Investigation of carbon nanotube nanocomposites based on the volume exclusion principle

It is generally known that in order to achieve conductive properties in nanocomposites, a percolating network must be formed by the conducting nanofiller particles, in this case carbon nanotubes (CNTs). For various reasons, the amount of added filler at which such a network is formed, defined as the percolation threshold, should be as low as possible. This requires a good dispersion of the CNT filler, which is often a problem due to the strong self-interaction of CNTs. For this purpose, specialized dispersion techniques are used in the preparation of these nancomposites, such as latex technology, where surfactants are used to form aqueous polymer and nanotube emulsions, which are subsequently mixed, freeze-dried and compression-molded.1,2 A complementary approach for lowering the percolation threshold is limiting the volume of the material that is accessible to CNTs. This requires a phase separated system such as a polymer blend. The study of polymer blend nanocomposites was performed using thermal analysis techniques, such as DSC and TA instruments rapid heating and cooling DSC (RHC) prototype, as well as surface characterization and rheology.

The first system studied based on this approach is a polystyrene (PS) / polymethylphenylsiloxane (PMPS) blend system, where the PMPS phase shows preferential interaction with CNTs. While an excellent dispersion of CNTs by polydimethylsiloxane (PDMS) was reported before, this is to our knowledge the first study on the related PMPS.3 In the case of a co-continuous phase separated morphology, the preferential interaction of CNTs with PMPS can lead to so called "double-percolation", lowering the percolation threshold.4 A study of PS/PMPS blends was therefore undertaken, indicating partial miscibility, unlike the strongly immiscible behaviour known for PS/PDMS blends.5 In a second system, prepared by TUe, CNTs were incorporated in a polypropylene (PP) matrix with a crosslinked ethylene propylene diene monomer (EPDM) rubber phase. Here the crosslinked material is not accessible to CNTs, again making it possible to lower the percolation threshold. As PP does not have the remarkable dispersive qualities of siloxanes, CNTs are incorporated using the latex methodology, yielding a more complex system in which surfactants play a role.


References

1. N. Grossiord, J. Loos, O. Regev, and C.E. Koning, Chem. Mater., 18, 1089 (2006).
2. O. Regev, P. ElKati, J. Loos, and C.E. Koning. Adv. Mater., 16, 248 (2004)
3. A. Beigbeder, M. Linares, M. Devalckenaere, P. Degee, M. Claes, D. Beljonne, R. Lazzaroni, and P. Dubois, Adv. Mater., 5, 1003 (2008)
4. A. Göldel, G. Kasaliwal, and P. Pötschke, Macromol. Rapid Commun. 30, 423 (2009)
5. M. Maric, and C. Macosko, J. Polym. Sci. Polym. Phys. 40, 346 (2002)