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.
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.
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 10-10 m3/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.