square wave bending modes. The above study reveals that the mechanoelectrical transduction is likely to be the consequence of ion polarization under a pressure gradient across the PEM thickness. To further evaluate the applicability of the PEM assemblies for harvesting energy from dynamic environments, oscillatory bending deformation is applied in the present study, whereby the complex flexoelectric coefficient corresponding to dynamic capacitance exhibits strong frequency dependence. At very high oscillatory bending frequencies, the ionic clouds inside the PEM assemblies cannot be fully polarized, and thus the corresponding energy output tends to become smaller. However, the PEM assemblies having higher ionic conductivities can enhance energy output at high frequencies. Of particular interest is that the incorporated ionic liquid (IL) is not only capable of effectively plasticizing the polymer network, but also expediting the ionic conductivity, thereby enhancing the electrical energy output, which in turn provides important design guidance for efficient polymer energy harvesters.

Flexo-Ionic Effect of Ionic Liquid Crystal Elastomers.

The first study of the flexo-ionic effect, i.e., mechanical deformation-induced electric signal, of the recently discovered ionic liquid crystal elastomers (iLCEs) is reported. The measured flexo-ionic coefficients were found to strongly depend on the director alignment of the iLCE films and can be over 200 µC/m. This value is orders of magnitude higher than the flexo-electric coefficient found in insulating liquid crystals and is comparable to the well-developed ionic polymers (iEAPs). The shortest response times, i.e., the largest bandwidth of the flexo-ionic responses, is achieved in planar alignment, when the director is uniformly parallel to the substrates. These results render high potential for iLCE-based devices for applications in sensors and wearable micropower generators.

Flexoelectricity in Flexoionic Polymer Electrolyte Membranes: Effect of Thiosiloxane Modification on Poly(ethylene glycol) Diacrylate and Ionic Liquid Electrolyte Composites.

The present article entails the generation of flexoelectricity during cantilever bending of a solid polymer electrolyte membrane (PEM), composed of poly(ethylene glycol) diacrylate (PEGDA) precursor and ionic liquid (hexylmethylimidazolium hexafluorophosphate). The effects of thiosiloxane modification of PEGDA precursor on glass transition, ionic conductivity, and flexoelectric performance have been explored as a function of PEM composition. The glass transition temperature (Tg) of the PEM declines with increasing thiosiloxane amount in the PEGDA co-network, while the ionic conductivity improves. The PEM/compliant carbonaceous electrodes assemblies were assembled to determine the flexoelectric coefficients by monitoring electrical voltage/current outputs for various PEM compositions under the intermittent square-wave and dynamic oscillatory sine-wave deformation modes. Of particular interest is that the room temperature flexoelectric coefficient exhibits strong frequency dependence in the vicinity of 0.01-10 Hz, suggesting that ion polarization and ion transport through the ion-dipole complexed networks can still be affected by the mobile side chain branches even in the elastic regime of the covalently bonded PEGDA network. The in-depth understanding of the effect of thiosiloxane side chain on flexoelectricity generation is anticipated to have impact on the development of mechanoelectrical energy conversion devices for energy harvesting applications from natural and dynamical environment.

Poly(ethylene glycol) Diacrylate Based Electro-Active Ionic Elastomer.

Preparation and low voltage induced bending (converse flexoelectricity) of crosslinked poly(ethylene glycol) diacrylate (PEGDA), modified with thiosiloxane (TS) and ionic liquid (1-hexyl-3-methylimidazolium hexafluorophosphate) (IL) are reported. In between 2µm PEDOT:PSS electrodes at 1 V, it provides durable (95% retention under 5000 cycles) and relatively fast (2 s switching time) actuation with the second largest strain observed so far in ionic electro-active polymers (iEAPs). In between 40 nm gold electrodes under 8 V DC voltage, the film can be completely curled up (270° bending angle) with 6% strain that, to the best of the knowledge, is unpreceded among iEAPs. These results render great potential for the TS/PEGDA/IL based electro-active actuators for soft robotic applications.

Mechanoelectrical Transduction of Polymer Electrolyte Membranes: Effect of Branched Networks.

The role of side-chain branching in flexoelectric properties of a flexible, ionic solid polymer electrolyte membrane (PEM) has been investigated subjected to mechanical bending. These PEMs were synthesized via photopolymerization of the bifunctional poly(ethylene glycol) diacrylate (PEGDA) network attached with monofunctional poly(ethylene glycol) methyl ether acrylate (PEGMEA) branches in their ternary mixtures with lithium bis(tri-fluoromethane sulfonyl) imide (LiTFSI) salt. Both the PEGDA polymer precursor and PEGDMA side branches are capable of ionizing the lithium salt, but the dissociated lithium cations can also form the complexation with ether oxygen of PEGDA. With increasing PEGMEA content, not only the glass transition temperature is lowered, but also the ionic conductivity increases with temperature, which may be attributed to plasticization by dangling PEGMEA side chains. The flexoelectric responses of PEMs were investigated under various intermittent and oscillatory cantilever bending modes as a function of PEM composition and frequency. Last but not the least, the mechanoelectrical energy conversion was evaluated for various PEGDA/PEGMEA compositions and discussed its potential applications in energy harvesting from natural resources such as wind and wave motions.

Electroresponsive Ionic Liquid Crystal Elastomers.

This paper describes the preparation, physical properties, and electric bending actuation of a new class of active materials-ionic liquid crystal elastomers (iLCEs). It is demonstrated that iLCEs can be actuated by low-frequency AC or DC voltages of less than 1 V. The bending strains of the unoptimized first iLCEs are already comparable to the well-developed ionic electroactive polymers. Additionally, iLCEs exhibit several novel and superior features, such as the alignment that increases the performance of actuation, the possibility of preprogrammed actuation patterns at the level of the cross-linking process, and dual (thermal and electric) actuations in hybrid samples. Since liquid crystal elastomers are also sensitive to magnetic fields and can also be light sensitive, iLCEs have far-reaching potentials toward multiresponsive actuations that may have so far unmatched properties in soft robotics, sensing, and biomedical applications.

Effect of Side-Chain Branching on Enhancement of Ionic Conductivity and Capacity Retention of a Solid Copolymer Electrolyte Membrane.

Low current drain driven by the low ionic conductivity of a solid polymer electrolyte is one of the major obstacles of solid-state battery. In an effort to improve the ionic conductivity of a solid polymer electrolyte membrane (PEM), polyethylene glycol diacrylate (PEGDA) and monofunctional polyethylene glycol methyl ether acrylate (PEGMEA) were copolymerized via photopolymerization to afford a PEGDA network with dangling PEGMEA side chains. By attaching PEGMEA side branches to the PEGDA network backbone, the glass transition temperature (Tg) was found to decrease, which may be controlled by relative amounts of PEGMEA and PEGDA. Concurrently, the ionic conductivity of a co-PEM consisting of lithium bis(trifluoromethane)sulfonylimide (LiTFSI) salt and a succinonitrile plasticizer in the PEGMEA-co-PEGDA copolymer network was enhanced with increasing PEGMEA side branching. The relationship between the network Tg and ionic conductivity of the branched co-PEM was analyzed in the context of the Vogel-Tammann-Fulcher equation. The plasticized branched co-PEM network exhibited room-temperature ionic conductivity at a superionic conductor level of 10-3 S/cm. Of particular importance is the fact that excellent capacity retention at a high current rate (2 C) in charge/discharge cyclings of Li4Ti5O12/co-PEM/Li and LiFePO4/co-PEM/Li half-cells was achieved. This improved charge retention may be attributed to lower frictional surfaces of the electrodes afforded by side brushes, which probably alleviates formation of irreversible reaction byproducts at the electrode/electrolyte interface.

Controlled Directional Crystallization of Oligothiophenes Using Zone Annealing of Preseeded Thin Films.

We demonstrate a simple route to directionally grow crystals of oligothiophenes, based on 2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene with degrees of polymerization of 2 (BTTT-2) and 4 (BTTT-4) via zone annealing (ZA) of preseeded films. ZA of spun-cast films of BTTT-2 does not yield highly aligned crystals. However, if the film is oven-annealed briefly prior to ZA, highly aligned crystals that are millimeters in length can be grown, whose length depends on the velocity of the ZA front. The precrystallized region provides existing nuclei that promote crystal growth and limit nucleation of new crystals in the melted region. Aligned crystals of BTTT-2 can be obtained even when the moving velocity for ZA is an order of magnitude greater than the crystal growth rate. The relative nucleation rate to the crystallization rate for BTTT-4 is greater than that for BTTT-2, which decreases the length over which BTTT-4 can be aligned to ∼500 µm for the conditions examined. The temperature gradient and moving velocity of ZA enable control of the length of the aligned crystalline structure at the macroscale.

In vitro investigation of antioxidant, anti-Inflammatory, and antiplatelet adhesion properties of genistein-modified poly(ethersulfone)/poly(vinylpyrrolidone) hemodialysis membranes.

Hemocompatibility of genistein-modified poly(ethersulfone)/poly(vinylpyrrolidone) (PES/PVP) hemodialysis (HD) membranes has been investigated in vitro with emphasis on evaluation of cell viability, antioxidant, anti-inflammatory, and antiplatelet adhesion properties. Genistein modified PES/PVP membranes reveal significant reduction of the reactive oxygen species and also considerable suppression of interleukin-1ß and tumor necrosis factor-α levels in whole blood, but to a lesser extent ininterleukin-6. The incorporation of PVP into the HD membrane reduces platelet adhesion by virtue of its hydrophilicity. Of particular importance is that platelet adhesion of the genistein modified membranes declines noticeably at low concentrations of genistein for about 5-10%, beyond which it raises the number of adhered platelets. The initial decline in the platelet adhesion is attributable to genistein's ability to inhibit intercellular and/or vascular cell adhesion, whereas the reversal of this adhesion trend with further increase of genistein loading is ascribed to the inherent hydrophobicity of the genistein modified HD membrane.

Effects of glucose on cell viability and antioxidant and anti-inflammatory properties of phytochemicals and phytochemically modified membranes.

By virtue of antioxidant and anti-inflammable properties, plant-derived phytochemicals such as mangiferin and genistein have attracted considerable attention for functionalization of polymeric hemodialysis (HD) membranes via solution blending. In-vitro dihydrorhodamine (DHR) assay of the genistein-modified membranes revealed drastic reduction in the level of the reactive oxygen species (ROS). In contrast, mangiferin-modified HD membrane manifested the pro-oxidant activity. We suspected that such difference in ROS generation may be attributed to the glucose unit on the xanthone backbone of mangiferin. This hypothesis was confirmed by comparing the ROS levels of genistein versus genistin, and mangiferin versus xanthone and 3,4,5,6-tetrahydroxyxanthone. Phytochemicals without the glucose unit show better antioxidant property related to the glycosides. Anti-inflammatory property was further conducted by measuring the level of TNF-α in blood after contacting with the same selected phytochemicals. Of particular interest is that the glucose unit promotes the generation of TNF-α.

Genistein-modified poly(ethylene oxide)/poly(D,L-lactic acid) electrospun mats with improved antioxidant and anti-inflammatory properties.

Genistein is a phytochemical with a broad range of desirable biological activity for wound healing. However, its poor bioavailability requires developing a new method for fabricating an appropriate carrier vehicle to deliver genistein in a sustained manner. Based on the guidance afforded by the ternary phase diagram of poly(D,L-lactic acid) (PDLLA), poly(ethylene oxide) (PEO), and genistein blends, certain selective compositions were electrospun. We obtained a uniformly smooth surface morphology in unmodified and genistein-modified PEO/PDLLA fibers, documented by scanning electron microscopy. Moreover, wide-angle X-ray diffraction and 1H NMR studies revealed that the genistein molecules, successfully incorporated in the blends, remained chemically stable after electrospinning. Besides surface wettability and dimensional stability of the electrospun mats, the released genistein amount has been evaluated as a function of PEO concentration. Our biocompatibility investigations suggest that genistein-modified PEO/PDLLA electrospun mats exhibit strong antioxidant and anti-inflammatory activities which indicate they have potential applications for wound dressings.

Solubilization of genistein in poly(oxyethylene) through eutectic crystal melting.

Solid-liquid phase diagrams of binary crystalline blends of genistein with poly(ethylene oxide) (PEO) and poly(ethylene glycol) (PEG) were established experimentally and theoretically based on the combined Flory-Huggins free energy of liquid-liquid phase separation and the phase field free energy pertaining to crystal solidification. The liquidus lines obtained self-consistently were found to agree well with trends of depressed crystal melting transitions in genistein/PEO and genistein/PEG blends, exhibiting eutectic phase behavior. Of particular importance is the lowering of the eutectic temperature of the genistein/PEO blend by about 60 °C upon switching to the genistein/PEG system. The occurrence of interspecies hydrogen bonding between genistein molecules and both PEO and PEG chains, albeit weak, was noticed by Fourier transform infrared spectroscopy. The improved solubility of genistein in PEG can be attributed not only to lowering of the molecular weight of PEG utilized, but also to its terminal hydroxyl groups. This eutectic melting approach by PEG solvent is sufficiently effective in solubilizing genistein crystals that development of genistein-containing drugs might be feasible for injection and/or oral administration.

In vitro evaluation of antioxidant and anti-inflammatory properties of genistein-modified hemodialysis membranes.

Genistein-modified poly(amide):poly(vinyl pyrrolidone) (PA:PVP/G) hemodialysis membranes have been fabricated by coagulation via solvent (dimethyl sulfoxide, DMSO)/nonsolvent (water) exchange. The antioxidant and anti-inflammatory properties of the unmodified PA:PVP membranes were evaluated in vitro using human blood. It was found that these unmodified PA:PVP membranes were noncytotoxic to peripheral blood mononuclear cells (PBMC) but raised intracellular reactive oxygen species (ROS) levels. Pure genistein (in DMSO solution) was not only nontoxic to PBMC, but also suppressed the ROS levels in a manner dependent on genistein dosage. A similar dose-dependent suppression of ROS was found in genistein-modified PA (i.e., PA/G) membranes. However, the PVP addition had little or no effect in the suppression of ROS levels for the ternary PA:PVP/G system; the membrane ROS suppression was largely controlled by the genistein dosage. The levels of tumor necrosis factor-α (TNF-α), interleukin-1ß (IL-1ß), and interleukin (IL-6) in whole blood were measured by ex vivo stimulation with lipopolysaccharide (LPS). The unmodified PA:PVP membranes drastically increased the level of TNF-α; however, the concentration of IL-1ß and IL-6 remained almost the same. The PA/G membranes reduced the concentration of IL-1ß and TNF-α even at very low genistein loadings, but it required a higher genistein loading to realize a similar effect in the case of IL-6. Of particular importance is that the genistein-modified blend membranes (PA:PVP/G) showed greater suppression of the concentrations of all three cytokines (TNF-α, IL-1ß, and IL-6) in comparison with those of the PA/G membranes, signifying the role of PVP in the enhanced anti-inflammatory properties of these genistein-modified membranes. Ultraviolet-visible (UV-vis) spectroscopy was employed to quantify any genistein leaching during the in vitro testing.

Induced smectic phases in phase diagrams of binary nematic liquid crystal mixtures.

To elucidate induced smectic A and smectic B phases in binary nematic liquid crystal mixtures, a generalized thermodynamic model has been developed in the framework of a combined Flory-Huggins free energy for isotropic mixing, Maier-Saupe free energy for orientational ordering, McMillan free energy for smectic ordering, Chandrasekhar-Clark free energy for hexagonal ordering, and phase field free energy for crystal solidification. Although nematic constituents have no smectic phase, the complexation between these constituent liquid crystal molecules in their mixture resulted in a more stable ordered phase such as smectic A or B phases. Various phase transitions of crystal-smectic, smectic-nematic, and nematic-isotropic phases have been determined by minimizing the above combined free energies with respect to each order parameter of these mesophases. By changing the strengths of anisotropic interaction and hexagonal interaction parameters, the present model captures the induced smectic A or smectic B phases of the binary nematic mixtures. Of particular importance is the fact that the calculated phase diagrams show remarkable agreement with the experimental phase diagrams of binary nematic liquid crystal mixtures involving induced smectic A or induced smectic B phase.

Effect of trans-cis photoisomerization on phase equilibria and phase transition of liquid-crystalline azobenzene chromophore and its blends with reactive mesogenic diacrylate.

Photoisomerization-induced phase transition of neat liquid-crystalline azobenzene chromophore (LCAC) and its effect on phase diagrams of its mixtures with reactive mesogenic diacrylate monomer (RM257) have been investigated experimentally and theoretically. Upon irradiation with ultraviolet light, the nematic phase of LCAC transformed to isotropic, while the crystal phase showed corrugated textures on the surface (i.e., ripples). The phase-transition temperatures and corresponding morphologies of the blends have been investigated by means of differential scanning calorimetry and optical microscopy. A theoretical phase diagram of a binary nematic and crystalline system was constructed by self-consistently solving the combined free energies of Flory-Huggins, Maier-Saupe, and phase-field theory. The calculation revealed various coexistence regions such as nematic + liquid (N1 + L2), crystal + liquid (Cr1 + L2), crystal + nematic (Cr1 + N2), and crystal + crystal (Cr1 + Cr2) over a broad range of compositions including the single-phase nematic (N1, N2) of the corresponding constituents. The calculated liquidus lines were in good accord with the depressed mesophase-isotropic transition points. The present paper demonstrates the effect of trans-cis photoisomerization on the mesophase transitions of neat LCAC and the phase diagram of LCAC-RM257 as well as on the ripple formation (i.e., periodic undulation) on the azobenzene crystals.

Photochromism and photopolymerization induced mesophase transitions in mixtures of spiropyran and mesogenic diacrylate.

A phase diagram of a binary mixture of photochromic molecule (spiropyran) and mesogenic diacrylate monomer has been established by means of differential scanning calorimetry and polarized optical microscopy. Subsequently, a theoretical phase diagram has been calculated by self-consistently solving the combined Flory-Huggins free energy for isotropic mixing, Maier-Saupe free energy for nematic ordering, and phase field free energy for crystal solidification. The phase diagram thus obtained consists of various coexistence regions involving single-phase crystals, pure nematic, crystal + liquid, crystal + nematic, and crystal + crystal coexistence gaps. Under UV irradiation, both SP and SP/RM257 mixtures showed the lowering trend of the melting points, which may be attributed to the plasticization effect by the merocyanine isomers. When UV light is illuminated on the 2/98 SP/RM257 mixture for an extended period, mesogenic diacrylate in the mixtures gets polymerized, showing the permanent fixation of isotropic and nematic structures due to the network formation of RM257 caused by the biradicals in the merocyanine intermediate.

Eutectic mesophase transitions and induced crystalline phase in mixtures of hexagonal columnar liquid crystal and mesogenic diacrylate.

Eutectic behavior and the induced crystalline phase in mixtures of hexagonal columnar liquid crystal, 2, 3, 6, 7, 10, 11-hexakis-(pentyloxy) triphenylene (HPTP)/reactive mesogenic diacrylate monomer, 4-(3-acryloyloxypropyloxy)-benzoic acid 2-methyl-1, 4-phenylene ester (RM257) have been investigated both experimentally and theoretically. To determine the theoretical phase boundaries, we combined the free energy of Flory-Huggins free energy for liquid-liquid demixing, Maier-Saupe free energy of nematic ordering, and Chandrasekhar-Clark free energy of hexagonal ordering. The calculated phase diagram of the HPTP/RM257 blend is essentially of a eutectic type that consists of isotropic (I), nematic (N), and hexagonal columnar (Col(h)) regions, and nematic + isotropic (N+I), hexagonal columnar + isotropic (Col(h)+I), and hexagonal columnar + nematic (Col(h)+N) coexistence regions, bound by the liquidus and solidus lines. Of particular interest is the formation of an induced crystalline phase (Cr(in)) in the intermediate compositions of the HPTP/RM257 mixtures, exhibiting Cr(2) (RM257) + Cr(in) in the RM257-rich and Cr(in) + Cr(1) (HPTP) in the HPTP-rich regions.