Simon J. Dünki

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.

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.

Elastomers with tunable dielectric and electromechanical properties

Novel electroresponsive silicone elastomers modified with nitrile groups are presented, whose dielectric permittivity (e′) is tuned from e′ = 4.3 to e′ = 17.4. They are prepared in a one-step process starting from a high molecular weight poly(methylvinylsiloxane) to which polar nitrile groups and cross-links are introduced in thin films. Different ratios of butanethiol/3-mercaptoproprionitrile are used to vary the amount of nitrile groups in these elastomers, while 2,2′-(ethylenedioxy)diethanethiol is used as a cross-linker. Because of the systematic nature of this investigation, we not only present promising elastic materials with remarkable dielectric, mechanical, and electromechanical properties but also provide a guideline for materials design aimed at dielectric elastomer actuator applications.

Synthesis of poly(ethylene-co-butylene)-block-poly(ethylene oxide) surfactant and its use in the synthesis of polyhydroxyethyl methacrylate nanoparticles containing azo-dye

A synthetic path to poly(ethylene-co-butylene)-block-poly(ethylene oxide), a substitute for the “KLE” surfactant was developed. For this purpose, mono hydroxyl end-functionalized poly(ethylene-co-butylene) (∼3800 g mol−1) was modified with a triple bond at its hydroxyl group, while two samples of mono hydroxyl end-functionalized poly(ethylene oxide) (∼2000 g mol−1 and 5000 g mol−1) were modified with an azide group. The two blocks were bound together by click chemistry and the products characterized by 1H NMR spectroscopy, EA and GPC. The ability of these new block copolymers to stabilize nanodroplets in inverse miniemulsion polymerization of 2-hydroxyethyl methacrylate (HEMA) was investigated. It was found that the molecular weight of the poly(ethylene oxide) block strongly influences the stability of the miniemulsions; only the copolymer with the short oligoethylene block gave satisfactory results. The possibility of encapsulating Disperse Red 1 (DR1) dye in polyhydroxyethyl methacrylate (PHEMA) was also investigated and the miniemulsions were optimized such as to prepare particles with a maximum DR1 loading of approximately 21 wt%. To avoid dye agglomeration and phase separation during polymerization, DR1 was equipped with a polymerizable group. The resulting particles were characterized by DLS, SEM, TGA, UV-vis and DSC. These particles are of importance as fillers in polydimethylsiloxane composites that are used as electrostrictive materials.

Silicones with enhanced permittivity for dielectric elastomer actuators

The research efforts for silicone based elastomers with high dielectric permittivity (Ɛ’) intensified significantly in the last years since such materials would allow the construction of dielectric elastomer actuators (DEA) with low operation voltages. Polar groups can be introduced to elastomers to adjust their permittivity. The results obtained regarding the functionalization of silicones with polar nitrile (CN) and trifluoropropyl (CF3) groups are presented. Those with CN groups were synthesized via anionic polymerization of nitrile containing cyclosiloxanes or via a post-polymerization modification of functional polysiloxanes. Polysiloxanes containing CF3 groups were prepared by anionic copolymerization of 1,3,5-tris(3,3,3-trifluoropropyl)-1,3,5-trimethylcyclosiloxane with octamethylcyclotetrasiloxane. Importantly, we have found that all polysiloxanes have glass transition temperatures (Tg) well below room temperature (<-50°C). This ensures that the materials turn into true elastomers after cross-linking. In addition to this, a linear increase in Ɛ’ with increasing content of polar groups was observed with maximum values of Ɛ’ = 18 and Ɛ’ = 8.8 for polysiloxanes modified at every repeating unit with either CN or CF3 groups, respectively.

Dielectric materials, design, and realization

It has been the dream of many scientists to create polymeric materials which exhibit simultaneously high dielectric permittivity, low glass transition temperature, and excellent elastic properties. Such materials would be a highly attractive dielectric in electromechanical transducers. Within this topic we are focusing on silicones because of their excellent elastic properties over wide temperature and frequency ranges combined with low glass transition temperatures. To increase their low permittivity, we followed different approaches which include: blending the matrix with highly polarizable conductive and polar nanofillers and chemical modification with polar side groups. This presentation will show the advantages and disadvantages of the two strategies we have been following and will provide an assessment of their future potential.

Dielectric elastomer actuators with increased dielectric permittivity and low leakage current capable of suppressing electromechanical instability

Dielectric elastomers with increased dielectric permittivity (e′), excellent insulating and mechanical properties have a broad application potential ranging from flexible electronics to dielectric elastomer transducers. Until now, several reports exist on elastomers with increased permittivity, but unfortunately in most cases this increase in permittivity is associated with an increase in conductivity, which detrimentally affects the insulating properties, such as the leakage current and dielectric breakdown. Here, novel polysiloxane based elastomers were prepared in three steps starting from a silanol end-terminated poly(methylvinyl)siloxane, whose vinyl groups were reacted with alkyl thiols via a thiol–ene reaction. The resulting polymers were cross-linked using condensation reactions and the dielectric, mechanical and electromechanical properties were evaluated. Eventually, an optimized material with a relative permittivity of e′ = 5.4, a conductivity of 4 × 10−11 S cm−1, a storage modulus of 300 kPa, and a mechanical loss factor below 0.05 was achieved. Actuators constructed from this optimized material showed no electromechanical instability in the absence of prestrain. An area actuation strain of about 200% at an electric field of 53 V μm−1 was achieved. This excellent electromechanical behavior can be rationalized by the increased permittivity due to the thioether side chain and its favorable strain-stiffening effect on the material when crosslinked with polar alkoxysilanes. The actuators gave a stable lateral actuation strain of 10% for more than 50 000 cycles when subjected to an electric field of 27 V μm−1 at a modulation frequency of 8 Hz. Due to the combined properties of this material such as a low leakage current density of 0.5 μA cm−2 at 27 V μm−1, an attractively low glass transition temperature, a very fast electromechanical response and an increased permittivity, the dielectric elastomers developed in this work may be considered as a future replacement for polydimethylsiloxane elastomers in dielectric elastomer transducers.