‘Fruitcake’ structure observed in organic polymers

The researchers analyzed the properties of an organic polymer with potential applications in flexible electronics and discovered variations in hardness at the nanoscale, the first time such a fine structure has been observed in this type of material.

The field of organic electronics has benefited from the discovery of new semiconducting polymers with molecular skeletons resistant to twisting and bending, which means that they can carry charges even when bent into different shapes.

It had been assumed that these materials resembled a plate of spaghetti at the molecular level, without any long term order. However, an international team of researchers has found that for at least one of these materials, there are tiny pockets of order inside. These orderly pockets, just a few ten-billionths of a meter in diameter, are stiffer than the rest of the material, giving it a “fruitcake” structure with harder and softer regions.

The work was carried out by the University of Cambridge and Park Systems UK Limited, along with KTH Stockholm in Sweden, the universities of Namur and Mons in Belgium and Wake Forest University in the United States. Their findings, reported in the journal Nature Communicationcould be used in the development of next-generation microelectronic and bioelectronic devices.

Studying and understanding the mechanical properties of these materials at the nanoscale – a field known as nanomechanics – could help scientists refine these properties and make the materials suitable for a wider range of applications.

“We know nature’s fabric at the nanoscale is not uniform, but finding uniformity and order where we didn’t expect to see it was a surprise,” said Dr Deepak. Venkateshvaran of Cambridge’s Cavendish Laboratory, who led the research.

The researchers used a imaging technique called higher eigenmode imaging to take nanoscale images of regions of order in a semiconducting polymer called indacenodithiophene-co-benzothiadiazole (C16-IDTBT). These images clearly showed how the individual polymer chains lined up next to each other in certain regions of the polymer film. These order regions measure between 10 and 20 nanometers in diameter.

“The sensitivity of these detection methods allowed us to map polymer self-organization down to individual molecular strands,” said co-author Dr. Leszek Spalek, also from the Cavendish lab. “Higher eigenmode imaging is a valuable method for characterizing the nanomechanical properties of materials, given the relatively easy sample preparation that is required.”

Further measurements of the material’s stiffness at the nanoscale showed that the areas where the polymers self-organized into ordered regions were harder, while the disordered regions of the material were softer. The experiments were carried out in ambient conditions as opposed to a high vacuumwhich had been a requirement in earlier studies.

“Organic polymers are normally studied for their applications in flexible, large-area, centimeter-scale electronics,” Venkateshvaran said. “Nanomechanics can complement these studies by developing an understanding of their mechanical properties at ultra-small scales with unprecedented resolutions.

“Together, the fundamental insights gained in both types of studies could inspire a new generation of flexible microelectronic and bioelectronic devices. These futuristic devices will combine the advantages of centimeter-scale flexibility, micrometer-scale homogeneity and the nanoscale electrically driven mechanical movement of polymer chains with superior biocompatibility.”