Adrian Bele

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.

Silicone–barium titanate composites with increased electromechanical sensitivity. The effects of the filler morphology

A high molecular weight polydimethylsiloxane-α,ω-diol, synthesized in the lab, was used as a matrix for nanocomposites. Barium titanate nanoparticles, either cubic or nanorods, were obtained by a hydrothermal procedure and used as a filler. In order to ensure a good compatibility with the matrix, the filler was surface treated with a commercial surfactant. A highly reactive trifunctional silane was used as crosslinking agent in the presence of organometallic catalyst. Two other samples, the first consisting of pure crosslinked polydimethylsiloxane and the second being the polymer matrix filled with commercial barium titanate, were prepared and used as references to evaluate the influence of the presence of barium titanate nanoparticles and their shape on some characteristics of the resulting crosslinked composites: morphology, thermal behavior, moisture sorption, mechanical and dielectric characteristics. The electromechanical sensitivity and energy output were calculated on the basis of appropriate experimental data in order to estimate the potential of the composites for future electromechanical applications.

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.

Bimodal silicone interpenetrating networks sequentially built as electroactive dielectric elastomers

Two polysiloxanes, a polydimethylsiloxane-α,ω-diol (PDMS) with Mn = 370 000 g mol−1, and α,ω-bis(vinyl)polydimethylsiloxane (Vi2PDMS) with Mn = 34 500 g mol−1, and appropriate crosslinking systems for each of them (tetraethyl orthosilicate–dibutyltindilaurate and α,ω-bis(trimethylsiloxy)poly(dimethylmethyl-H-siloxane)–Speiers 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.

Preparation of electromechanically active silicone composites and some evaluations of their suitability for biomedical applications

Some films based on electromechanically active polymer composites have been prepared. Polydimethylsiloxane-α,ω-diols (PDMSs) having different molecular masses (Mv=60 700 and Mv=44 200) were used as matrix in which two different active fillers were incorporated: titanium dioxide in situ generated from its titanium isopropoxide precursor and silica particles functionalized with polar aminopropyl groups on surface. A reference sample based on simple crosslinked PDMS was also prepared. The composites processed as films were investigated to evaluate their ability to act as efficient electromechanical actuators for potential biomedical application. Thus, the surface morphology of interest for electrodes compliance was analysed by atomic force microscopy. Mechanical and dielectric characteristics were evaluated by tensile tests and dielectric spectroscopy, respectively. Electromechanical actuation responses were measured by interferometry. The biocompatibility of the obtained materials has been verified through tests in vitro and, for valuable films, in vivo. The experimental, clinical and anatomopathological evaluation of the in vivo tested samples did not reveal significant pathological modifications.

Dielectric elastomers with dual piezo-electrostatic response optimized through chemical design for electromechanical transducers

A new class of elastomers that simultaneously shows sensing, actuation and energy conversion functionalities is synthesized to meet the current requirements for electroactive materials. These new materials consist of a silicone network (polydimethylsiloxane-α,ω-diol crosslinked through chain ends) semi-interpenetrated with different percentages (2, 5, and 10 wt%) of polyimide derivatives stepwise modified by different strategies to improve the compatibility with the silicone core network. By addressing the right chemical pathway, the resulting semi-interpenetrated structures (S-IPNs) show noticeable dielectric permittivity, eps′ (up to 11), and breakdown strength, Ebd (up to 88 μm V−1), improvements as compared with the starting polymers (silicone with eps′ = 2.9 and Ebd = 38 μm V−1 and our best polyimide with eps′ = 6.2 and Ebd = 23 μm V−1). The S-IPNs with 10 wt% polyimide are able to gain energy up to 132 mJ cm−3 at 100% strain and up to 164 mJ cm−3 at maximum strain to develop large actuation strain (up to 8.7%) and show very good piezo-response (up to 44 pm V−1), making them highly suitable for cutting-edge electromechanical applications. For a better evaluation, S-IPNs are compared with one of the commercially available dielectric elastomers, most often used for this purpose.

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.

Experimental testing on free vibration behaviour for silicone rubbers proposed within lumbar disc prosthesis.

This research was focused on the damping capacity study of two types of silicone rubbers proposed as layers within total lumbar disc prostheses of ball-and-socket model. In order to investigate the damping capacity, the two silicone rubber types mainly differing by the molecular mass of polymeric matrix and the filler content, as was emphasized by scanning electron microscopy and differential scanning calorimetry, were subjected to free vibration testing. Using an adapted experimental installation, three kinds of damping testing were realised: tests without samples and tests with three samples of each type of silicone rubber (69 ShA and 99 ShA). The free vibration tests were performed at a frequency of about 6 Hz using a weight of 11.8 kg. The relative damping coefficient was determined by measuring of two successive amplitudes on the vibrogram and calculating of the logarithmic decrement. The test results with silicone rubber samples showed a relative damping coefficient of 0.058 and respectively 0.077, whilst test results without samples showed a relative damping coefficient of 0.042. These silicone rubbers were found to have acceptable damping properties to be used as layers placed inside the prosthetic components.

All-silicone elastic composites with counter-intuitive piezoelectric response, designed for electromechanical applications

New all-silicone composite materials were prepared using polar silicone particles dispersed within a high molecular weight polydimethylsiloxane (PDMS) matrix. Polar silicones with either CN or Cl groups attached to the polysiloxane chain in similar proportions were prepared by appropriate post-polymerization reactions and submicron particles were obtained and stabilized with a hydrophobic commercial surfactant, Pluronic L81, while the polar polymers were cross-linked within the particles in most cases. The mechanical and dielectric properties of the all-silicone composites were measured and discussed as a function of the nature and number of polar groups in the filler particles and in correlation with morphological aspects. Soft elastomeric materials with a Young’s modulus of 0.12–0.5 MPa, dielectric permittivity up to 4.7 at 104 Hz and dielectric strength up to 63 V μm−1 were obtained, and promising electromechanical performance resulted from theoretical calculations. Dielectric relaxation spectrometry at varying temperatures revealed dynamic relaxations in agreement with DSC data, and high dielectric relaxation strength values (Δe) were calculated. We also report for the first time piezoelectric behaviour of all-polymer composites containing amorphous components, measured using Piezoelectric Force Microscopy (PFM) at ambient temperature (above Tg) without poling. Average longitudinal piezoelectric coefficients (d33) of 24 and 13 pm V−1 were found for composites with cyano- and chloro-silicones, respectively. The variation of d33 with stretching was also followed and explained in correlation with morphological aspects. The promising properties of the all-silicone composites create the premises for possible applications as stretchable electronics, including tactile sensors, acoustic transducers and wearable devices.