Fully carboxy-functionalized polyhedral silsesquioxanes as polar fillers to enhance the performance of dielectric silicone elastomers

Octakis(2-carboxyethyl-thioethyl)silsesquioxane (T8-COOH), a structurally well-defined organo-functionalized nano-silica, was used as an addition in different percentages (up to 15 wt.%) to a high molecular mass PDMS, obtaining composites that were processed as films by solution casting and stabilized by condensation crosslinking. SEM images showed a common aggregation phenomenon within polymeric matrix at high filler concentration. The presence of T8-COOH pushed the main stage of thermal decomposition to higher temperatures, generally around 500 °C, as compared to 400 °C for pure PDMS. The mechanical strength of the obtained composites was also enhanced by increased addition of filler that acts as a reinforcing agent. Tensile toughness, strains at break, and Young's modulus increased with increasing filler content. The viscoelastic behavior of the silicone composites was evaluated in five cyclic stress-strain tests. All materials showed relatively large hysteresis loops for the first cycle due to Mullin's effect, while the subsequent hysteresis became smaller from the second cycle. The composites showed dielectric behavior with low loss (<10−1), relative permittivity superior to pure siloxane (4–5 vs. 3.4), and slightly increased electrical breakdown strength (30–36 V/μm vs. 27 V/μm). The applicative potential of these composites in electromechanical devices was estimated by calculated figures of merit, based on mechanical and dielectric parameters, which indicate their suitability for energy harvesting/capacitive sensors rather than for actuators. In addition, the composite with the highest ε′ was tested as capacitive pressure sensor, showing a high sensitivity at low pressure, with potential application in wearable devices.