Thermoplastic liquid crystal elastomer soft actuators
Sean Lugger successfully defended his thesis on September 20th at the department of Chemical Engineering and Chemistry.
Over the past decade, a substantial amount of research has been devoted to fabricating stimuli-responsive polymers with large, reversible deformations. The scientific fascination for nature’s responsive actuating systems has fueled the development of soft actuators that are readily applied in real-world applications, including artificial muscles, soft robotics, smart textiles, and devices exhibiting stimuli-triggered deformations triggered by changes in humidity, temperature, and light, for example. Although these developments are promising, soft actuators are typically thermosets containing permanent crosslinks, thus virtually unrecyclable and not moldable. This thesis focuses on developing thermoplastic shape-changing polymers to endow smart materials with recyclable and moldable features, affording sustainable soft actuators with a staggering variety of functionalities.
For this, melt-processable thermoplastic soft actuators are introduced, demonstrating immediate, reversible responses and weightlifting capabilities with large deformations. The thermoplastic material could be recycled, reprogrammed into 3D actuators, and welded into an actuator assembly with different deformation modes.
Industrial-relevant processing methods are key to future applications of freestanding actuators and will open new application spaces. A 3D-printing approach is developed for preparing soft actuators with reversible contractions or bending and curling motions. Furthermore, the thermoplastic ink enables recyclability, as shown by cutting and printing the actuators five times.
Given the 3D printing results, we prepared light-responsive thermoplastic LCEs for generating untethered light- and temperature-responsive soft actuators, showing reversible contracting and bending deformations both in air and underwater. The freedom provided by DIW allowed for printing honeycomb and spiral structures, demonstrating complex and reversible deformations.
To expand versatility, light-driven soft actuators with fully reprogrammable, pre-designed rest states and shape deformations from the same polymer films are explored. For this, a switchable near-infrared dye is embedded in the polymer film enabling photothermal actuation of the material. The photothermal dye may be dissociated in specific regions of the film, allowing selective heating and localized actuation of the films upon exposure to near-infrared light. Moreover, the capability of completely reforming the shape, color, and actuation mode of the LCE provides a smart material with unprecedented application versatility.
To further push the applicability of soft actuators toward standard industrial processes, melt-extrusion and drawing of thermoplastic actuator fibers on a large scale are shown. To illustrate scalability, plied fibers are fabricated as a triple helical twisted rope capable of reversibly opening and lifting a screw cap vial. Besides the programmability, the thermoplastic material employed lends itself to self-healing and complete reprocessing into other configurations in contrast to thermosets.
Overall, the findings outlined in this thesis establish a new class of thermoplastic LCEs achieving more sustainable soft actuators with melt-processable properties and recyclable, reusable capabilities. It is envisioned that these materials will serve as starting point for industrially-relevant smart materials that are widely applicable in our society for a myriad of applications such as soft robotics, smart textiles, artificial muscles, and deployable soft actuating devices.
Title of PhD-thesis: Thermoplastic liquid crystal elastomer soft actuators. Supervisors: Prof. dr. Albert Schenning and dr. Dirk Jan Mulder. Other main parties involved: European Union’s Horizon 2020 Research and Innovation Programme under grant agreement no. 829010 (PRIME).