Measuring strain at the nanoscale is important for understanding how nano-structuring affects the strength of polymer nanocomposites. However, there aren’t many methods around for measuring mechanical properties on such a small scale.

One technique is digital image correlation. This needs the material to have a randomly patterned surface. With this in mind, Michael Myrick and colleagues used pattern stamping to apply a random pattern to the surface of a polymer.

The University of South Carolina researchers employed a porous alumina membrane with 200 nm diameter pores. They sputtered a gold coating up to 100 nm thick onto the membrane. Then they heated an ethylene vinyl acetate-based hot melt polymer to 190 °C and left it to cool slightly before placing the alumina stamp on to its surface. Once the polymer had cooled completely, the team dissolved the alumina stamp with a solution of sodium hydroxide.

This resulted in the gold pattern transferring to the polymer, typically imprinted around 1-2 µm beneath the polymer surface. The scientists found that they could alter the depth of the imprint by changing the viscosity of the polymer or the force applied to the stamp.

The gold network remained in good contact with the polymer surface even under a surface strain of around 10%.

“Making the imprinted patterns worked more or less as we’d hoped, but microscopy and electrical characterization were difficult because the pattern is embedded beneath the surface of the polymer,” said Myrick.

To make electrical connections to the gold, the team applied lines of silver paint to the polymer surface. The paint travelled through the polymer’s pores to contact the gold under the surface.

The gold network proved to be electrically conductive. What’s more, as the mechanical strain increased above a threshold value of about 1%, the network’s resistance rose. And, following removal of the load, the post-strain resistance was around 4-7 times higher than for an unstrained specimen. This means that measuring the pattern’s resistance could find a use in structural health monitoring: the networks could detect whether a polymer has exceeded a critical strain. The gold network could also be used for surface heating.

The researchers reported their work in Nanotechnology.