Dielectric elastomers based on silicones filled with transitional metal complexes

New composite dielectric elastomers with improved dielectric properties were prepared on the basis of polydimethylsiloxane rubber filled with new types of metal (Mn, Fe, Cr) complexes of the bis-azomethine derived from the condensation of a siloxane diamine, 1,3-bis(aminopropyl)tetramethyldisiloxane, with 3,5-di-bromo-2-hydroxybenzaldehyde. The tetramethyldisiloxane fragment from the structure of the complexes creates the premise for a good compatibility with the silicone matrix without the need for other surface treatments while the complexed metal unit through its polar character changes the dielectric properties of the material. The resulted composites crosslinked at room temperature as dielectric elastomer films were investigated in order to establish if such materials are suitable for use in the structure of electromechanical devices. The introduction of metal complexes in the polymer matrix has led to a slight decrease of the elastic domain but increased the relative dielectric permittivity with up to 100% and the electromechanical sensitivity of the materials, with negligeable changes of the thermal behavior and overall moisture sorption capacity, thus preserving the chemical stability and hydrophobic character of siloxanes

Interpenetrating poly(urethane-urea)–polydimethylsiloxane networks designed as active elements in electromechanical transducers

A poly(urethane-urea-siloxane) was prepared in a two-step procedure involving the synthesis of a bis-isocyanate prepolymer on the basis of 4,4′-diphenylmethane diisocyanate, a polyether glycol and dimethylol propionic acid, and its extension by reacting with 1,3-bis(3-aminopropyl)tetramethyldisiloxane. The resulted polymer was used in different percentages to prepare three series of interpenetrating networks (IPNs) with polydimethylsiloxane-α,ω-diols with molecular masses, Mn, of 70000, 230000 and 370000 g mol−1. A polydimethylsiloxane–polyethyleneoxide graft copolymer was added as a compatibilizing agent. The IPN precursors were mixed in solution and processed as films. During solvent evaporation, the chemical crosslinking of the polydimethylsiloxane-α,ω-diols occurs with tetraethyl orthosilicate in the presence of dibutyltin dilaurate, while in the case of poly(urethane-urea-siloxane) only physical crosslinking by hydrogen bonds is expected to occur. The morphology and thermal transitions of the resulted networks were examined by scanning electron microscopy, differential scanning calorimetry with dynamic mechanical analysis. The mechanical and dielectric characteristics (dielectric permittivity, loss, strength) of the aged films were studied. Their responsiveness to an external stimulus in the form of an increasing electric field was assessed by electromechanical measurements and expressed as lateral strain. The results were critically analyzed with respect to each other as a correlation with their composition and compared with those obtained for three common commercially available dielectric elastomers

Bentonite as an active natural filler for silicone leading to piezoelectric-like response material

Raw sodium bentonite (Bent) without preliminary treatments is incorporated as a filler in a silicone matrix, from 5 to 100 parts per hundred (pph), by weight, by simple mixing in solution. The mixtures are processed as films and stabilized by condensation crosslinking at room temperature. Besides being environmentally safe and non-toxic, bentonite is 30 times cheaper than polydimethylsiloxane (PDMS), so the cost price of composites can be reduced by over 40%. Studies on the effects of bentonite addition as filler on the properties of composites reveal that thermal stability is not significantly affected, while an increase in the amount of inorganic residue with an increase of Bent content is recorded. More importantly, the mechanical and dielectric properties are significantly influenced by the Bent content in the PDMS matrix. The Young's modulus increases, while the elongation decreases, indicating a stiffening of the material and a decrease in its elasticity as the Bent load increases. Most notably, the dielectric permittivity increases up to more than five times at 103 Hz by adding 100 pph Bent, while the dielectric losses remain acceptable, especially at high frequencies for all composites. Furthermore, the study of composite films through Piezoresponse Force Microscopy and piezoelectric testing system reveals an outstanding piezoelectric-like response for composites with a high Bent content. The wideangle X-ray diffraction indicates an increase of the crystalline fraction - the main factor that influences the apparent piezoelectric coefficient - with increasing the Bent loa

Nanomaterials Developed by Processing Iron Coordination Compounds for Biomedical Application

The iron oxides, widespread in nature, are used in numerous applications in practice due to their well-known properties. These properties can be modified by size lowering at nanoscale. Some applications, such as biomedical, require a rigorous selection of nanoparticles by size, shape, and surface functionality. In other applications, such as catalysis or magnetism, the composition (generally mixed oxides) and morphology of the nanoparticles are of high importance. The preparation of iron oxide nanoparticles (IONPs) is a complex process whose control raises a number of issues. The first challenge is finding the optimal experimental conditions, which would lead to the preparation of monodisperse nanoparticles. Another issue is the selection or setting a reproducible and clean manufacturing process without a need of complex purification. Even though at the moment several methods for preparing IONPs are known, there are still concerns in the scientific world to further improve existing methods or create new protocols. Therefore, the establishment of optimal methods for preparing IONPs with predetermined structural, dimensional, and morphological characteristics is an important task of scientists. Most of the methods reported in literature for the preparation of IONPs use proper metal salts as precursors. Recently, the use of the organometallic and coordination compounds of iron as precursors for IONPs has emerged as an alternative for a better control of these. Here, achievements reported in the literature in this direction are reviewed and critically analysed in relation to the conventional method based on iron salts

Bimodal silicone interpenetrating networks sequentially built as electroactive dielectric elastomers

Two polysiloxanes, a polydimethylsiloxane-α,ω-diol (PDMS) with Mn = 370000 g mol−1, and α,ω-bis(vinyl)polydimethylsiloxane (Vi2PDMS) with Mn = 34500 g mol−1, and appropriate crosslinking systems for each of them (tetraethyl orthosilicate–dibutyltindilaurate and α,ω-bis(trimethylsiloxy)poly(dimethylmethyl-H-siloxane)–Speier's catalyst, respectively), were mixed together in various weight ratios (1:0.1, 1:0.2, 1:0.3, 1:0.5) and cast into films. These were sequentially crosslinked by different mechanisms. A determined pre-stretch was applied to the first network after its formation followed by thermal treatment for curing the second network. Non-prestreched networks were also prepared in parallel for comparison. The aged films were characterized from the point of view of the soluble fraction content, and analysed by differential scanning calorimetry, water vapour sorption in dynamic regime, dielectric spectroscopy and tensile tests. Dielectric strength and actuation strain were measured to estimate the suitability of the samples for electromechanical applications. The results revealed that through this approach one can relatively easily obtain very simple and homogeneous materials suitable for use as dielectric elastomer transducers

Goethite nanorods as a cheap and effective filler for siloxane nanocomposite elastomers

Iron oxide (goethite) with a nanorod morphology was prepared by a chemical precipitation method and characterized by FTIR, EDX, TEM, WAXD. This was used as an active filler to prepare dielectric elastomer nanocomposites by its incorporation, besides silica, in a silicone matrix consisting in a high molecular weight polydimethylsiloxane-α,ω-diol (PDMS). The nanocomposites were processed as films, stabilized by peroxidic crosslinking at high temperature, and their properties of interest for potential use in the structure of electromechanical devices were studied. It is for the first time that such composites, based on PDMS and iron oxide in a well-defined type (goethite) and shape (nanorods) are approached from the perspective of dielectric elastomers. The introduction of iron oxide nanoparticles into the polymer matrix resulted in improvements in both mechanical and dielectric properties. Thus the breaking strain and the dielectric constant values increased in comparison with those of the pure polymer sample, while the dielectric loss preserved low values specific for dielectrics

From iron coordination compounds to metal oxide nanoparticles

Various types, shapes and sizes of iron oxide nanoparticles were obtained depending on the nature of the precursor, preparation method and reaction conditions. The mixed valence trinuclear iron acetate, [Fe2IIIFeIIO(CH3COO)6(H2O)3]·2H2O (FeAc1), μ3-oxo trinuclear iron(III) acetate, [Fe3O(CH3COO)6(H2O)3]NO3∙4H2O (FeAc2), iron furoate, [Fe3O(C4H3OCOO)6(CH3OH)3]NO3∙2CH3OH (FeF), iron chromium furoate, FeCr2O(C4H3OCOO)6(CH3OH)3]NO3∙2CH3OH (FeCrF), and an iron complex with an original macromolecular ligand (FePAZ) were used as precursors for the corresponding oxide nanoparticles. Five series of nanoparticle samples were prepared employing either a classical thermal pathway (i.e., thermal decomposition in solution, solvothermal method, dry thermal decomposition/calcination) or using a nonconventional energy source (i.e., microwave or ultrasonic treatment) to convert precursors into iron oxides. The resulting materials were structurally characterized by wide-angle X-ray diffraction and Fourier transform infrared, Raman, energy-dispersive X-ray, and X-ray fluorescence spectroscopies, as well as thermogravimetric analysis. The morphology was characterized by transmission electron microscopy, atomic force microscopy and dynamic light scattering. The parameters were varied within each route to fine tune the size and shape of the formed nanoparticles

From Amorphous Silicones to Si-Containing Highly Ordered Polymers: Some Romanian Contributions in the Field

Polydimethylsiloxane (PDMS), in spite of its well-defined helical structure, is an amorphous fluid even at extremely high molecular weights. The cause of this behavior is the high flexibility of the siloxane backbone and the lack of intermolecular interactions attributed to the presence of methyl groups. These make PDMS incompatible with almost any organic or inorganic component leading to phase separation in siloxane-siloxane copolymers containing blocks with polar organic groups and in siloxane-organic copolymers, where dimethylsiloxane segments co-exist with organic ones. Self-assembly at the micro- or nanometric scale is common in certain mixed structures, including micelles, vesicles, et cetera, manifesting reversibly in response to an external stimulus. Polymers with a very high degree of ordering in the form of high-quality crystals were obtained when siloxane/silane segments co-exist with coordinated metal blocks in the polymer chain. While in the case of coordination of secondary building units (SBUs) with siloxane ligands 1D chains are formed; when coordination is achieved in the presence of a mixture of ligands, siloxane and organic, 2D structures are formed in most cases. The Romanian research group’s results regarding these aspects are reviewed: from the synthesis of classic, amorphous silicone products, to their adaptation for use in emerging fields and to new self-assembled or highly ordered structures with properties that create perspectives for the use of silicones in hitherto unexpected areas