Conducting polymers are becoming viable engineering materials and are gradually being integrated into a wide range of
devices. Parallel efforts conducted to characterize their electromechanical behavior, understand the factors that affect
actuation performance, mechanically process films, and address the engineering obstacles that must be overcome to
generate the forces and displacements required in real-world applications have made it possible to begin using
conducting polymers in devices that cannot be made optimal using traditional actuators and materials. The use of
conducting polymers has allowed us to take better advantage of biological architectures for robotic applications and has
enabled us to pursue the development of novel sensors, motors, and medical diagnostic technologies. This paper uses the
application of conducting polymer actuators to a biorobotic fin for unmanned undersea vehicles (UUVs) as a vehicle for
discussing the efforts in our laboratory to develop conducting polymers into a suite of useful actuators and engineering
Poly(3,4-ethylenedioxythiophene), or PEDOT, freestanding films were synthesized and characterized as conducting polymer linear actuators. Variations of solvent and electrolyte led to the observation of strains greater than 4% with maximum strain rates of 0.2%/s during electrochemical interrogation in an ionic liquid environment. The ionic liquid 1,3-butylmethylimidazolium hexafluorophosphate, BMIMPF6, enabled the largest strains to be observed repeatedly while the polymer was held at a stress of 1.0MPa over tens of cycles. The ionic liquid environment also produced a single polarity in the relationship between charge and strain. This single polarity suggested that only the imidazolium cation was actively intercalating into and out of the polymer film. The possible sources and consequences of such a mechanism as compared to actuation in conventional solvents and electrolytes which show dual polarity of charge and strain is discussed.
A typical limitation of polypyrrole based conducting polymer actuators is the low achievable active linear strains (2 % recoverable at 10 MPa, 7 % max) that these active materials exhibit when activated in a common propylene carbonate / tetraethylammonium hexafluorophosphate electrolyte. Mammalian skeletal muscle, on the other hand, exhibits large recoverable linear strains on the order of 20%. Such large linear strains are desirable for applications in life-like robotics, artificial prostheses or medical devices. We report herein the measurement of recoverable linear strains in excess of 14 % at 2.5 MPa (20 % max) for polypyrrole activated in the 1-butyl-3-methyl imidazolium tetrafluoroborate liquid salt electrolyte. This advancement in conducting polymer actuator technology will impact many engineering fields, where a lightweight, large displacement actuator is needed. Benefits and trade offs of utilizing ionic liquid electrolytes for higher performance polypyrrole actuation are discussed.
A new class of molecular actuators where bulk actuation mechanisms such as ion intercalation are enhanced by controllable single molecule conformational rearrangements offers great promise to exhibit large active strains at moderate stresses. Initial activation of poly(quarterthiophene) based molecular muscles, for example, show active strains in the order of 20%. Molecular rearrangements in these conjugated polymers are believed to be driven by the formation of pi-dimers (e.g. the tendency of pi orbitals to align due to Pauli’s exclusion principle) upon oxidation of the material creating thermodynamically stable molecular aggregates. Such thiophene based polymers, however, suffer from being brittle and difficult to handle. Polymer composites of the active polymer with a sulfated polymeric anion were therefore created and studied to increase the mechanical robustness of the films. This additional polyelectrolyte is a Sulfated Poly-Beta-Hydroxy Ether (S-PHE) designed to form a supporting elastic matrix for the new contractile compounds. Co-deposition of the polyanion with the conducting polymer material provides an elastic mechanical support to the relatively stiff conjugated polymer molecules, thus reducing film brittleness. The active properties of such poly(quarterthiophene)/S-PHE polymer actuator composites based on intrinsic molecular contractile units are presented and discussed.
Freestanding films of poly(3,4-ethylenedioxythiopene), PEDOT, were synthesized electrochemically from a solution containing EDOT monomer, tetrabutylammonium hexafluorophosphate, and water in propylene carbonate. The films were tested mechanically under constant stresses ranging from 0.6 to 2.1 MPa and subjected to various electrochemical waveforms while immersed in a bath containing propylene carbonate and an electrolyte. The characterization resulted in observations of ultimate linear strains of 2%, strain rates of 0.003 Hz, and strain to charge densities of 4 x 10<sup>-10</sup> m<sup>3</sup>/C, comparable to the conventional conducting polymer polypyrrole. In addition to the quantitative analysis, evidence of both anionic and cationic intercalation into the polymer is presented with a discussion of prospective mechanisms and consequences.
Traditional conducting polymer actuators such as polypyrrole offer tremendous active stress at low actuation voltages but with moderate strain, strain-rate and efficiency. We report the synthesis of novel thiophene based conducting polymer molecular actuators, exhibiting electrically triggered molecular conformational transitions. In this new class of materials, actuation is the result of conformational rearrangement of the polymer backbone at the molecular level and is not simply due to ion intercalation in the bulk polymer chain upon electrochemical activation. Molecular actuation mechanisms results from (pi) $min(pi) stacking of thiophene oligomers upon oxidation, producing a reversible molecular displacement which is expected to lead to surprising material properties such as electrically controllable porosity and large strains. The hypothesis of active molecular conformational changes is supported by in situ electrochemical data. Single molecule techniques are considered for molecular actuator characterization. Mechanical properties of these new materials are currently being assessed.