Isolating droplet behaviour - Electroactive polymers of the future

Materials World magazine
26 Nov 2012

The behaviour of droplets – or defects – in polymers under electrical voltage has been observed for the first time. Researchers at Duke University in North Carolina, USA, claim their discovery could lead to improved insulation materials that are better equipped to deal with increasing loads of electricity.

Droplets are present in most materials as bubbles, cavities, or tiny, unavoidable defects that occur when the material is made. These droplets change shape from a sphere into a more tubular, spheroid shape when the insulating material is heated above its glass transition temperature, resulting in instability and making the material susceptible to failure.  

Until now, exactly how this happens has not been known, despite the parallel behaviour of droplets in liquids and gases being well-documented. Xuanhe Zhao, Assistant Professor of Mechanical Engineering and Materials Science at Duke University, says this knowledge gap existed because it was ‘hard to capture this phenomenon experimentally’.  

‘Before you can observe significant deformation, usually the material fails due to electrical breakdown,’ he says. ‘If you have a bubble [under observation] and apply a voltage to the polymer, once the bubble becomes unstable the material is shorted, it breaks down and you cannot observe further phenomenon. The resistant force in liquid is usually the surface tension of the bubble, but in solids the dominant resistant force is the elasticity of the material.’  

As a solution, Zhao and his team developed a technique that isolates the bubble using two rigid insulating layers either side of the polymer, enabling them to observe the reaction. ‘We sandwich the polymer so that when the bubble begins to deform or becomes unstable, there is always an insulating layer protecting the whole system. Even though the bubble can become unstable, the whole system is not shorted.’ Zhao’s team tested the theory with silicon rubber, a material widely used in insulating materials.  

The research would help create materials that are more resistant to failure. These could be achieved using one of two approaches, according to Zhao. One would be to simply eliminate the defects from the material. ‘It’s obvious but sometimes hard to do – often the defects are unavoidable.’ The second approach would be to enhance the mechanical properties of the material, such as its elasticity or toughness, which would increase the electrical energy density of the material. ‘There could be drops or defects but if you make the material stronger, tougher, more rigid, the breakdown due to instability can be significantly delayed.’  

This improved understanding of polymer behaviour might inform the next-generation of electronics says Zhao, in particular high energy density capacitors and storage devices. ‘In terms of storage and transmission, people want to put a higher voltage through them, so the requirements of the polymers become higher and higher.’ He adds that the research could also help develop new materials to be used in the electroactive polymers of the future, for actuators, energy harvesters or even artificial muscles for robots.