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(methyl-vinylsiloxanes) 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

Dielectric properties of silver nanoparticles coated with silica shells of different thicknesses

Core/shell nanoparticles having metallic silver nanoparticle cores of similar to 38 nm in diameter and silica shells of different thicknesses ranging from similar to 3.6-20 nm were prepared. For the silica coating, a slightly modified Stober method was used which allowed preparing grams of core/shell nanoparticles for the first time. The particles were characterized by UV-vis spectroscopy, dynamic light scattering, scanning electron microscopy, transmission electron microscopy, and energy-dispersive X-ray scattering. Their dielectric properties were measured as pellets in parallel-plate capacitors. It was found that the permittivity is much influenced by the silica shell thickness with an increase in permittivity for thinner shells. A shell thickness of 20 +/- 2 nm allowed fabrication of capacitors which have similar characteristics to those of silica, thus, there is no influence of the metal core on the dielectric properties anymore. However, by decreasing the silica shell to 17 +/- 2, 8 +/- 1.5, and 6.6 +/- 1.5 nm the permittivity at high frequencies is increasing from 10, 34, to 41, respectively. The insulator to metal transition was observed for a silica shell thickness of 3.6 +/- 1 nm. Functionalization of the silica surface with a hydrophobic coating removes surface adsorbed water as observed by the flat dielectric permittivity over a large frequency domain

A Versatile Synthetic Path to Thiol Containing Polysiloxanes

A synthetic strategy to polydimethylsiloxanes and polymethylsiloxanes containing thiol functions as end- or side-groups, respectively, is presented. Such polymers are important starting materials for elastomeric networks and postpolymerization modifications. The synthesis starts either with vinyl end-functionalized polydimethylsiloxanes or with polymethylvinylsiloxanes. The vinyl groups are reacted either with thioacetic acid or with a thioacetic acid/butanethiol mixture via a UV-initiated thiol-ene reaction to form the respective thioester quantitatively within few minutes. The thioesters are subsequently deprotected to the respective thiols by reduction with LiAlH4. The resulting thiol containing polysiloxanes can be used for the formation of networks or another functionalization. (C) 2016 Wiley Periodicals, Inc

Synthesis of solvent-free processable and on-demand cross-linkable dielectric elastomers for actuators

Dielectric elastomer actuators (DEAs) have gained growing interest during the last decade from both the scientific community and industry. However, the high operating voltages needed to drive these actuators reduce their application potential. To date, several reports have been published on high permittivity elastomers that can be operated at low electric fields; however, little attention has been given to their processability into thin films and reliability as dielectrics in actuators. This work presents the synthesis of low molecular weight polar polysiloxanes, which have low viscosity and can, therefore, be processed into thin films by the solvent-free doctor blade deposition technique. Additionally, the prepared polysiloxanes have vinyl end-groups which are subsequently used for cross-linking into thin films, where multifunctional thiol cross-linkers and a UV initiator are used. The prepared films are elastic and show a permittivity of about two times higher as compared to commercial polydimethylsiloxane elastomers. In addition, their elastic modulus is easily tuned between 1.2 and 0.4 MPa using a dithiol as a chain prolongation reagent, while their viscoelastic behavior is comparable to commercial silicone elastomers. When used as dielectrics in DEAs the developed elastomers allow reliable operation over 180 000 actuation cycles at reduced voltages

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 degrees 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 mu m(-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 mu m(-1) and are therefore promising for applications as artificial muscles in soft robotics.ISSN:2198-384

Elastomers with tunable dielectric and electromechanical properties

Novel electroresponsive silicone elastomers modified with nitrile groups are presented, whose dielectric permittivity (epsilon') is tuned from epsilon' = 4.3 to epsilon' = 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 2,20-(ethylenedioxy) diethanethiol is used as a crosslinker. 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

Charge generation by ultra-stretchable elastomeric electrets

Novel piezoelectric elastomers are synthesized and their long term stability and piezoelectric properties are investigated. They are thin film composites of specially designed polymer nanoparticles with high glass transition temperatures (T-g) and side groups with large permanent dipoles, which are embedded in chemically crosslinked polydimethylsiloxane matrices. To obtain a piezoelectric material, the initially randomly oriented polar groups in the nanoparticles are poled in a strong electric field while the film is heated above the T-g of the nanoparticles. Under these conditions the polar groups orient in the direction of the electric field and the achieved orientation is subsequently frozen-in by cooling the material back to room temperature. A permanent polarization responsible for the piezoelectricity is induced in the elastomers. All composites are elastic and can be strained up to 600%. The lateral piezoelectric coefficient d(31) of the composites are found to first decrease over a period of several days before ultimately stabilizing. The largest d(31) value obtained, 12.1 pC N-1, is comparable to commercially available materials which are not elastic. Two composites exhibit promising thermal stability at 50 degrees C and generate a maxim of 2.5 V when strained. The novel elastic electret materials described in this paper are likely to find application as stretchable sensors, soft electronics, transducers and energy harvesters. Another important aspect of this work is the abundantly available combinations of elastic matrices and high T-g polar polymers, which will allow the creation of elastic electrets with tailor-made properties in the future

Mild synthesis of mercaptonitriles from vinyl nitriles and their cyclization reactions

Thiol-ene addition of thioacetic acid A is widely used in the synthesis of thiols from vinyl precursors, but so far has not been conducted on non-conjugated vinyl nitriles. The challenge when vinyl nitriles are used is to selectively conduct the thiol-ene addition, while avoiding the nucleophilic addition of A to the nitrile group. We have found that vinyl nitriles give selective UV-induced thiol-ene addition in the presence of photoinitiators as long as a stoichiometric amount of A to the vinyl group and sterically unhindered vinyls are used. In contrast, when a sterically hindered vinyl is used, the nucleophilic addition of the nitrile is favoured. The prepared mercaptonitriles can easily undergo cyclization reactions in basic and acidic conditions as well as in the presence of silica gel. This illustrates the high reactivity of nitriles towards thiol addition. 1,2-Ethanedithiol B is presented as an alternative reagent to A as it allows conversion of vinyl nitriles directly into mercaptonitriles under mild and non-acidic reaction conditions