Dorina M. Opris

Frequency dependent dielectric and mechanical behavior of elastomers for actuator applications

The low frequency mechanical and dielectric behavior of three different elastomers has been investigated by dynamic mechanical analysis and dielectric spectroscopy, with the aim of accounting for the frequency dependence of the characteristics of the corresponding dielectric elastomer actuators. Satisfactory agreement was obtained between the dynamic response of the actuators and a simple model based on the experimental data for the elastomers, assuming that the relatively large prestrains employed in the actuators to have little influence on the frequency dependence of their effective moduli. It was thus demonstrated that the frequency dependence of the actuator strain is dominated by that of the mechanical response of the elastomer, and that the frequency dependence of the dielectric properties has a relatively minor influence on the actuator performance.

Synthesis and characterization of silicones containing cyanopropyl groups and their use in dielectric elastomer actuators

Polydimethylsiloxanes (PDMS) have recently been used as dielectric elastomer materials in electromechanical actuators. When they are soft enough, electric fields can change their shape. However, due to their low dielectric permittivity, large electric fields are required to induce a change. The approach presented here is to chemically modify silicones with cyanopropyl groups in order to increase their permittivity. Samples containing repeat units with cyanopropyl groups from 3 to 23% were synthesized, different methods being employed. The prepared polymers were cross-linked into thin films. The dielectric permittivity of these films increased from 2.4 (for the silicone matrix) to 6.5 for a film containing about 23% of cyanopropyl repeat units. The most promising materials were further optimized to meet the requirements for actuators and their electromechanical properties were investigated. Material A for example, which is a blend of PDMS and cyanopropyl-modified silicone, has a permittivity of 3.5 and higher moduli of elasticity as compared to the matrix but nevertheless shows 10% actuation strain at 40 V μm−1 which is a factor of 3.8 larger as compared to the matrix (2.6% actuation strain at the same voltage).

Highly stretchable dielectric elastomer composites containing high volume fractions of silver nanoparticles

The dielectric permittivity (e′) of a polymeric material can be significantly increased when blended with conductive fillers at concentrations approaching the percolation threshold. However, reproducible synthesis of such composites is after decades of research still a major challenge and a bottleneck for their application. Difficulties arise in controlling the size and shape of the filler as well as in its homogenous distribution within the composite. These parameters strongly affect the dielectric as well as mechanical properties of the composite. While a substantial amount of literature deals with the influence of conductive fillers on the dielectric properties of composites, little is known about their mechanical properties. It is therefore still an important goal to synthesize materials with simultaneously high e′ and good mechanical properties. Here, we report the synthesis of dielectric elastomers that combine key properties such as high flexibility and stretchability, high thermal stability, increased e′, low dielectric loss and conductivity. Such materials were prepared by solution processing using quasi-spherical silver nanoparticles (AgNPs) of a defined size in a polydimethylsiloxane matrix (Mw = 692 kDa). To prevent percolation, the AgNPs were coated with a thin silica shell (<4 nm). To increase their compatibility with the silicone matrix, these core–shell nanoparticles were passivated with a silane reagent. The insulating silica shell around the particles precisely defines the minimum approach distance between the cores as twice the shell thickness. The dielectric properties of the passivated particles (filler) were measured in pellets and found to have an almost frequency independent value of e′ = 90 and a very low loss factor tan δ = 0.023 at high frequencies. When such particles were used as fillers in a polydimethylsiloxane matrix, composites with low dielectric losses were obtained. A composite containing a 31 vol% filler with e′ = 21 and tan δ = 0.03 at ∼1 kHz was achieved. At a AgNP volume fraction of 20%, the composite has e′ = 5.9 at ∼1 kHz, a dielectric strength of 13.4 V μm−1, an elastic modulus as low as 350 kPa at 100% strain, and a strain at break of 800%. Due to the high specific energy density per volume at low electric fields, these composites are attractive materials in applications involving low electric fields.

Chemical modification of polysiloxanes with polar pendant groups by co-hydrosilylation

New polymers with tuneable dielectric properties were prepared by modifying trimethylsilyl end-terminated poly(methylhydro)siloxane with polar γ-cyanopropyl groups. The amount of polar groups was tuned by adjusting the allyl cyanide/n-hexene ratio in poly(methylhydro)siloxane co-hydrosilylation. The copolymers were characterized by FTIR and NMR spectroscopy. The distribution of the polar groups along the chain was evaluated based on 1H NMR spectroscopy. The influence of the amount of polar γ-cyanopropyl on the glass transition temperature (Tg) and on the dielectric properties was investigated by DSC and impedance spectrometry. All polymers showed Tgs well below room temperature. A linear increase in permittivity (e′) with increasing amount of γ-cyanopropyl groups was observed. A maximum e′ value of 15.9 for the copolymer containing 89 mol% polar groups was achieved, which is 6-fold higher than polydimethylsiloxane. The incomplete conversion of Si–H groups observed in all hydrosilylation reactions with allyl cyanide opened up the possibility of using the prepared copolymers as cross-linkers.

Improved performance of cyanine solar cells with polyaniline anodes

Significant progress is being made in the photovoltaic energy conversion using soluble small organic molecules. We report the fabrication of layered heterojunction solar cells with 3% power conversion efficiency consisting of a solution-processed cyanine dye, C60 and doped polyaniline anode layers that match the cyanine energy level and facilitate hole extraction.

Self-healing electrodes for dielectric elastomer actuators

A conductive, printable and stretchable composite based on 20 wt% reduced graphite nanoplatelets in silicone to be used as the electrode in dielectric elastomer actuators was developed. It has a sheet resistance of 0.1 kΩ □−1 and a low modulus of elasticity. Additionally, it is able to self-heal the actuator after a breakdown and thus increases significantly its lifetime and reliability. Such an actuator can be operated repeatedly and reversibly at voltages below and above the voltage of the first breakdown. The actuator can suffer many breakdowns and is able to self-heal again and again without loss of performance.

Fine-tuning of the dielectric properties of polysiloxanes by chemical modification

A series of novel polysiloxanes is presented, the glass transition temperatures and dielectric properties of which are systematically fine-tuned by utilizing thiol-ene post-polymerization reactions. The pendant vinyl groups of a high molecular weight polymethylvinylsiloxane P1 were exhaustively reacted with the thiol compounds 1-butanethiol (2) and 3-mercaptopropionitrile 3 both separately to give polymers P2 and P3, respectively, as well as in various ratios x, y so as to create materials P2xP3y with greatly differing contents of the polar nitrile group (y). All modifications proceeded quantitatively as was confirmed by 1H NMR spectroscopy. Because of the presence of the polarizable thioether and nitrile groups, the resulting siloxane polymers exhibit permittivity ranging from e′ = 4.7 to 18.4 for P2 and P3, respectively. The e′ values of all polymers P2xP3y carrying more than one kind of thiolether group lie within this range. Additionally, broadband dielectric spectroscopy measurements of P2, P21P31 and P3 have been conducted in the temperature range from −150 °C to 60 °C and the frequency range from 0.1 Hz to 1 MHz. Due to their high permittivity, polymers P2, P2xP3y and P3 are attractive candidates for dielectric elastomer actuators and flexible electronics.

Synthesis of silicone elastomers containing trifluoropropyl groups and their use in dielectric elastomer transducers

Vinyl end-functionalized polysiloxanes Px containing varying mol% of trifluoropropyl groups (x) were prepared starting from 1,3,5-tris(3,3,3-trifluoropropyl)-1,3,5-trimethylcyclotrisiloxane (F3) and octamethylcyclotetrasiloxane (D4) via anionic polymerization in the presence of tetramethylammonium hydroxide (TMAH) and 1,3-divinyl-1,1,3,3-tetramethyldisiloxane end-capping reagent. Their structures were determined by 1H NMR spectroscopy and their molecular weights and distributions were measured by GPC. The various Px were cross-linked in thin films via hydrosilylation of the vinyl groups with tetrakis(dimethylsiloxy)silane cross-linker in the presence of Karstedt catalyst. The mechanical, dielectric and electromechanical properties of the prepared films were investigated. An increase in the permittivity (e′) with increasing content of polar trifluoropropyl groups was observed with a maximum value of e′ = 6.4 for P58(0). A maximum lateral actuation strain of 5.4% at an electric field as low as 7.8 V μm−1 was measured for a material prepared by cross-linking P53.

Polar–nonpolar interconnected elastic networks with increased permittivity and high breakdown fields for dielectric elastomer transducers

Elastic materials with increased permittivity (e′) were obtained in a three-step process starting from a hydroxyl end-functionalized polydimethylsiloxane (PDMS) of a high molecular weight (Mw = 139 kDa), trimethylsilyl end-blocked silicones that carry hydrosilane, cyanopropyl and hexyl groups Px (where x represents the mol% of cyanopropyl groups), and tetraethoxysilane (TEOS). The hydrosilane groups of Px were first hydrolyzed and the formed hydroxyl groups were subsequently reacted with partially hydrolyzed TEOS and further used as high e′ components, cross-linkers, and reinforcing agents for the PDMS matrix. A high wt% of the polar component Px was incorporated into the nonpolar PDMS matrix by forming interconnected networks. Thermal (DSC, DMA) and morphological investigations (SEM) show the biphasic morphology of the networks. The dielectric, mechanical, and electromechanical properties of the films were investigated. Materials with good elastic properties, increased e′, and a high breakdown field (Eb) were obtained. The best material has an elastic modulus of 800 kPa at 10% strain, an e′ = 4.5, and a maximum actuation strain of 8% at Eb = 56 V μm−1.

A facile synthetic strategy to polysiloxanes containing sulfonyl side groups with high dielectric permittivity

The chemical modification of polymers with lateral polar groups increases their dielectric permittivity above the glass transition temperature, making them attractive materials for dielectric elastomer actuators. Despite the large dipole moment of the sulfonyl moiety, its usefulness as a substituent in high permittivity polysiloxanes has not been explored so far. This work explores two post-polymerization synthetic strategies to reach such a goal, namely the oxidation of the thioether groups present in polysiloxanes which carry thioether side groups at every repeat unit and the modification of the vinyl groups of poly(methylvinylsiloxanes) with sulfonyl groups via thiol–ene chemistry. While both strategies in principle work, the oxidation of the thioether groups results in an undesired shortening of the polysiloxane chains. In contrast, the thiol–ene reactions give the target polymer in a clean and highly efficient process. For this reason the access to two sulfonyl containing thiols, to be employed in the thiol–ene reaction, was improved to the degree that they are now available on the 50 g scale as pure compounds. The sulfonyl content of the polysiloxanes was systematically varied by the use of two different thiols in the thiol–ene post-polymerization modification, one of which carried the sulfonyl group, the other a (dummy) butyl group instead. The prepared polymers were characterized by NMR, DSC, TGA, GPC, and impedance spectroscopy. All polymers show glass transition temperatures below room temperature. Dielectric permittivity measurements at room temperature show that the permittivity of the polymers at the frequency with minimal losses can be fine-tuned from about 5 up to 22.7. Because of their high dielectric permittivity, low glass transition temperatures, and easy and scalable synthesis from cheap materials, these novel polymers are attractive components for high permittivity elastomers to be employed in actuators, capacitors, and flexible electronics.