Perfectly imperfect: why it pays to mix it up with nanoparticles

Polystyrene C60 Composites

Investigation of the effect of the feeding formulation (premixed powders of pure components versus solvent-blended mixture) of polystyrene–C60 composites on the dispersion and reagglomeration phenomena developing along the barrel of a twin-screw extruder. Reprinted (adapted) with permission from Gaspar H et al. A Journey along the Extruder with Polystyrene:C60 Nanocomposites: Convergence of Feeding Formulations into a Similar Nanomorphology. Macromolecules (2017). Copyright 2017 Americal Chemical Society.
View full-size image

The discovery of bucky balls (the spherical molecules of Buckminsterfullerene, C60) in 1985 led to a Nobel Prize (in 1996) and the development of a new branch of chemistry, sending ripples through disciplines as diverse as astrochemistry, superconductivity and materials science.

In the 21st century, fullerenes sit alongside other carbon nanoparticles - graphenes and carbon nanotubes - in offering significant benefits in polymer nanocomposites. Not only can they significantly enhance the mechanical, electrical and thermal properties of the pure polymers, but their inclusion can create novel applications. It is possible to create lightweight, conductive polymers, for example, that could be of particular interest to the aerospace industry. However, the dispersion of carbon nanoparticles in polymers has proved difficult to control, with both thermodynamic and kinetic effects in play.

A paper recently published in Macromolecules reveals the results of detailed investigations into the dispersion of C60 nanoparticles in extruded polystyrene composites. The interdisciplinary study brought together researchers with widely different skillsets from several institutions: the University of Minho in Portugal, the University of Sheffield, Delft University of Technology in the Netherlands, and ISIS. The team performed sequential sampling of a molten polymer nanocomposite as it passed along an extruder, characterizing the corresponding degrees of dispersion using a battery of direct and indirect imaging techniques - optical microscopy, spin-echo small-angle neutron scattering (SESANS), small-angle neutron scattering (SANS), small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) - covering length scales from several micrometers down to the nanometre.

Although it may be counter-intuitive, when it comes to dispersing carbon nanoparticles in a polymer, there is such as thing as too well mixed. The dispersion needs to be good, but not perfect, as this can reduce desirable properties. And whilst an extruder is naturally thought of as a mixing device, it turns out it can also behave as a de-mixer! But, crucially, depending on the actual initial feed formulation, both processes can be used to achieve the desired level of dispersion.

For these experiments, the team investigated two very different starting scenarios, feeding either a very well mixed formulation into the extruder, or a not very well mixed formulation. What they found was that the dispersion of the nanoparticles in these two scenarios converged during the extrusion process, leading to polymers with similar nanomorphology. The results strongly suggest that the final morphology is dictated by the interaction of thermodynamics and flow, leading to a steady state that is independent of initial conditions. It looks as though starting with a well-dispersed formulation and using the extruder to promote the desired degree of de-mixing is a good processing strategy for tailoring nanomorphology in these materials. The next step for this research is to bring a small extruder to ISIS and do some real-time in situ measurements of the nanomorphology of the polymer during the extrusion process.

The SANS measurements were performed on the LOQ instrument at ISIS, which has been in operation for more than 30 years, generating more than 200,000 data sets and around 600 scientific papers, making it the most successful time-of-flight SANS instrument in the world. The SESANS measurements in this study were performed at the Reactor Institute Delft, but an Anglo-Dutch collaboration will be bringing this state-of-the-art capability to the LARMOR instrument at ISIS later in 2017.

Emma Cooper

Research date: April 2017

Further Information

Gaspar H et al. A Journey along the Extruder with Polystyrene:C60 Nanocomposites: Convergence of Feeding Formulations into a Similar Nanomorphology.  Macromolecules (2017). DOI:10.1021/acs.macromol.6b02283

Bookmark and Share
Skip to the top of the page