Highly stretchable dielectric elastomer composites containing high volume fractions of silver nanoparticles

Abstract

The dielectric permittivity (ε') of a polymeric material can be significantly increased when blended with conductive fillers at concentrations approaching percolation threshold. However, reproducible synthesis of such composites is after decades of research still a major challenge and a bottleneck for their application. Difficulties arise in controlling size and shape of the filler as well as in its homogenous distribution within the composite. These parameters strongly affect the dielectric as well as the mechanical properties of the composite. While a substantial amount of literature is dealing with the influence of conductive filler on the dielectric properties of composites, little is known about their mechanical properties. It is therefore still an important goal to synthesize materials with simultaneously high ε' and good mechanical properties. Here, we report the synthesis of dielectric elastomers that combine key properties such as high flexibility and stretchability, high thermal stability, increased ε', low dielectric loss and conductivity. Such materials were prepared by solution processing using quasi-spherical silver nanoparticles (AgNPs) of defined size in a polydimethylsiloxane matrix (M w = 692 kDa). To prevent percolation, the AgNPs were coated with a thin silica shell (<4 nm). To increase their compatibility with the silicone matrix, these core/shell nanoparticles were passivated with a silane reagent. The insulating silica shell around the particles precisely defines the minimum approach distance between the cores as twice the shell thickness. The dielectric properties of those passivated particles (filler) were measured in pellets and found to have an almost frequency independent value of ε' = 90 and a very low loss factor tan δ = 0.023 at high frequencies. When such particles were used as filler in a polydimethylsiloxane matrix, composites with low dielectric losses were obtained. A composite containing 31 vol% filler with ε' = 21 and a tan δ = 0.03 at ~1 kHz was achieved. At a AgNPs volume fraction of 20%, the composite has a ε' = 5.9 at ~1 kHz, a dielectric strength of 13.4 V/µm, elastic modulus as low as 350 kPa at 100% strain, and a strain at break of 800%. Due to the high specific energy density per volume at low electric fields, these composites are attractive materials in applications involving low electric fields.A composite of Ag/SiO 2 core/shell nanoparticles 20 vol% in polydimethylsiloxane with ε' = 5.9, E b = 13.4 V/µm, Y 100% = 350 kPa, a strain at break of 800% is presented.

The Current State of Silicone-Based Dielectric Elastomer Transducers

“…A subsequent polydopamine layer could be used to control the thickness of the isolating layer, and thereby the permittivity of the system, where an optimum level was found with a dopamine layer of 15 nm and a loading of up to 50 vol%, thus providing a dielectric permittivity of up to ε' = 50. Recently, Quinsaat et al [ 173,174 ] have also shown how Ag particles covered by a SiO 2 shell could be used to obtain a permittivity of ε' = 5.9. As with other conductive fi llers, they observed a reduced dielectric breakdown strength of 13.4 V μm −1 , as also mentioned above for the other conductive fi llers.…”

Optimization Techniques for Improving the Performance of Silicone‐Based Dielectric Elastomers

“…The traditional choice of fillers for silicones is silica particles, due to their inherent compatibility. These fillers can eithervia the simplest processbe blended directly into one or both of the elastomer premixes [33][34][35][36][37][38][39][40][41][42] or alternativelyvia the more demanding processincorporated through in situ condensation reactions during the crosslinking condensation reaction, [43,44] therebyin both casesyielding a morphology, as illustrated in Figure 2. [32] Silica, however, does not contribute positively to the dielectric permittivity of the resulting elastomer, since the relative permittivities of silica and silicone are similar.…”

“…[33] Therefore, high-permittivity fillers such as metal oxide fillers are required in order to meet the demand for enhanced dielectric elastomer energy density. These fillers can eithervia the simplest processbe blended directly into one or both of the elastomer premixes [33][34][35][36][37][38][39][40][41][42] or alternativelyvia the more demanding processincorporated through in situ condensation reactions during the crosslinking condensation reaction, [43,44] therebyin both casesyielding a morphology, as illustrated in Figure 2.…”

Polysiloxanes with Increased Permittivity as Artificial Muscles

“…An attractive way to reduce the driving voltage is to increase the dielectric permittivity of the elastomer used. Our attempts towards developing such materials went through several phases, [1,2] and resulted in polysiloxanes and polysiloxane-based composite materials with driving electric fields of about 30 V/µm, strains at break of higher than 500% and elastic moduli below 200 kPa. While these properties pointed in the right direction, in particular the driving voltage was still not where we wanted it to be.…”