Multifunctional nanocomposites are used in application areas ranging from sensors and plasmonics to stretchable electronics and smart coatings, as well as in energy conversion and biomedicine. Researchers are particularly interested in polymeric nanocomposites that can adapt to physical or chemical stimuli in their environment since such materials can be controlled via molecular design.

A subset of such functional systems is based on superparamagnetic iron-oxide nanoparticles (SPIONs), also known as magnetite, whose properties can be modified using an external magnetic field. The problem is, however, that most of the SPION-based composites made so far are inhomogeneous – that is, they contain nanoparticles of varying sizes and shapes as well as aggregates. This ultimately leads to the magnetic properties of these materials being inhomogeneous, which means they are difficult to control.

Crosslinking of the polymer blocks is important

A team led by Horst Weller at the Institute of Physical Chemistry and the Hamburg Center for Ultrafast Imaging at the University of Hamburg has now overcome this problem. The researchers employed a wet chemical technique to control the size and shape of magnetite nanocrystals, encapsulating them in stable micelles with a covalently bound outer corona of an embedding polymer matrix, and then dispersed them in the final step of the process. The intermediate step of micellar encapsulation ensures that the nanoparticles are homogenously distributed in the polymer melt, explains Weller.

The researchers characterized their composites using state-of-the-art transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS), two techniques that complement each other perfectly. They found that it is in fact the crosslinking of the polymer blocks surrounding the SPION that is crucial for homogenously distributing the nanocrystals within the polymer.

Cycling magnetic and mechanical properties on demand

"Homogenously dispersed nanocrystals in a polymer matrix could make for composites with extreme hardness and stiffness," Weller tells "Our approach will allow us to modify the material's rheological response, and its mechanical and magnetic properties, using external magnetic fields. To do this, we heat the nanocomposites above their glass temperature and then apply a magnetic field to align the magnetic nanocrystals.

"As a result, we can control their viscosity (which is important for extruding polymers during manufacturing)," he adds. "We can also trap the particles in their aligned form by cooling and then dispersing them again by heating. We can thus cycle their magnetic and mechanical properties on demand."

Spurred on by their preliminary results, the researchers say that they will now be looking at how to optimize the magnetic and mechanical properties of their nanocomposites by changing the amount of the nanocrystal filler, and the size and shape of the nanoparticles.

Their present work is detailed in ACS Nano DOI: 10.1021/acsnano.6b08441.