Bistable electroactive polymers (BSEP) usually exhibit glass transition that spans a rather broad temperature range and are normally actuated above 70 °C. High actuation temperature limits the BSEP for wearable and personal assistive applications. A phase-changing polymer is synthesized and employed as BSEP having a narrow rigid-to-rubbery transition temperature range. Shape memory effect with both fixation and recovery rate close to 100% was observed. Diaphragm actuators of the BSEP can be electrically actuated at 50 °C up to 70% strain, and the deformed shape was fixed after cooling the BSEP below the transition temperature. The rigid-to-rigid actuation can be repeated for at least 10,000 cycles.
During the past decade, inorganic nanoparticles/polymer nanocomposites have been intensively studied to provide a low cost, high performance alternative for gamma scintillation. However, the aggregation of nanoparticles often occurs even at low nanoparticle concentrations and thus deteriorates the transparency and performance of these nanocomposite scintillators. Here we report an efficient fabrication protocol of transparent nanocomposite monoliths based on surface modified hafnium oxide nanoparticles. Using hafnium oxide nanoparticles with surface-grafted methacrylate groups, highly transparent bulk-size nanocomposite monoliths (2 mm thick, transmittance at 550 nm >75%) are fabricated with nanoparticle loadings up to 40 wt% (net hafnium wt% up to 28.5%). These nanocomposite monoliths of 1 cm diameter and 2 mm thickness are capable of producing a full energy photopeak for 662 keV gamma rays, with the best deconvoluted photopeak energy resolution reaching 8%.
Dielectric elastomers are useful for large-strain actuation and energy harvesting. Their application has been limited by their low dielectric constants and consequently high driving voltage. Various fillers with high dielectric constants have been incorporated into different elastomer systems to improve the actuation strain, force output and energy density of the compliant actuators and generators. However, agglomeration may happen in these nanocomposites, resulting in a decrease of dielectric strength, an increase of leakage current, and in many instances the degree of enhancement of the dielectric constant. In this work, we investigated aluminum nanoparticles as nanofillers for acrylate copolymers. This metallic nanoparticle was chosen because the availability of free electrons could potentially provide an infinite value of dielectric constant as opposed to dielectric materials including ferroelectric nanocrystals. Moreover, aluminum nanoparticles have a self-passivated oxide shell effectively preventing the formation of conductive path. The surfaces of the aluminum nanoparticles were functionalized with methacrylate groups to assist the uniform dispersion in organic solutions and additionally enable copolymerization with acrylate copolymer matrix during bulk polymerization, and thus to suppress large range drifting of the nanoparticles. The resulting Al nanoparticle-acrylate copolymer nanocomposites were found to exhibit higher dielectric constant and increased stiffness. The leakage current under high electric fields were significantly lower than nanocomposites synthesized without proper nanoparticle surface modification. The dielectric strengths of the composites were comparable with the pristine polymers. In dielectric actuation evaluation, the actuation force output and energy specific work density were enhanced in the nanocomposites compared to the pristine copolymer.
ABSTRACT: Bistable electroactive polymers (BSEP) amalgamating electrically induced large-strain actuation and shape memory effect present a unique opportunity for refreshable Braille displays. A new BSEP material with long-chain crosslinkers to achieve prolonged cycle lifetime of refreshable Braille displays is reported here. The modulus of the BSEP material decreases by more than three orders of magnitude from a rigid, plastic state to a rubbery state when heated above the polymer’s glass transition temperature. In its rubbery state, the polymer film can be electrically actuated to buckle convexly when a high voltage is applied across a circular active area. Modifying the concentration of long-chain crosslinkers in the polymer allows not only for fine-tuning of the polymer’s glass transition temperature and elasticity in the rubbery state, but also enhancement of the actuation stability. For a raised height of 0.4 mm by a Braille dot with a 1.3 mm diameter, actuation can be repeated over 2000 cycles at 70°C in the rubbery state. The actuated dome shape can be fixed by cooling the polymer below the glass transition temperature. This refreshable rigid-to-rigid actuation simultaneously provides large-strain actuation and large force support. Devices capable of displaying Braille characters over a page-size area consisting of 324 Braille cells have been fabricated.
The best performing dielectric elastomer materials reported thus far are commercial products manufactured for applications unrelated to electro-mechanical transduction. Prestretching is commonly employed to obtain high actuation strain and energy density. The limited knowledge of the polymers’ chemical structure makes it difficult to re-formulate the polymers for significantly improved overall material performance. We report the synthesis of new acrylate-based dielectric elastomers that exhibit high actuation strain without prestretching. Mixtures of commercial acrylate monomers and other additives are copolymerized by UV-initiated radical polymerization to form thin dielectric elastomer membranes. This processing can readily be scaled up to fabricate multi-layer stacked actuators with 11% linear actuation strain.
Dielectric Elastomers (DEs) can be actuated under high electric field to produce large strains. Most high-performing DE materials such as the 3M™ VHB™ membranes are commercial products designed for industrial pressure-sensitive adhesives. The limited knowledge of the exact chemical structures of these commercial materials has made it difficult to understand the relationship between molecular structures and electromechanical properties. In this work, new acrylic elastomers based on n-butyl acrylate and acrylic acid were synthesized from monomer solutions by UV-initiated bulk polymerization. The new acrylic copolymers have a potential to obtain high dielectric constant, actuation strain, dielectric strength, and a high energy density. Silicone and ester oligomer diacrylates were also added onto the copolymer structures to suppress crystallization and to crosslink the polymer chains. Four acrylic formulations were developed with different amounts of acrylic acid. This gives a tunable stiffness, while the dielectric constant is varied from 4.3 to 7.1. The figure-of-merit performance of the best formulation is 186 % area strain, 222 MV/m of dielectric strength, and 2.7 MJ/m3 of energy density. To overcome electromechanical instability, different prestrain ratios were investigated, and under the optimized prestrain, the material has a lifetime of thousands of cycles at 120 % area strain.
Polymer composite electrodes based on silver nanowires or carbon nanotubes have been prepared with
transparency and surface conductivity approaching those of ITO/glass and better than ITO/PET. The
conductive surface has an average roughness less than 10 nm, better than ITO/glass. Depending on the
polymer matrix selected, the composite electrodes can be made rigid, flexible like polycarbonate, or
stretchable like a rubber. Various polymer light emitting diodes,light emitting electrochemical cells and
polymer solar cells have been fabricated using the composite electrode as anode, exhibiting
electroluminescent efficiencies generally higher than control devices fabricated on ITO/glass. These
polymer light emitting devices are all highly flexible and can be bent to less than 3 mm radius without
loss of performance. With further modification of the composite electrodes, we have also demonstrated
stretchable OLEDs wherein the emissive area can be elongated by as much as 50%.
Dielectric elastomer actuators (DEA) and bistable electroactive polymers (BSEP) both require compliant
electrodes with rubbery elasticity and high conductivity at large strains. Stretchable opto-electronic
devices additionally require the compliant electrodes to be optically transparent. Many candidate
materials have been investigated. We report a new approach to mechanically robust, stretchable compliant
electrodes. A facile in-situ composite synthesis and transfer technique is employed, and the resulting
composite electrodes retain the high surface conductivity of the original conductive network formed by
nanowires or nanotubes, while exhibiting the mechanical flexibility of the matrix polymer. The composite
electrodes have high transparency and low surface roughness useful for the fabrication of polymer thinfilm
electronic devices. The new electrodes are suitable for high-strain actuation, as a complaint resistive
heating element to administer the temperature of shape memory polymers, and as the charge injection
electrodes for flexible/stretchable polymer light emitting diodes. Bistable electroactive polymers
employing the composite electrodes can be actuated to large strains via heating-actuation-cooling cycles.
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