Responsive Thermoplastic Elastomer Gels: Applications in Electroactive, Shape-Memory and Thermal Energy Management Materials

Thermoplastic elastomers are a class of rubbery polymeric materials that exhibit solidlike properties due to physically associating moieties. Block copolymers are often used as the network forming component of thermoplastic elastomers. Additionally, block copolymers can be modified with block selective solvents that contribute a specific functionality to the system; these solvent modified systems will be referred to throughout as thermoplastic elastomer gels. Thermoplastic elastomers and their gels have a long history of applications as specialty materials for passive systems where traditional rubbers cannot meet the required design criteria—often properties of softness, toughness and low hysteresis are of interest. Herein, we discuss the use of thermoplastic elastomer gels as active materials that respond to external stimuli to change their mechanical and thermal properties. First, the text will introduce concepts of phase behavior and resultant physical behavior of block copolymers in the presence of a selective solvent. Included are specific details pertinent to materials used in experimental discussions presented in this work. Following this broad discussion, the introduction of a specific class of smart and responsive materials, known as dielectric elastomer actuators, is detailed in a survey of recent technological developments in the field. The main body of the text describes multiple applications of thermoplastic elastomer gels. It begins with an entirely novel use of a semi-crystalline olefin block polymer gel as a dielectric elastomer actuator exhibiting programmable anisotropy and promising actuation behavior. The subsequent study uses specific control over the architecture of a polydimethylsiloxane elastomer to make ultra-soft films for exceptional dielectric elastomers. These so-called bottlebrush elastomers are formed from heavily grafted polymer backbones that reduce entanglements resulting in incredibly soft elastomers. As dielectric elastomers, these materials operate with no mechanical prestrain and achieve strains greater than 300% by area. This is followed by the use of a traditional ABA triblock copolymer (poly[styrene-bethylene- co-butylene-b-styrene]) with a crystallizing selective solvent to impart shape memory behavior. This is the first demonstration of a dielectric elastomer utilizing crystallization for electroactive strain fixation. Finally, we conclude with the discussion of thermoplastic copolyester based gels as form-stable phase change materials. These phase change gels have applications in passive thermal energy management systems and compete with existing commercial technologies