Mihaela Dascalu

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.

Polar silicones: structure-dielectric properties relationship

Abstract A series of polar silicones was synthesized in order to compare their dielectric properties. Different substituents with high dipole moment (epoxy, pyridyl, aldehyde, cyano-, nitroazobenzene) were attached by hydrosilylation to a poly(dimethyl-methylhydro)siloxane. Thiol-ene addition on a dimethyl-methylvinyl siloxane copolymer with similar composition was also used for chemical modifications with chloro- or carboxy- derivatives. This approach allowed comparison of properties with emphasis on dielectric behavior measured in liquid state, as a preliminary step in design and preparation of materials suitable for dielectric elastomers. Although a relatively low content of polar groups was used (8%), permittivity values of 5.4 and even 7.4 were achieved (at 10 kHz), either due to the large dipole moment or to the presence of important amounts of moisture. The water sorption capacity of the polar silicones was investigated by dynamic vapor sorption, while structural parameters of model molecules were calculated, in order to correlate the dielectric properties with the polarity/hydrophilicity of the substituents to the silicone chain. A combined effect of the calculated dipole moment, molar polarizability, molar volume, and the measured water sorption capacity on dielectric permittivity was observed.

Highly stretchable composites from PDMS and polyazomethine fine particles

Polyazomethine (PAZ) submicron (fine) particles were obtained by polycondensation reactions occurring in reverse micelles of an amphiphilic siloxane oligomer. These particles were used as a dispersed phase of 10–40 wt% in a high molecular mass PDMS to obtain all-polymer composites. The new materials were characterized by scanning electron microscopy (SEM), differential scanning calorimetry (DSC), dielectric relaxation spectroscopy (DRS), contact angle, breakdown and stress–strain measurements. Thin films with a uniform morphology all over the cross-section were obtained. The dielectric permittivity markedly increased (up to 300% at 1 Hz) compared to the pristine PDMS. The PAZ fine particles act as reinforcing fillers for the PDMS matrix. Low loading levels (10–20%) allow the material to be kept within the range of soft elastomers, with a maximum strain of 600–800% and a low Young’s modulus, while higher amounts of fillers limit the strain to around 350%. The dielectric and mechanical properties can be tuned depending on the composition and structure of the dispersed phase. Such materials may be interesting as dielectric elastomer transducers or as highly flexible PDLCs.

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.

Synthesis and characterization of metal-containing poly(siloxane-urethane) crosslinked structures derived from siloxane diols and ferrocene diisocyanate

Three new poly(siloxane-urethane) crosslinked structures (PSFUs) were prepared from 1,1′-diisocyanatoferrocene (Fc(NCO)2), and two siloxane diols in different proportions, i.e. α,ω-bis(hydroxybutyl)oligodimethylsiloxane (HS-1) and a hybrid diol containing hydrolysable triethoxy (–Si(–O–Et)3) groups (HS-2). The formation of the urethane groups along the polymer backbone was first confirmed by the presence of the specific bands in FTIR spectra. The resulting materials were characterized using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), dielectric spectroscopy (DE) and X-ray photoelectron spectroscopy (XPS). Cyclic voltammograms of the compounds showed quasi-reversible oxidation–reduction waves making them suitable for sensing applications.