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.

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.

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.

Enhanced Room-Temperature Photoluminescence Quantum Yield in Morphology Controlled J-Aggregates.

Supramolecular assemblies from organic dyes forming J-aggregates are known to exhibit narrowband photoluminescence with full-width at half maximum of ≈9 nm (260 cm-1). Applications of these high color purity emitters, however, are hampered by the rather low photoluminescence quantum yields reported for cyanine J-aggregates, even when formed in solution. Here, it is demonstrated that cyanine J-aggregates can reach an order of magnitude higher photoluminescence quantum yield (increase from 5% to 60%) in blend solutions of water and alkylamines at room temperature. By means of time-resolved photoluminescence studies, an increase in the exciton lifetime as a result of the suppression of non-radiative processes is shown. Small-angle neutron scattering studies suggest a necessary condition for the formation of such highly emissive J-aggregates: the presence of a sharp water/amine interface for J-aggregate assembly and the coexistence of nanoscale-sized water and amine domains to restrict the J-aggregate size and solubilize monomers, respectively.

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.

Cyanine platelet single crystals: growth, crystal structure and optical spectra.

Crystalline organic semiconducting materials are much in demand for multiple electronic and optoelectronic device applications. Here, solution grown ultrathin rhombic crystals of a trimethine carbocyanine anionic dye are used to establish relationships between structural and optical properties. The dye crystallized in the monoclinic space group P21/c featuring alternating layers of molecules in two different herringbone type patterns, with perchlorate counterions located mostly within one of the two layers. Micro transmittance spectroscopy revealed a broadened spectrum compared to those obtained in solution and in an amorphous thin film. Using polarized light, transmission spectroscopy revealed strong low-energy and weak high-energy bands polarized along the crystallographic b- and c-axis, respectively. Using the extended dipole approximation, significant exciton couplings are predicted between neighboring molecules in the crystal, of the order of the intrinsic monomer reorganization energies associated with nuclear relaxation after excitation, depicting a complex spectral scenario. The exciton coupling pattern explains the relative energies of the b- and c-polarized components but the observed intensities are opposite to expectations based on chromophore alignment within the crystal.

One-Dimensional Organic-Inorganic Hybrid Perovskite Incorporating Near-Infrared-Absorbing Cyanine Cations.

Hybrid perovskite crystals with organic and inorganic structural components are able to combine desirable properties from both classes of materials. Electronic interactions between the anionic inorganic framework and functional organic cations (such as chromophores or semiconductors) can give rise to unusual photophysical properties. Cyanine dyes are a well known class of cationic organic dyes with high extinction coefficients and tunable absorption maxima all over the visible and near-infrared spectrum. Here we present the synthesis and characterization of an original 1D hybrid perovskite composed of NIR-absorbing cyanine cations and polyanionic lead halide chains. This first demonstration of a cyanine-perovskite hybrid material is paving the way to a new class of compounds with great potential for applications in photonic devices.

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.

Resonance light scattering in dye-aggregates forming in dewetting droplets.

Small organic semiconducting molecules assembling into supramolecular J- and H- aggregates have attracted much attention due to outstanding optoelectronic properties. However, their easy and reproducible fabrication is not yet sufficiently developed for industrial applications, except for silver halide photography. Here we present a method based on aggregate precipitation during the phase separation and dewetting of the evaporating dye precursor solution. The smaller the precursor droplets, the more pronounced the J-aggregation. The aggregates cause the films to resonantly scatter incoming light. Because the dye aggregate extinction resonances have narrowest bandwidths, a wavelength selectivity is observed that exceeds the selectivity of localized surface plasmon resonances. The aggregation mechanism can be easily applied to periodically structured substrates, making the method appealing for photonic applications. We demonstrate this point with a 2D grating, where the narrow absorption range of the aggregates leads to wavelength specific (one color only) scattering.

Interface dipoles for tuning energy level alignment in organic thin film devices.

One of the key features of organic optoelectronic and electronic devices resides in the multilayer architecture of the device stack. The performance of the latter strongly depends on the interface quality between organic layers or at the electrode heterojunction. Apart from interface thermodynamics governing adhesion and wetting, the electronic energy levels of the organic semiconductor are affected by the interface properties in a drastic way. This mini review gives a short overview on the possibilities to adjust frontier orbital energy levels using oriented electrical dipoles at the interfaces.

Sequential printing by laser-induced forward transfer to fabricate a polymer light-emitting diode pixel.

Patterned deposition of polymer light-emitting diode (PLED) pixels is a challenge for electronic display applications. PLEDs have additional problems requiring solvent orthogonality of different materials in adjacent layers. We present the fabrication of a PLED pixel by the sequential deposition of two different layers with laser-induced forward transfer (LIFT), a "dry" deposition technique. This novel use of LIFT has been compared to "normal" LIFT, where all the layers are transferred in a single step, and a conventional PLED fabrication process. For the sequential LIFT, a 50-nm film of an alcohol-soluble polyfluorene (PFN) is transferred onto a receiver with a transparent anode, before an aluminum cathode is transferred on top. Both steps use a triazene polymer dynamic release layer and are performed in a medium vacuum (1 mbar) across a 15 µm gap. The rough morphologies of the single-layer PFN pixels and the PLED device characteristics have been investigated and compared to both bilayer Al/PFN pixels fabricated by normal LIFT and conventionally fabricated devices. The functionality of the sequential LIFT pixels (0.003 cd/A, up to 200 mA/cm(2), at 30-40 V) demonstrates the suitability of LIFT for sequential patterned printing of different thin-film layers.