Pyroelectricity in poled all-organic polar polynorbornene/polydimethylsiloxane-based stretchable electrets.

Pyroelectricity in a recently developed all-organic composite electret with a polar polynorbornene-based filler and polydimethylsiloxane (PDMS) matrix has been studied with the help of thermal and dielectric techniques. Measurement of the pyroelectric p coefficient using a quasi-static periodic temperature variation at RT shows a non-linear dependence with the applied poling field, which is uncharacteristic of amorphous materials. Dielectric relaxation spectroscopy (DRS) and the thermally stimulated depolarization current (TSDC) technique reveal that this behaviour can be attributed to Maxwell-Wagner interface (MWI) polarization that occurs at the filler-matrix interface. These charges released during the onset of dipolar α and ß relaxations of the filler particles contribute majorly to the observed pyroelectricity at RT. The saturation of both MWI TSDC shoulders and spontaneous polarization at higher electric fields correlates with the p coefficient value reaching a plateau at these applied fields. A maximum p coefficient of 0.54 µC m-2 K-1 is calculated for a poling field of 30 V µm-1.

Synthesis of Bottlebrush Polymers with Spontaneous Self-Assembly for Dielectric Generators.

Cross-linked bottlebrush polymers received significant attention as dielectrics in transducers due to their unique softness and strain stiffening caused by their structure. Despite some progress, there is still a great challenge in increasing their dielectric permittivity beyond 3.5 and cross-linking them to defect-free ultrathin films efficiently under ambient conditions. Here, we report the synthesis of bottlebrush copolymers based on ring-opening metathesis polymerization (ROMP) starting from a 5-norbornene-2-carbonitrile and a norbornene modified with a poly(dimethylsiloxane) (PDMS) chain as a macromonomer. The resulting copolymer was subjected to a postpolymerization modification, whereby the double bonds were used both for functionalization with thiopropionitrile and subsequent cross-linking via a thiol-ene reaction. The solutions of both bottlebrush copolymers formed free-standing elastic films by simple casting. DMA and broadband impedance spectroscopy revealed two glass transition temperatures uncommon for a random copolymer. The self-segregation of the nonpolar PDMS chains and the polynorbornane backbone is responsible for this and is supported by the interfacial polarization observed in broadband impedance spectroscopy and the scattering peaks observed in small-angle X-ray scattering (SAXS). Additionally, the modified bottlebrush copolymer was cross-linked to an elastomer that exhibits increased dielectric permittivity and good mechanical properties with significant strain stiffening, an attractive property of dielectric elastomer generators. It has a relative permittivity of 5.24, strain at break of 290%, elastic modulus at 10% strain of 380 kPa, a breakdown field of 62 V µm-1, and a small actuation of 5% at high electric fields of 48.5 V µm-1. All of these characteristics are attractive for dielectric elastomer generator applications. The current work is a milestone in designing functional elastomers based on bottlebrush polymers for transducer applications.

Nitrile-functionalized Poly(siloxane) as Electrolytes for High-Energy-Density Solid-State Li Batteries.

In the quest to replace liquid Li-ion electrolytes with safer and non-toxic solid counterparts for Li-ion batteries, polysiloxane polymers have attracted considerable attention as they offer low glass transition temperatures, stability with metallic lithium, and versatility in chemical functionalization of the backbone. Herein, we present the synthesis of Li-ion conductive polysiloxane-based polymers functionalized with 60 % nitrile groups per chain unit. The synthesis procedure is based on the reaction of poly-(dimethylsiloxane-co-methylvinylsiloxane) polymer with 2-cyanoethanethiol, followed by the addition of lithium bis (trifluoromethanesulfonyl) imide. The presented polysiloxane-based polymers exhibit exceptionally high ionic conductivity up to 0.375 mS cm-1 at 60 °C and Li+ ion transfer number of 0.73, one of the highest reported for polymer Li-ion conducting electrolytes. Their electrochemical performance was evaluated in both symmetrical and full-cell configurations to test the utility of synthesized polymers as electrolytes in Li-ion batteries.

Synthesis of polysiloxane elastomers modified with sulfonyl side groups and their electromechanical response.

Dielectric elastomer transducers are elastic capacitors that respond to mechanical or electrical stress. They can be used in applications such as millimeter-sized soft robots and harvesters of the energy contained in ocean waves. The dielectric component of these capacitors is a thin elastic film, preferably made of a material having a high dielectric permittivity. When properly designed, these materials convert electrical energy into mechanical energy and vice versa, as well as thermal energy into electrical energy and vice versa. Whether a polymer can be used for one or the other application is determined by its glass transition temperature (Tg), which should be significantly below room temperature for the former and around room temperature for the latter function. Herein, we report a polysiloxane elastomer modified with polar sulfonyl side groups to contribute to this field with a powerful new material. This material has a dielectric permittivity as high as 18.4 at 10 kHz and 20 °C, a relatively low conductivity of 5 × 10-10 S cm-1, and a large actuation strain of 12% at an electric field of 11.4 V µm-1 (0.25 Hz and 400 V). At 0.5 Hz and 400 V, the actuator showed a stable actuation of 9% over 1000 cycles. The material exhibited a Tg of -13.6 °C, which although is well below room temperature affected the material's response in actuators, which shows significant differences in the response at different frequencies and temperatures and in films with different thicknesses.

On-Demand Cross-Linkable Bottlebrush Polymers for Voltage-Driven Artificial Muscles.

Dielectric elastomer actuators (DEAs) generate motion resembling natural muscles in reliability, adaptability, elongation, and frequency of operation. They are highly attractive in implantable soft robots or artificial organs. However, many applications of such devices are hindered by the high driving voltage required for operation, which exceeds the safety threshold for the human body. Although the driving voltage can be reduced by decreasing the thickness and the elastic modulus, soft materials are prone to electromechanical instability (EMI), which causes dielectric breakdown. The elastomers made by cross-linking bottlebrush polymers are promising for achieving DEAs that suppress EMI. In previous work, they were chemically cross-linked using an in situ free-radical UV-induced polymerization, which is oxygen-sensitive and does not allow the formation of thin films. Therefore, the respective actuators were operated at voltages above 4000 V. Herein, macromonomers that can be polymerized by ring-opening metathesis polymerization and subsequently cross-linked via a UV-induced thiol-ene click reaction are developed. They allow us to fast cross-link defect-free thin films with a thickness below 100 µm. The dielectric films give up to 12% lateral actuation at 1000 V and survive more than 10,000 cycles at frequencies up to 10 Hz. The easy and efficient preparation approach of the defect-free thin films under air provides easy accessibility to bottlebrush polymeric materials for future research. Additionally, the desired properties, actuation under low voltage, and long lifetime revealed the potential of the developed materials in soft robotic implantable devices. Furthermore, the C-C double bonds in the polymer backbone allow for chemical modification with polar groups and increase the materials' dielectric permittivity to a value of 5.5, which is the highest value of dielectric permittivity for a cross-linked bottlebrush polymer.

Transient Elastomers with High Dielectric Permittivity for Actuators, Sensors, and Beyond.

Dielectric elastomers (DEs) are key materials in actuators, sensors, energy harvesters, and stretchable electronics. These devices find applications in important emerging fields such as personalized medicine, renewable energy, and soft robotics. However, even after years of research, it is still a great challenge to achieve DEs with increased dielectric permittivity and fast recovery of initial shape when subjected to mechanical and electrical stress. Additionally, high dielectric permittivity elastomers that show reliable performance but disintegrate under normal environmental conditions are not known. Here, we show that polysiloxanes modified with amide groups give elastomers with a dielectric permittivity of 21, which is 7 times higher than regular silicone rubber, a strain at break that can reach 150%, and a mechanical loss factor tan δ below 0.05 at low frequencies. Actuators constructed from these elastomers respond to a low electric field of 6.2 V µm-1, giving reliable lateral actuation of 4% for more than 30 000 cycles at 5 Hz. One survived 450 000 cycles at 10 Hz and 3.6 V µm-1. The best actuator shows 10% lateral strain at 7.5 V µm-1. Capacitive sensors offer a more than a 6-fold increase in sensitivity compared to standard silicone elastomers. The disintegrated material can be re-cross-linked when heated to elevated temperatures. In the future, our material could be used as dielectric in transient actuators, sensors, security devices, and disposable electronic patches for health monitoring.

Self-Healable, Self-Repairable, and Recyclable Electrically Responsive Artificial Muscles.

Elastomers with high dielectric permittivity that self-heal after electric breakdown and mechanical damage are important in the emerging field of artificial muscles. Here, a one-step process toward self-healable, silicone-based elastomers with large and tunable permittivity is reported. Anionic ring-opening polymerization of cyanopropyl-substituted cyclic siloxanes yields elastomers with polar side chains. The equilibrated product is composed of networks, linear chains, and cyclic compounds. The ratio between the components varies with temperature and allows realizing materials with largely different properties. The silanolate end groups remain active, which is the key to self-healing. Elastomeric behavior is observed at room temperature, while viscous flow dominates at higher temperatures (typically 80 °C). The elasticity is essential for reversible actuation and the thermoreversible softening allows for self-healing and recycling. The dielectric permittivity can be increased to a maximum value of 18.1 by varying the polar group content. Single-layer actuators show 3.8% lateral actuation at 5.2 V µm-1 and self-repair after a breakdown, while damaged ones can be recycled integrally. Stack actuators reach an actuation strain of 5.4 ± 0.2% at electric fields as low as 3.2 V µm-1 and are therefore promising for applications as artificial muscles in soft robotics.

Steering Self-Assembly of Three-Dimensional Iptycenes on Au(111) by Tuning Molecule-Surface Interactions.

Self-assembly of three-dimensional molecules is scarcely studied on surfaces. Their modes of adsorption can exhibit far greater variability compared to (nearly) planar molecules that adsorb mostly flat on surfaces. This additional degree of freedom can have decisive consequences for the expression of intermolecular binding motifs, hence the formation of supramolecular structures. The determining molecule-surface interactions can be widely tuned, thereby providing a new powerful lever for crystal engineering in two dimensions. Here, we study the self-assembly of triptycene derivatives with anthracene blades on Au(111) by Scanning Tunneling Microscopy, Near Edge X-ray Absorption Fine Structure and Density Functional Theory. The impact of molecule-surface interactions was experimentally tested by comparing pristine with iodine-passivated Au(111) surfaces. Thereby, we observed a fundamental change of the adsorption mode that triggered self-assembly of an entirely different structure.

Synthesis of polar polynorbornenes with high dielectric relaxation strength as candidate materials for dielectric applications.

Materials with high dielectric permittivity and dielectric relaxation strength are sought for thermal and pressure sensors and electrical energy generators. However, most polymers have either too low dielectric permittivity or are so polar that their glass transition temperature (T g) is too high and thus decomposition and side reactions occur before an electric field can polarize the polar groups. Here, we use the power and versatility of ring-opening metathesis polymerization (ROMP) to synthesize polar polymers with high dielectric relaxation strength and T g significantly below the decomposition temperature. We first synthesized six polar norbornene monomers by conventional esterification, which were then polymerized by ROMP using Grubbs first- and third-generation catalysts. The structure of the polynorbornenes obtained were verified by multinuclear NMR spectroscopy, molecular weights determined by gel permeation chromatography (GPC), and thermal properties evaluated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Additionally, their dielectric permittivity, conductivity, and dielectric losses were measured at different temperatures and frequencies ranging between 0.1 and 106 Hz.

Solvent-Free Synthesis and Processing of Conductive Elastomer Composites for Green Dielectric Elastomer Transducers.

Stretchable electrodes are more suitable for dielectric elastomer transducers (DET) the closer the mechanical characteristics of the electrodes and elastomer are. Here, a solvent-free synthesis and processing of conductive composites with excellent electrical and mechanical properties for transducers are presented. The composites are prepared by in situ polymerization of cyclosiloxane monomers in the presence of graphene nanoplatelets. The low viscosity of the monomer allows for easy dispersion of the filler, eliminating the need for a solvent. After the polymerization, a cross-linking agent is added at room temperature, the composite is solvent-free screen-printed, and the cross-linking reaction is initiated by heating. The best material shows conductivity σ = 8.2 S cm-1 , Young's modulus Y10%  = 167 kPa, and strain at break s = 305%. The electrode withstands large strains without delamination, shows no conductivity losses during repeated operation for 500 000 cycles, and has an excellent recovery of electrical properties upon being stretched at strains of up to 180%. Reliable prototype capacitive sensors and stack actuators are manufactured by screen-printing the conductive composite on the dielectric film. Stack actuators manufactured from dielectric and conductive materials that are synthesized solvent-free are demonstrated. The stack actuators even self-repair after a breakdown event.

Electrically Driven Artificial Muscles Using Novel Polysiloxane Elastomers Modified with Nitroaniline Push-Pull Moieties.

The synthesis of novel dielectric elastomers that show a muscle-like actuation when exposed to a low electric field represents a major challenge in materials science. Silicone elastomers modified with polar side groups are among the most attractive dielectrics for such a purpose because of their high polarizability over a wide temperature and frequency range. Nitroaniline (NA) has a strong dipole moment, and therefore, its incorporation into silicone networks should allow the formation of elastomers with increased dielectric permittivity. However, incorporation of a large amount of NA into silicone needed to increase the dielectric permittivity is still challenging. In this work, we present the synthesis of polysiloxane elastomers modified with a large fraction of the nitroaniline (NA) polar group, following two different synthetic strategies. Both approaches allowed the formation of homogenous elastomers at the molecular level. These yellowish materials have a dielectric permittivity three times higher as compared to the reported NA-modified silicones. Additionally, they have excellent mechanical properties with low viscoelastic losses and a strain at break reaching 300%. Furthermore, the mechanical properties of these elastomers can be easily tuned by the content of cross-linkers used. The developed elastomers are highly stable in electromechanical tests and show an actuation strain of 8% at unprecedentedly low electric fields of 7.5 V/µm. The combination of properties such as high dielectric permittivity, large strain at break, low viscoelastic losses, fast and reversible actuation, and actuation at low electric fields is crucial for the new generation of dielectric elastomer materials that will find their way in applications ranging from artificial muscles, soft robots, sensors, and haptic displays to electronic skin.

Development of carbohydrate functionalized magnetic nanoparticles for aminoglycosides magnetic solid phase extraction.

Magnetic nanoparticles decorated with d-galactose and galactitol (Fe3O4@SiN-galactose and Fe3O4@SiN-galactitol) were synthesized and employed as sorbent in a magnetic solid phase extraction (MSPE) procedure prior the analysis of aminoglycosides (AGs) in honey samples by LC-MS/MS. AGs are broad spectrum antibiotics, characterized by aminosugars, widespread used in therapeutic and veterinary applications. AGs can be found in the environment and food of animal origin. Fe3O4@SiN-galactose and Fe3O4@SiN-galactitol were synthesized via copper catalyzed alkyne azide cycloaddition and the synthesis was efficiently followed by infrared spectroscopy. They were characterized by electron microscopy, thermogravimetric analysis and magnetization curves. The nature of the loading (acetonitrile:water, 50:50 v/v) and elution solution (formic acid 190 mM) were studied in order to optimize the MSPE. Quantitative difference between MSPE with Fe3O4@SiN-galactose and MSPE with Fe3O4@SiN-galactitol in terms of recovery was found. The final optimized method using Fe3O4@SiN-galactose and Fe3O4@SiN-galactitol was applied in the determination of AGs in honey. The MSPE performance of Fe3O4@SiN-galactitol was found to be superior to that of MSPE with Fe3O4@SiN-galactose. The limits of quantification were between 2 and 19 µg kg-1 for amikacin, dihydrostreptomycin, tobramicyn and gentamycin. A good correlation between predicted and nominal values of AGs in honey was found (trueness from 84% to 109%). This MSPE procedure not only requires a minimum amount of sorbent (1 mg) and sample (0.2 g), but it can also be accomplish in a rather short time.

Conductive silicone elastomers electrodes processable by screen printing.

Conductive inks consisting of graphene and carbon black conductive fillers into a polydimethylsiloxane (PDMS) matrix, which can be processed into thin films by screen printing are developed. The influence of filler composition and content on mechanical and electrical properties of the conductive composites is investigated. The best composites were evaluated as electrode material for dielectric elastomer actuators and for piezoelectric sensors. With increasing filler content, the electrical properties of the resulting composites of graphite nanoplates (GNPs) or a binary mixture of GNPs and carbon black (CB) with PDMS (Mw = 139 kg/mol) are enhanced. Hence, PDMS composites filled with GNPs (42 wt.%) or a binary mixture of GNPs/CB (300/150 ratio, 30 wt.% of total filler loading) exhibited constant contact resistance values of 0.5 and 5 Ω determined in life-cycle test, respectively, thus rendering them suitable as electrode materials for piezosensors. On the other hand, dielectric elastomer actuators require more flexible electrode materials, which could be tuned by varying the polymer molecular weight and by reducing the filler content. Therefore, a composite consisting of PDMS (Mw = 692 kg/mol) and a binary filler mixture of GNPs/CB (150/75 ratio, 18 wt.% of total filler loading) was used for producing the electrodes of dielectric elastomer transducers (DETs). The produced DETs with different electrode thicknesses were characterized in terms of their performance. The negligible hysteresis of the electrode materials is favorable for sensor and actuator applications.

Artificial Muscles: Dielectric Elastomers Responsive to Low Voltages.

The lack of soft high-dielectric-permittivity elastomers responsive to a low voltage has been a long-standing obstacle for the industrialization of dielectric elastomer actuators (DEA) technology. Here, elastomers that not only possess a high dielectric permittivity of 18 and good elastic and insulating properties but are also processable in very thin films by conventional techniques are reported. Additionally, the elastic modulus can be easily tuned. A soft elastomer with a storage modulus of E = 350 kPa, a tanδ = 0.007 at 0.05 Hz, and a lateral actuation strain of 13% at 13 V µm-1 is prepared. A stable lateral actuation over 50 000 cycles at 10 Hz is demonstrated. A stiffer elastomer with an E = 790 kPa, a tanδ = 0.0018 at 0.05 Hz, a large out-of-plane actuation at 41 V µm-1 , and breakdown fields of almost 100 V µm-1 is also developed. Such breakdown fields are the highest ever reported for a high-permittivity elastomer. Additionally, actuators operable at a voltage as low as 200 V are also demonstrated. Because the materials used are cheap and easily available, and the chemical reactions leading to them allow upscaling, they have the potential to advance the DEA technology.

Synthesis of novel cyclosiloxane monomers containing push-pull moieties and their anionic ring opening polymerization.

The synthesis of three novel tetracyclosiloxane monomers modified either with a nitroaniline (NA) or with a Disperse Red 1 (DR1) push-pull group and their ring opening polymerization reaction in the presence of tetramethylammonium hydroxide are presented. The prepared monomers and polymers were characterized by different spectral methods and gel permeation chromatography. For the crystalline monomers, the structures were further proven by single crystal X-ray diffraction. Dynamic scanning calorimetry shows that the polymers that carry NA groups have a glass transition temperature (T g) well below room temperature (RT), while the one that carries DR1 groups melts at 55 °C. The transition temperatures have a strong effect on permittivity as indicated by broadband impedance spectroscopy measurements conducted at different temperatures and frequencies. The polymers modified with NA groups have a high permittivity (maximum value of 17.3) at RT, suggesting the polar groups to be mobile and orientation polarization to be effective. However, the polar groups of the polymer modified with DR1 are frozen and thus cannot contribute to the permittivity via orientation polarization. Consequently, the permittivity is only 8.8 at RT, but increases to 22 above the melting temperature, where the dipoles are mobile. Because of the high dielectric permittivity and rather low T g, the polymers modified with NA are attractive as active dielectric materials in actuators, capacitors, and stretchable electronics, whereas the polymer modified with DR1 may be of interest in nonlinear optical devices.

Polar Elastomers as Novel Materials for Electromechanical Actuator Applications.

Dielectric elastomer actuators are stretchable capacitors capable of a musclelike actuation when charged. They will one day be used to replace malfunctioning muscles supposing the driving voltage can be reduced below 24 V. This focus here is on polar dielectric elastomers and their behavior under an electric field. Emphasis is placed on all the features that are correlated with the molecular structure, its synthetic realization, and its impact on properties. Regarding the polymer class, the focus, to some degree, is on polysiloxanes because of their attractively low glass transition temperatures. This enables introduction of highly polar groups to the backbone while maintaining soft elastic properties. The goal is to provide a few guidelines for future research in this emerging field that may be useful for those considering entering this fascinating endeavor. Because of the large number of materials available, a few restrictions in the selection have to be applied.

An All-Organic Elastomeric Electret Composite.

Copolymer nanoparticles with a highly polar repeating unit are blended in an elastic matrix and poled at elevated temperatures. The composite exhibits piezoelectricity due to the overall polarization imparted by the particles, which can be easily modulated thanks to the soft matrix.

A set of homologous hetarylenediyne macrocycles by oxidative acetylene-acetylene coupling.

The synthesis of a set of bipyridine-containing macrocycles by oxidative acetylene-acetylene dimerization is described. The cycles are separated by preparative GPC, and the smallest homologue is analyzed by single-crystal X-ray diffraction, which shows a layered structure with channels that are partially filled with parts of the flexible chains of adjacent macrocycles. The cyclic trimer has a D3h symmetry and is a possible candidate for the formation of metal organic supramolecular assemblies on surfaces.

Photoinduced energy- and electron-transfer processes in dinuclear Ru(II)-Os(II), Ru(II)-Os(III), and Ru(III)-Os(II) trisbipyridine complexes containing a shape-persistent macrocyclic spacer.

The PF6- salt of the dinuclear [(bpy)2Ru(1)Os(bpy)2]4+ complex, where 1 is a phenylacetylene macrocycle which incorporates two 2,2'-bipyridine (bpy) chelating units in opposite sites of its shape-persistent structure, was prepared. In acetonitrile solution, the Ru- and Os-based units display their characteristic absorption spectra and electrochemical properties as in the parent homodinuclear compounds. The luminescence spectrum, however, shows that the emission band of the Ru(II) unit is almost completely quenched with concomitant sensitization of the emission of the Os(II) unit. Electronic energy transfer from the Ru(II) to the Os(II) unit takes place by two distinct processes (k(en) = 2.0x10(8) and 2.2x10(7) s(-1) at 298 K). Oxidation of the Os(II) unit of [(bpy)2Ru(1)Os(bpy)2]4+ by Ce(IV) or nitric acid leads quantitatively to the [(bpy)2Ru(II)(1)Os(III)(bpy)2]5+ complex which exhibits a bpy-to-Os(III) charge-transfer band at 720 nm (epsilon(max) = 250 M(-1) cm(-1)). Light excitation of the Ru(II) unit of [(bpy)2Ru(II)(1)Os(III)(bpy)2]5+ is followed by electron transfer from the Ru(II) to the Os(III) unit (k(el,f) = 1.6x10(8) and 2.7x10(7) s(-1)), resulting in the transient formation of the [(bpy)2Ru(III)(1)Os(II)(bpy)2]5+ complex. The latter species relaxes to the [(bpy)2Ru(II)(1)Os(III)(bpy)2]5+ one by back electron transfer (k(el,b) = 9.1x10(7) and 1.2x10(7) s(-1)). The biexponential decays of the [(bpy)2*Ru(II)(1)Os(II)(bpy)2]4+, [(bpy)2*Ru(II)(1)Os(III)(bpy)2]5+, and [(bpy)2Ru(III)(1)Os(II)(bpy)2]5+ species are related to the presence of two conformers, as expected because of the steric hindrance between hydrogen atoms of the pyridine and phenyl rings. Comparison of the results obtained with those previously reported for other Ru-Os polypyridine complexes shows that the macrocyclic ligand 1 is a relatively poor conducting bridge.