Polymers have many attractive features but traditionally conductivity isn’t one of them. Now a lot of work is going into making polymers that are conductive to exploit their mechanical flexibility and plasticity in electronic devices.
“We decided to work on graphene – it’s a natural material and it can really change many physical properties of plastics – mechanical , electrical and thermal,” says Athanassia Athanassiou, senior researcher responsible for the Smart Materials Group of the Istituto Italiano di Tecnologia (IIT) in Genoa. Athanassiou and her group are not alone in turning to the all-carbon conducting wonder material to bring smart properties to plain polymers, but it turns out that with graphene you can have too much of a good thing.
“The point is if you want to work with graphene you need to overload the matrix,” Athanassiou tells Physics World Materials, as she explains how anything short of overloading the matrix leaves you with a composite that is little better at electrical conduction than the original polymer. “But if you overload the graphene to make the polymer conductive then you might get to loadings that counteract the mechanical properties – making the matrix brittle.” In fact while graphene can enhance plasticity at specific loadings, going past that can have the opposite effect on the mechanical properties.
Going for gold
The solution the IIT team worked on was to supplement the graphene-polymer composite with gold nanoparticles. Again they were not the first to try this either but where their approach had the magic touch was by growing the gold nanoparticles in situ.
“Graphene is not just electrical but thermally conducting,” explains Athannasiou. This means that when they add a precursor of gold to the polymer and graphene flakes and heat it, the gold nanoparticles grow directly on the graphene flakes. While electrons cannot move through pure polymer or a polymer composite at such low loadings of graphene or gold nanoparticles alone, the in situ grown nanoparticles surround the graphene flake, making conduction possible.
The in situ growth also prevents clustering, which can be a real problem when trying to enhance polymer properties with gold nanoparticles. Graphene flakes are also increasingly prone to clustering the higher the concentration used. With the in situ grown gold nanoparticles the concentration was just 1 wt% – as opposed to the 3 wt% or more usually needed – so the graphene flakes do not cluster either.
As well as preserving the mechanical properties and avoiding clustering, there are cost savings associated with using less graphene as it is still an expensive material. “We really hoped it would work like that and somehow we got the experiment just right – usually that doesn’t happen,” says Athanassiou.
The future of nanoparticle enhancements in situ
Athanassiou points out that there may be useful enhancements to the thermal properties too that they have not yet checked, since thermal and electrical properties often go hand in hand. Other future work will focus on improving the method and materials that reap the enhanced effects.
The researchers worked with PMMA a common polymer that is widely used and well known. Next they plan to develop an extrusion method for producing graphene PMMA composites with in situ grown gold nanoparticles. Solvent processes require a lot of protective equipment for workers so for industry extrusion processes that produce composites from pellets are preferred. They are also looking at whether different metal nanoparticles and polymer matrices may give improved results, as well as the effect of using graphene or carbon nanotubes for the carbon additive, or a mixture of both. For future work they are very focused on the use of biodegradable bioplastics.
Full details are reported in Nano Futures.
