By adding semiconducting nanoparticles to polymers, the Materials + Technologies Research Group (GMT) of the Polytechnical College of San Sebastian of the UPV/EHU-University of the Basque Country has created nanostructured composite materials with specific optical and electrical properties that vary according to size. These properties allow researchers to synthesise particles of the size corresponding to the desired properties, and by adding these particles to polymers, to give the final product one specific property or another.
At the UPV/EHU's Polytechnical College of San Sebastian they are working with particles that act like quantum dots, specifically with cadmium and selenium composite nanoparticles. One of the characteristics of quantum dots is that the optical and electrical properties of the particle vary according to size.
In the case of the cadmium and selenium composite particles, this variation takes place in nanoparticles of less than 10 nanometres -- a nanometre is equal to one millionth of a millimetre -- , and, "therefore it is not the same to have a nanoparticle of 3 nanometres or one of 6 nanometres," explains Haritz Etxeberria, researcher in the UPV/EHU's department of Chemical Engineering and the Environment and author of the research. This allows nanoparticles with very specific properties to be synthesised, and subsequently when these nanoparticles are incorporated into other materials, the researcher 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. Finally, by uniting the properties of some of them, materials with new properties are obtained," says Etxeberria.
In opto-electronics, biomedicine and in the field of solar panels the authors are seeking applications for the particles that function like quantum dots.
The work done by Etxeberria consisted of synthesising composite cadmium and selenium nanoparticles, and subsequently, of analysing methods for inserting these nanoparticles into a polymer. The main challenge tends to be in fact dispersing the nanoparticles properly throughout the polymer; failure to achieve this means that the composite material will not have the properties that one wants to give it by means of the nanoparticles. "Because the nanoparticles are so small, they tend to group together. 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," stresses Etxeberria.
In the first phase of the research work, Etxeberria synthesized nanoparticles of cadmium selenide of different sizes and, bearing in mind the importance of size in the properties of the particle, he analysed various synthesis parameters to optimize the synthesis of the nanoparticles and obtain nanoparticles of cadmium selenide of the desired size and properties.
In the second phase, he analysed methodologies for inserting and dispersing nanoparticles of a specific size (of between 3 and 4 nanometres) throughout the polymer. For this purpose he worked with a block copolymer made of polystyrene and polybutadiene. "We used block copolymers because they allow the phases to be obtained. 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," explains Etxeberria.
Etxeberria was aiming to disperse the cadmium selenide nanoparticles in the polystyrene phase. To do this, he tried out 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 by means of the technique known as "grafting through." "Using the grafting through technique, the nanoparticles are placed in the environment in which styrene polymerization takes place. That way, the polymer sometimes grows from the nanoparticle surface, other particles are trapped between the polymer chains, and free polymer is also created," explains Etxeberria. The result is a material that has an affinity with polystyrene, and which produces the desired homogenous dispersion when blended with the block copolymer.
This has been demonstrated by the measurements carried out on the composite material created: the composite material has the same optical and electrical characteristics that the nanoparticles had initially. In view of the good results of the technique, Etxeberria is now working on other materials, like cellulose.
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