Carmen Racles

Synthesis and characterization of silicones containing cyanopropyl groups and their use in dielectric elastomer actuators

Polydimethylsiloxanes (PDMS) have recently been used as dielectric elastomer materials in electromechanical actuators. When they are soft enough, electric fields can change their shape. However, due to their low dielectric permittivity, large electric fields are required to induce a change. The approach presented here is to chemically modify silicones with cyanopropyl groups in order to increase their permittivity. Samples containing repeat units with cyanopropyl groups from 3 to 23% were synthesized, different methods being employed. The prepared polymers were cross-linked into thin films. The dielectric permittivity of these films increased from 2.4 (for the silicone matrix) to 6.5 for a film containing about 23% of cyanopropyl repeat units. The most promising materials were further optimized to meet the requirements for actuators and their electromechanical properties were investigated. Material A for example, which is a blend of PDMS and cyanopropyl-modified silicone, has a permittivity of 3.5 and higher moduli of elasticity as compared to the matrix but nevertheless shows 10% actuation strain at 40 V μm−1 which is a factor of 3.8 larger as compared to the matrix (2.6% actuation strain at the same voltage).

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.

Synthesis of functional telechelic polydimethylsiloxanes by ion-exchangers catalysis

Abstract Telechelic siloxane oligomers with vinyl-, aminopropyl- methylmethacryloxy- and carboxypropyl- groups were obtained by ion-exchanger catalysed equilibration reactions between octamethylcyclotetrasiloxane and proper 1,3-difunctional disiloxanes. Depending on the nature of the functional group (i.e. acid or basic) either cation- or anion-exchangers were employed, in order to prevent catalyst deactivation and side reactions. The conditions in which oligomers with desired molecular weights can be obtained have been established.

Silicone-based Composite for Relining of Removable Dental Prosthesis

A high molecular weight dimethylmethylvinylsiloxane copolymer (Mv = 450, 000) has been synthesized, analyzed, and used as matrix to prepare a composite by mixing with a proper amount of silica. A mixture of Fe2O 3/TiO2 was added to obtain a color close to the natural. 2,4-Dichlorobenzoyl peroxide was also incorporated as a crosslinking catalyst. This heat curable silicone-based formulation was used for relining removable dental prosthesis. The optimum conditions to appliance and hardening of the silicone composite on the prosthesis were established. The main surface (morphology, water contact angle, and water vapor sorption capacity) and mechanical characteristics were evaluated for the crosslinked silicone processed as thick film. The prosthesis relined with silicone was tested on a machine simulating mandible biomechanics both from kinematics and dynamic points of view. The results revealed a good behavior of our composite that remained unmodified after 700,000 mastication cycles.

Kinetics of the nematic ordered phase growth during a temperature quench of an isotropic siloxane‐azomethine polymer

Kinetics of the nucleus growth during a deep temperature quench across the isotropic to nematic phase transition was experimentally investigated for a siloxane‐azomethine polyether at cooling rates of 10 and 20°C min−1. Nematic droplets revealed in the optical images during the phase separation were treated statistically and the resulting statistical size distributions were described using the model of reversible aggregation. Analysis of the time‐dependent distribution parameters allowed two processes involved in liquid crystal phase ordering to be identified: nucleus growth and nucleus coarsening. Both regimes are quantitatively described using the universal growth law.

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.

influence of chemical structure on processing and thermotropic properties of poly(siloxane-azomethine)s

A disiloxane dialdehyde was obtained from bis (chloromethyl)disiloxane and p-hydroxybenzal-dehyde and it was used in an equilibration (redistribution) reaction to synthesize an oligomeric dialdehyde. By solution polycondensation, starting from bis(formyl-p-phenoxymethyl)tetramethyldisiloxane and different organic diamines, poly(azomethine)s were obtained. In the case of aromatic diamines with high rigidity, which gave insoluble polymers, different approaches were tested in order to improve the solubility: the use of the dialdehyde having longer siloxane chain, or of a siloxane diamine in a copoly-condensation reaction. The polymers’ structures were confirmed by IR and 1H NMR spectroscopy and by elemental analysis. The structure-properties relationship was studied in terms of solubility, thermal and thermotropic behavior. Most of the obtained poly(azomethine)s has mesomorphic properties, which were studied by polarized optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction. The influence of aromatic diamines’ chemical structure on the processability of the siloxane-organic poly(azomethine)s was emphasized.

Poly(siloxaneimide)s 2. Polycondensation of some imidic diacid chlorides with aminoalkylsiloxanes

Abstract Some new poly(siloxaneimide) copolymers with good solubilities have been synthesized by solution polycondensation of aromatic diacid chlorides containing preformed imide rings with disiloxanes and siloxane oligomers having α,ω-aminopropyl functionalities. The polycondensation reactions were carried out using equimolecular amounts of the two monomers, in polar aprotic solvents and inert atmosphere. The obtained compounds were characterized by elemental C, H, and Si analysis, solubility measurements, IR and 1 H-NMR spectrometry. Thermogravimetric curves were also recorded. All data agree with the proposed structures.

Kinetics of the nematic phase growth across the isotropic-nematic phase transition in polymer-dispersed liquid crystals

Polymer-dispersed liquid crystals (PDLCs) composed of poly(dimethyl siloxane), cured poly(dimethyl siloxane) and polysulfone (as matrices), and an azomethine compound (as an embedded mesogen varying in weight from 5 to 80%) were prepared via solvent-induced phase separation. After preparation, they were heated to the melt and then cooled; phase transitions upon both heating and cooling were detected with a differential scanning calorimeter and a polarising optical microscope (POM). The nematic droplets appearing in the POM images across the isotropic-nematic phase transition were treated statistically and described with principles of irreversible thermodynamics. Furthermore, kinetics of the nematic phase growth across this phase transition was studied and described analytically with the universal law for cluster growth. Both flexibility of the polymer matrix and the mesogen content in PDLCs were shown to influence the processes studied.

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.