Effect of Janus Particles on Performance of Electroactive Polymer Films

Dielectric elastomers (DEs) have become a popular material, because they can efficiently convert between electrical and mechanical energy. This property enables their use as electromechanical actuators in applications such as mini-robots, artificial muscles, or dynamic tactile screens. Owing to the reversibility of the mechanical-to-electrical energy conversion, DEs can also be employed in modern green energy technology to harvest natural forces from wind and ocean waves to generate green energy thereby satisfying a continuously increasing energy demand. The dielectric elastomer, poly(ethylene glycol phenyl ether acrylate), p(EGPEA) for short, is used in this thesis. p(EGPEA) responds according to the Maxwell effect when stressed, i.e., the strain is proportional to the dielectric constant and the square of the external electric field and inversely proportional to its Young's Modulus. p(EGPEA) is a soft polymer with a dielectric constant (6.59 at 20 Hz) that shows an actuation in the 0~2 % range at external fields of 12 V/m. This low actuation response at these relatively high actuation fields limits its applicability in technology that involves interfaces with humans. One way to improve the electromechanical actuation is the increase of p(EGPEA)'s dielectric constant and/or lowering of its Young's Modulus through the addition of fillers, i.e., materials with higher dielectric constants or lower Young's Moduli, respectively. One kind of filler particle is the Janus particle, a particle that carries a metallic cap on one half of its surface. Janus particles are interesting filler particles due to their anisotropic structure. Here, 500 nm SiO2 particles are assembled into monolayers and coated with 5/20 nm Ti/Au on one hemisphere. 0.5 vol% of the resulting Janus particles are added to the p(EGPEA) films and their effect on the Young's Modulus, the dielectric constant, and the actuation behavior of p(EGPEA) is studied. Interestingly, an astonishingly high overall dielectric constant of 10.20 for JP SiO2-loaded p(EGPEA) composite films at 20 Hz compared to the dielectric value of 6.59 for pure p(EGPEA) is measured. Most surprisingly, addition of 0.5 vol% Janus particles results in an unexpected lowering of the Young's Modulus, an observation that does not agree with the mixing theory of Guth and Simha. An additional set of experiments ranging from swelling via UV/Vis spectroscopy and thermogravimetric analaysis (TGA) to nanoindentation measurements is used to better understand the effect of the gold caps on the curing behavior and the dielectric constant. We find that the presence of gold caps contributes to the inhibition of the 365 nm UV light exposure resulting in a less thoroughly crosslinked and softer polymer in the vicinity of the caps. Further, we also show that the enhancement of the dielectric constant cannot be explained by the reduced curing alone, but that it also has to involve the anisotropic nature of the Janus particles. Owing to the larger dielectric constant and the decreased Young's Modulus, the electrostriction coefficient of JP SiO2-loaded p(EGPEA) composite films is measured as Sexp = 55x10-16 m2/V2 compared to that of pure p(EGPEA) films Sexp = 0.69x10-16 m2/V2 indicating a 10 times stronger response or a potential reduction in the actuation E-field required to obtain the same electromechanical strain response. Discrepancy is found between the measured Sexp and theoretical Stheo values calculated from the Young's Modulus and dielectric values pointing towards a potentially more complex electrostriction mechanism.