Polydimethylsiloxane–barium titanate composites: Preparation and evaluation of the morphology, moisture, thermal, mechanical and dielectric behavior
Introduction
Silicone rubber is a well-known dielectric elastomer which is used in electromechanical transducers, devices able to convert electrical energy into mechanical one and vice versa [1]. A promising application of these materials, which is currently being investigated, is the generation of electricity from ocean waves energy [2], [3]. For such applications, large displacement with high precision and speed are required, together with durability and reliability. In order to obtain high actuation strain, a material with low stiffness (low Young’s modulus), high breakdown strength, and high permittivity is desired [4]. Silicones have highly desired elastic behavior, their representative polydimethylsiloxane (PDMS) being known for its unusual rheological/flow properties [5]. The polarizability of the Si–O bond that constitutes a premise for a high dielectric constant is higher as compared to organic nonpolar polymers (e.g., polyethylene), but in reality, this is not so much due to the side methyl groups (in the case of PDMS), which prevent Si–O dipoles from approaching each other too closely) [6]. In order to increase the dielectric constant, the polysiloxanes are chemically modified by attaching polar groups, such as N-allyl-N-methyl-4-nitroaniline [7] and cyanalkyl [8] to the silicon atoms. Fillers are often used to enhance the dielectric as well as the mechanical properties of the silicones [9]. The use of high permittivity inorganic fillers is a well-established technique to improve the dielectric constant of a polymer matrix. Various permittivity values can be achieved by changing the type and percentage of the filler in the substrate. Based on the literature data, it can be identified mainly three types of fillers used to improve the permittivity of the dielectric elastomer actuator: ceramic particles with a high dielectric constant, such as titanium dioxide, barium titanate, magnesium niobate, lead magnesium niobate–lead titanate, and strontium titanate [9], [10], [11], [12], [13]; conductive particles, such as carbon nanotube, carbon black, copper–phthalocyanine/polyaniline [14] and short fibers [11]; highly polarizable conjugated polymer, such as undoped poly(3-hexyltiophene), polyaniline, polythiophene incorporated by blending or as nanoparticles [9], [11]. BaTiO3, a ferroelectric crystal which exhibits spontaneous polarization and high electrical breakdown strength is often used in this aim [15]. Thus, it has been incorporated in different polymeric matrix [12], like epoxides, polystyrenes [16], polyimides, polyetherimides [17], poly-ethylene–glycol-diacrylate (PEGDA) [18], polyurethane [19], acrylics [20], [21], etc. This was also used as filler for PDMS [22]. The effect of BaTiO3 nanoparticles on electrical and mechanical properties were extensively studied and found that dielectric constant of nanocomposites increases significantly with the increase in BaTiO3 concentration where as the volume resistivity decreases continuously [22]. Commercially available barium titanate with particles of different shape and size or prepared by certain procedure to obtain a certain size and shape were used [23], [24]. There are a few commercial dielectric elastomer materials available, these mainly including acrylic VHB (Very High Bond – a 3M tape) foil, silicones, polyurethanes, and some polystyrene/polybutadiene copolymers, acrylics and acrylonitrile butadiene rubber [13], [25]. In general, commercially available room temperature vulcanization formulations based on low molecular weight polydimethylsiloxane were used as matrix in such cases and either addition (hydrosilylation) or condensation (with tetrafunctional silanes) mechanisms were used to convert the fluid compounds in silicone elastomers.
Different from the literature, in this paper, home prepared PDMSs of different molecular masses, higher as compared with those used in other studies, were used as polymeric matrix in which surface-treated barium titanate was incorporated in different percents. The composites were processed as films and crosslinked by condensation at room temperature with a trifunctional silane, methyltriacetoxysilane. The morphology, moisture sorption, thermal, mechanical and dielectric properties were investigated.
Section snippets
Materials
The polydimethylsiloxane-α,ω-diols, PDMSs, were synthesized according to the already described procedure [26]: cationic ring-opening polymerization of octamethylcyclotetrasiloxane in the presence of a cation exchanger as catalyst. Molecular masses were estimated on the basis of GPC analysis as being those presented in Table 1. Barium titanate (BaTiO3), CO, with particle size <3 μm according to supplier (Fluka AG) was investigated through SEM and TEM to determine the particle size and shape (Fig.
Results and discussions
PDMSs of different molecular masses were used as a matrix for the incorporation of different amounts of barium titanate, according to Table 1. Commercial barium titanate (with particles <3 μm in overall dimensions) was used in this study. The main difficulties concerning to the incorporation of such fillers in the silicone matrix are due to the known low compatibility of the silicones with almost any organic or inorganic partner. The improvement of the fillers compatibility with the polymer
Conclusion
Siloxane composites based on polydimethylsiloxane-α,ω-diols with different chain lengths as matrices and barium titanate powder as filler added in different percents (1, 2, and 5 wt%) were prepared and crosslinked in film forms. Samples without filler were prepared for comparison in similar conditions. DSC study reveals that the presence and amount of the filler do not affect the thermal transitions of the crosslinked structures, these being influenced mainly by the polymer chain length. A
Acknowledgments
The work presented in this paper is developed in the context of the project PolyWEC (www.polywec.org, prj. ref. 309139), a FET-Energy project that is partially funded by the 7th Framework Programme of European Community. Support by the COST Action (MP1003) “European Scientific Network for Artificial Muscles (ESNAM)”, through Short-Term Scientific Missions COST-STSM-MP1003-14671 and 14672, and PERCRO Laboratory – TeCIP Institute – Scuola Superiore Sant’Anna Pisa Italy for technical support are
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