Valentina Musteata

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.

Well-defined silicone–titania composites with good performances in actuation and energy harvesting

Although silicones possess low dielectric constant, they are between the most used polymers in actuation due to their appropriate mechanical properties (low modulus and high elongation). These can be easily tuned by the preparation strategy: proper choice of the molecular mass and microstructure of the polymer matrix; adding or not of more or less active fillers; whether these are incorporated in the polymeric matrix (ex situ) or generated in situ; crosslinking mode (through the side or ending functional groups) or mechanism (condensation, radicalic or by hydrosilylation). A relatively low cost and easy scalable procedure was used in this article to prepare silicone composites based on high molecular weight polydiorganosiloxane copolymer and hydrophobized silica and titania nanoparticles. The matrix polymer was synthesized by bulk ring opening copolymerization of different substituted cyclosiloxanes and characterized by FTIR, 1H NMR and gel permeation chromatographic analysis. The composites prepared by the mechanical incorporation of the fillers were crosslinked by radicalic mechanism and investigated by dielectrical spectroscopy, mechanical tests, dynamo-mechanical analysis and dynamic vapor sorption. The actuation measurements revealed displacement values in the range 0.04–5.09 nm/V/mm, while energy harvesting measurements revealed impulse electrical voltage in the range 6–20 V for a dynamic force of 0.1–1 Kgf. The robustness of these composites supported by their thermal, mechanical and surface properties recommends them for use inclusively in harsh environmental conditions, when their behavior is not significantly affected.

Thermal, dynamic mechanical, and dielectric analyses of some polyurethane biocomposites

Polymer biocomposites based on segmented poly(ester urethane) and extracellular matrix components have been prepared for the development of tissue engineering applications with improved biological characteristics of the materials in contact with blood and tissues for long periods. Thermal, dynamical, and dielectrical analyses were employed to study the molecular dynamics of these materials and the influence of changing the physical network morphology and hydrogen bond interactions accompanied by phase transitions, interfacial effects, and polarization or conductivity. All phenomena that concur in the tested materials are evaluated by cross-examination of the dynamic mechanical characteristic properties (storage modulus, loss modulus, and loss factor) and dielectric properties (relative permittivity, relative loss factor, and loss tangent) as a function of temperature. Comparative aspects were elucidated by calculating the apparent activation energies of multiplex experiments.

Properties of some azo-copolyimide thin films used in the formation of photoinduced surface relief gratings

Thin free standing films have been obtained by casting from dimethylacetamide solutions of some azo-copolyimides. The dynamo-mechanical and dielectric properties, and the effect of the chemical structure of polymers on the physical properties are investigated. The incorporation of substituted azobenzene groups and hexafluoroisopropylidene units in the macromolecular chain allowed the patterning of the materials under different irradiation conditions. The azo-copolyimide thin films showed high thermal stability, low dielectric constant, good dynamo-mechanical characteristics and uniform surface relief gratings.

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.

Novel bio-based flexible epoxy resin from diglycidyl ether of bisphenol A cured with castor oil maleate

The paper deals with the synthesis, thermal and electrical behavior study of a flexible epoxy resin based on castor oil maleate and diglycidyl ether of bisphenol A. Castor oil maleate was obtained via esterification of castor oil with maleic anhydride. The chemical structures of the castor oil maleate and epoxy network were confirmed by FTIR and 1H-NMR spectroscopy. Thermal decomposition of the synthesized flexible network has been studied by means of dynamic thermogravimetry in a nitrogen atmosphere up to 600 °C. Global kinetic parameter values of the thermal decomposition process were determined using the isoconversional method of Friedman. Simultaneous TG-DTA analysis indicated that the thermal decomposition process occurred in three stages, characterized by diffusion, n order and Avrami–Erofeev reaction models. Kinetic parameter determination, corresponding to each individual stage of the thermal decomposition, was also possible through a multivariate non-linear regression method. A good correlation was found between the experimental and simulated data. Evolved gas analysis was monitored by a simultaneous TG/DTA-FTIR-MS technique. Dielectric relaxation spectroscopy measurements were also undertaken.

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.

A silicon-containing polyazomethine and derived metal complexes: synthesis, characterization, and evaluation of the properties

A new dialdehyde-containing silicon was prepared by a reaction of Williamson-type and structurally characterized. By its subsequent condensation with 2,5-bis(p-aminophenyl)-1,3,4-oxadiazole, a multifunctional polyazomethine-containing imine, phenylene, oxadiazole, ether, and carbosilane groups within the chain was prepared. The complexes of this polymer with Co(II), Cu(II), and Zn(II) ions formed on the basis of electron-donor ability of the oxadiazole and azomethine groups were prepared in a one-step approach by mixing the two precursors of polyazomethine (dialdehyde and diamine) with corresponding metal salt. The ligand and metal complexes were characterized by spectral (Fourier transform infrared (FTIR), 1H, and 13C NMR) methods. The thermal, optical, electrochemical, and dielectric properties were studied and the effects of the metal insertion in dependence on its nature over these properties were evaluated. All compounds are soluble in common organic solvents and show low glass transitions due to the flexible ether and Si–C bonds, are fluorescent, mainly due to the oxadiazole ring, are redox active and have high dielectric constant due to the presence of oxadiazole, azomethine metal units.

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.

New dielectric elastomers with improved properties for energy harvesting and actuation

New materials with large value for dielectric constant were obtained by using siloxane and chemically modified lignin. The modified lignin does not act as a stiffening filler material for the siloxane but acts as bulk filler, preserving the softness and low value of Young’s modulus specific for silicones. The measured values for dielectric constant compare positively with the ones for previously tested dielectric elastomers based on siloxane rubber or acrylic rubber loaded with ceramic nanoparticles. The new materials use the well-known silicone chemistry and lignin which is available worldwide in large amounts as a by-product of pulp and paper industry, making its manufacturing affordable. The prepared dielectric elastomers were tested for possible applications for wave, wind and kinetic body motion energy harvesting. Siloxane, lignin, dielectric