‘Fruitcake’ structure observed in organic polymers

Researchers have analyzed the properties of an organic polymer with potential applications in flexible electronics and have 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 molecular backbone semiconducting polymers that are resistant to twisting and bending, meaning they can transport charge even when bent into different shapes.

These materials were believed to resemble a plate of spaghetti on a molecular scale, without any long-term order. However, an international team of researchers found that for at least one such material there are small pockets of order. Measuring only a few ten billionths of a meter in diameter, these ordered bags are stiffer than the rest of the material, giving it a ‘fruitcake’ structure with harder and softer parts.

The work was led by the University of Cambridge and Park Systems UK Limited, with KTH Stockholm in Sweden, the Universities of Namur and Bergen in Belgium and Wake Forest University in the US. Their results, 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 — an area known as nanomechanics — could help scientists refine those properties and make the materials suitable for a wider range of applications.

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

The researchers used an imaging technique called higher eigenmode imaging to take nanoscale pictures of the regions of order within a semiconducting polymer called indacenodithiophene-co-benzothiadiazole (C16-IDTBT). These pictures clearly showed how individual polymer chains are adjacent to each other in some parts of the polymer film. These order regions are between 10 and 20 nanometers in size.

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

Further measurements of the material’s stiffness at the nanoscale showed that the regions where the polymers self-organized into ordered regions were harder, while the disordered regions of the material were softer. The experiments were performed under ambient conditions as opposed to an ultra-high vacuum, which was a requirement in previous studies.

“Organic polymers are normally studied for their applications in large areas, centimeters, flexible electronics,” Venkateshvaran says. “Nanomechanics can extend these studies by understanding their mechanical properties at an ultra-small scale at unprecedented resolutions.

Together, the fundamental knowledge gained from both types of studies could inspire a new generation of soft microelectronic and bioelectronic devices. These futuristic devices combine the advantages of centimeter-scale flexibility, micrometer-scale homogeneity and electrically controlled mechanical movement of nanometer-scale polymer chains. . with superior biocompatibility.”

The research was funded in part by the Royal Society.

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