Mechanically durable and damage resilient dielectric elastomer actuators

• Main Author: Tan, Matthew Wei Ming
• Other Authors: Lee Pooi See
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Engineering::Materials::Organic/Polymer electronics; Engineering::Materials::Mechanical strength of materials
• Online Access: https://hdl.handle.net/10356/166347
• Institution: Nanyang Technological University

Robots have become ubiquitous and integral to the daily lives of people. These robots are often composed of rigid structural materials that enable them to apply large loads and perform tasks with precision. However, these rigid robots lack compliance and adaptability, making them unsafe for human interactions and operating under unpredictable conditions. This has led to the emergence of soft robots that are composed of materials with similar moduli as biological materials. As such, soft robots demonstrate embodied intelligence principles at which their compliance allows them to adapt to various shapes. Soft actuators are a major component of these robots, being responsible for generating mechanical motion. While various soft actuators have been designed, dielectric elastomer actuators (DEAs) have become prominent due to their rapid response times, large actuation strains, and energy densities. However, electric fields to drive actuation require kilovolt range inputs that pose a safety concern and limits their compatibility with portable power sources. Owing to their soft nature, these DEAs are also susceptible to premature failure generated by physical damages within dynamic environments. Therefore, the objective of this thesis is to develop next-generation DEAs with mechanical durability and damage resilience. In pursuit of these properties, actuation performances will be concurrently enhanced, ensuring the retention of their functionality. Particularly, mechanical durability refers to imparting material properties such as tensile strength, mechanical toughness, and puncture resistance, preventing damage from being formed. Damage resilience involves enabling the device to continue its operation when damage has occurred through fracture toughness and self-healing capabilities. To achieve this, we hypothesize that polar crosslinkers can be introduced to polyurethane elastomers to meet this objective. To impart DEAs with high tensile strengths and improved actuation performance, polyurethane acrylate was copolymerized with a polar chemical crosslinker, polyethylene glycol diacrylate (PEGDA). The inclusion of PEGDA polar crosslinkers increased the tensile strength and dielectric permittivity of polyurethane acrylate (PUA) from 1.65 to 2.33 MPa and 5.5 to 9.4 respectively. Crosslinks anchored polymer chains to prevent chain slipping, allowing the viscoelastic effects to be effectively tuned for better dynamic actuation performance. Compared to commercial acrylic VHB films that are often used for DEAs, the designed elastomer exhibited rapid response times (< 1 s) with actuation minimal drifts. Also, actuated area strains (71.4%) were found to be higher than low viscoelastic silicones due to improved dielectric permittivity and breakdown fields. To achieve both mechanical durability and damage resilience, polyurethanes were functionalized with ureido-4[1H]-pyrimidinone (UPy) and carboxyl groups. This enabled the formation of physical crosslinks through multiple hydrogen bonds, allowing elastomers with high mechanical toughness (75.85 MJ m-3) and tensile strengths (9.44 MPa) to be realized. Owing to the dynamic nature of hydrogen bonds, elastomers were imparted with self-healing capabilities, at which bond reformation can be accelerated by incorporating polar solvents. As such, high toughness (41.74 MJ m−3) and tensile strength (5.57 MPa) were recovered within only 12 h. The polar nature of physical crosslinking groups provided the elastomer with a high dielectric constant (~ 9) that allowed DEAs to achieve actuated area strains of ~ 68%. Combined with self-healing, area strain can be recovered by 97% after mechanical damage, achieving the best-performing self-healable DEA. To gain greater commercial interest, the synthesis of mechanical durable, and damage resilient materials was simplified to a single-step reaction. By tuning hard-to-soft segment ratios and molecular weights of polydiols, carboxyl polyurethanes (PULM0) attained large mechanical toughness (96.5 MJ m-3) and puncture resistance. However, these mechanical enhancements often lead to higher stiffness that deteriorates actuation strains. Thus, liquid metal nanoparticles are introduced to PULM0 to sensitize the DEA with responsiveness to electric fields and NIR light. Under NIR illumination (0.2 W cm-2), self-healing was enhanced to retain high mechanical toughness (55 MJ m-3) within 2 h. When co-stimulated, photothermal effects modulate the elastomer moduli to lower driving electric fields of DEAs (~40% electric field reduction to drive ~50% area strains). Therefore, by adopting co-stimulation strategies, DEAs can not only have mechanical durability and damage resilience but also high actuation performances.