Composite Created with Specific Optical Properties

When it comes to creating nanostructured composite materials with specific optical and electrical properties, size matters.

By adding semiconducting nanoparticles to polymers, scientists at the UPV/EHU-University of the Basque Country have created nanostructured composite materials with specific optical and electrical properties that vary according to size. These properties allow researchers to synthesize particles of the size corresponding to the desired properties, and by adding these particles to polymers, give the final product one specific property or another. The particles could find use in optoelectronics, biomedicine and solar panel applications.

The Materials + Technologies Research group at the university’s Polytechnical College of San Sebastian worked with cadmium and selenium composite nanoparticles, which act like quantum dots whose optical and electrical properties vary according to size. In the composite particles, the variation takes place in nanoparticles of less than 10 nm and, “therefore it is not the same to have a nanoparticle of 3 nm or one of 6 nm,” said Haritz Etxeberria, a researcher in the university’s chemical engineering and the environment department and author of the study that appeared online in Colloid and Polymer Science (doi: 10.1007/s00396-013-2927-8).

This enables nanoparticles with very specific properties to be synthesized, and subsequently when these nanoparticles are incorporated into other materials, the researchers can prepare new composite materials with pre-selected properties.

“Through nanocharges it is possible to add other properties to the intrinsic properties of the basic materials: nanoparticles, nanoclays, fibres, etc.,” Etxeberria said. “Finally, by uniting the properties of some of them, materials with new properties are obtained.”

Scientists at the University of the Basque Country are working with particles that act like quantum dots, specifically with cadmium and selenium composite nanoparticles, to create nanostructured composite materials with specific optical and electrical properties that vary according to size. Courtesy of Basque Research.

The investigators first synthesized composite cadmium and selenium nanoparticles, and then analyzed methods for inserting these particles into a polymer. The main obstacle they encountered was how to disperse the nanoparticles throughout the polymer to achieve the properties they desired.

“Because the nanoparticles are so small, they tend to group together,” Etxeberria said. “So large agglomerates are obtained and they appear mixed in different phases. But when their size is increased, they lose the properties they have as nanoparticles.”

He analyzed different methodologies for inserting and dispersing nanoparticles between 3 and 4 nm in size throughout the polymer. In the experiment, his team worked with a block copolymer made of polystyrene and polybutadiene.

“We used block copolymers because they allow the phases to be obtained,” he said. “They share immiscible ingredients, but because they are bonded to each other, they create phase arrangements on a nanometric level, and allow the adding of nanoparticles that have an affinity with one phase or another.”

Etxeberria aimed to disperse the cadmium selenide nanoparticles in the polystyrene phase using various functionalization techniques. Functionalization means that molecules that will render the nanoparticles miscible in the selected phase are added to their surface so that they can be properly dispersed throughout the polymer. The best results were obtained using a “grafting through” technique.

“Using the grafting through technique, the nanoparticles are placed in the environment in which styrene polymerization takes place,” he said. “That way, the polymer sometimes grows from the nanoparticle surface, other particles are trapped between the polymer chains, and free polymer is also created.”

The resulting material has a likeness to polystyrene and produces the desired homogenous dispersion when blended with the block copolymer. This was demonstrated by the measurements carried out on the newly created composite material: the material has the same optical and electrical characteristics that the nanoparticles had initially.

Etxeberria is now using the technique to work with other materials such as cellulose.

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