NOVEL ELECTROACTIVE SOFT ACTUATORS BASED ON IONIC GEL/GOLD NANOCOMPOSITES PRODUCED BY SUPERSONIC CLUSTER BEAM IMPLANTATION

Ionic electro-active polymers (IEAPs) constitute a promising solution for developing self-regulating, flexible and adaptive mechanical actuators in the area of soft robotics, micromanipulation and rehabilitation. These smart materials have the ability to undergo large bending deformations as a function of a low applied voltage (1 to 5 V), as a result of the ions migration through their inner structure when the network is liquid filled. Among this broad family of materials, ionic-polymer-metal composites (IPMC) based on DuPont’s Nafion® have attracted an increasing interest for the production of light weight controllable soft machines due to their easiness to be metalized (e.g. by mean of electroless plating), fast response and capability of working exposed to air. However, the high cost of the material, its relatively low working density (i.e. the maximum mechanical work output per unit volume of active material that drives the actuation) and weak force output, as well as the considerable fatigue effects endured by the surface electrodes upon cycling, is limiting the performance of these IPMC actuators and hindering their implementation in traditional mechatronic and robotic systems. On the other hand, ionic hydrogels, such as poly(acrylic acid) (PAA) and poly-styrene sulfonate (PSS) based polymers, exhibit controllable mechanical properties and porosity and have shown to be excellent candidates to be used as electrically triggered artificial muscles and miniaturized robots operating in aqueous environments. Although the relatively low cost of these materials render them appealing for mass production scale up, the applicability of these polymeric actuators is limited to a liquid environment, which is intrinsically facilitating the solvent evaporation when the hydrogels are exposed to air. Furthermore, because of the difficulty encountered in fabricating stable and anchored metal structures on these polymer surfaces, these smart soft systems operate in a non-contact configuration with respect to the pilot electrodes, therefore increasing the actuators response time up to few tenths of seconds. In order to achieve an efficient electromechanical transduction along with a stable and durable performance for electro-active actuators operating in air, two main interplaying characteristics must be tailored when designing the system. On one hand side, the need of electrodes that are physically interpenetrating with the polymeric basis is of absolute priority, since the intercalation of ions into the electrode layers and the resulting material volumetric change are fundamental for strain generation. On the other hand, the formulation and engineering of new low cost materials able to merge highly elastic properties and efficient ionic transport features is of crucial importance. The present thesis work deals with the formulation, synthesis and manufacturing of a novel ionic gel/metal nanocomposite (IGMN) that was designed and developed to merge the advantageous properties of both IPMCs and ionic hydrogel actuators and to contextually overcome many of the above mentioned drawbacks characteristic of these two families of polymers. These composites were obtained by mean of Supersonic Cluster Beam Implantation (SCBI). This technique, developed in-house, relies on the use of supersonically accelerated gas-phase metal cluster beams directed onto a polymeric substrate in order to generate thin conductive layers (few tenths to few hundreds of nanometers thick) anchored to the polymer. This scalable approach already proved to be suitable for the manufacturing of elastomer/metal functional nanocomposites, and, as described in this work, it enabled the production of cluster-assembled gold electrodes (100 nm thick) interpenetrating with an engineered ionic gel matrix. This novel approach led to the fabrication of highly conductive metal nanostructures, large surface area for ions storage and providing minimal interfacial stresses between the metal layer and the polymeric basis upon deformation. The key features of this novel system comprise the control on the polymer elasticity, bending actuation in air from 0.1V to 5V, fast response time (< 300 ms), high displacement (> 5 cm), high work density ( >10 J/cm3), minimal electrodes fatigue upon cycling and low manufacturing costs. A bottom-up approach was firstly adopted to engineer and produce Uv photo-cross-linked ionic co-polymers (iongel) with tailored mechanical properties and provided with inorganic nano-structures embedded in the macromolecular matrix which show excellent long-term performance. The polymer is based on poly(acrylic acid)-co-poly(acrylonitrile) (PAA-co-PAN) co-polymers, which are chemically cross-linked in a hydrogel-like fashion and swollen with suitable imidazolium-based ionic liquid. The materials are produced as 100 um freestanding layers using a one-pot synthesis and a simple molding process. Due to the incommensurably low vapor pressure of the ionic liquid, issues concerning the shrinkage of traditional water swollen gels operating exposed to air could be avoided. An organic cation (tetraethyl ammonium, TEA+) is stably coordinated to the carboxyl groups of the PAA and free to move in the polymer sieve-like structure when a small voltage is applied at the electrodes. PAN was introduced to enhance the elastic properties of whole polymer. In the bulk polymer, halloysite nanoclays (HNC) are physically embedded into the gel in order to both improve the toughness of the gel and to improve the ionic conductivity of the system. In fact, the nanostructures interacts with the imidazolium cation of the ionic liquid through an oxygen reduction reaction, and therefore the latter is able to contribute to the charge transport phenomena induced by the electric field due to the solvent partial dissociation. Furthermore, the porosity of the polymer, tailored by the cross-linker, creates physical channels to favor the mobility of positive ions when an electric field is applied. The contribution of both the positive charged species (TEA+ and cations of ionic liquid) that accumulates at the nanostructured electrode in a double layer capacitance regime generates a differential swelling at the opposite sides of the actuator, which bends towards the anode. As it will be shown in the next sections, the actuation mechanism of the IGMN could be modeled according to both the material structure and design, as well as to the experimental data on its electrochemical and electro-mechanical properties.Comparing with traditional soft polymers incompatibility with current metallization processes, like electroless plating or surface silver laminated electrodes fabrication, which are not suitable to guarantee long-term actuation of the components, SCBI demonstrated to be a suitable technique for the production of next generation electro-active soft actuators. The IGMN-based actuators showed superior performance, such as large bending displacement, fast response time, long durability in a low voltage regime during the actuation process. The combination of the SCBI fabrication technology with the ionic gel synthesis and fabrication renders the manufacturing of these systems time-saving and costs-effective, and the unique properties of these actuators render them good candidates for potential scale up and for applications in micro-electromechanical systems, microfluidics, soft robotics, and rehabilitation.