High Permittivity Polysiloxanes and Polyphosphazenes for Actuator Applications

In the modern world, soft robots are gaining importance with technological advances. Compared to rigid robots, their elasticity enables safer and more adaptable integration into various systems, making them suitable for wearable devices, biomedical engineering, and robotics. As demand increases for lightweight, energy efficient, and elastic materials, both academia and industry are focusing more on soft actuators. Among these, dielectric elastomer actuators (DEAs) stand out for their large deformation, low power consumption, silent operation, and fast response. However, most DEA materials require high voltages for actuation which limits their application areas.

This thesis aims to develop high dielectric permittivity elastomers with good mechanical performance for DEA applications, focusing on polysiloxanes and polyphosphazenes. Polysiloxanes are well-established, known for their flexibility, processability, and environmental stability. Polyphosphazenes, though newer to DEA research, are elastic, biocompatible, and chemically resistant. Both, however, have relatively low dielectric permittivity. To address this, chemical modifications with highly polar functional groups were carried out to enhance their dielectric properties.

In the first study, a single-layer DEA was developed using polyphosphazene. While polyphosphazenes are well known, their actuator applications is underexplored. Polydichlorophosphazene (PDCP) was synthesized via living cationic polymerization and modified to create a stable polymer. Trifluoroethoxy groups were introduced as polar substituents to increase permittivity, leveraging fluorine's electron withdrawing nature. The resulting polymer showed a dielectric permittivity of 5.65 at 1 MHz. Varying cross-linker concentrations allowed tuning of mechanical properties, and the best per-forming material achieved 5.8% actuation at 80 V µm-1 with excellent stability across frequencies.

In the second study, both single-layer and stack actuators were made using a novel polysiloxane. The backbone was functionalized with ethyl sulfone groups, while butane thiol groups were varied to tune the glass transition temperature. Several polymers were synthesized, and the best material combined high dielectric permittivity with low mechanical losses. An increase in ethyl sulfone content led to higher permittivity. The best performing sample showed a dielectric permittivity of 16.2 at 10 kHz and lateral actuation strain of 13% at 8.2 V µm-1. A stack actuator made from this material showed 3% strain at 14.5 V µm-1 and maintained stable performance over 2000 cycles at 1 Hz.

The third study focused on another modified polysiloxane, incorporating 3-mercaptosulfolane as a polar group. Due to its bulky structure, the resulting polymer was thermoplastic. To adjust the Tg, butane thiol groups were introduced in different ratios, and the optimal material was selected through electromechanical testing. The DEA achieved 5% strain at 1.00 kV and 7.34%