A team of researchers from Tufts University perform a systematic quantitative characterization of the mechanical and dielectric properties of poly(L-lactic acid) electrospun nanofibers (PLLA). They do this using a combination of Atomic Force Microscopy (AFM) techniques. They also extend this to composites of PLLA fibers with embedded multiwall carbon nanotubes (MWCNT-PLLA).

Elastic moduli

The elastic moduli of the nanofibers are measured via AFM nanoindentation. The dielectric constants are determined by measuring the phase shifts obtained via Electrostatic Force Microscopy (EFM). These measurements show that the transverse elastic moduli of the MWCNT-PLLA composites are significantly lower than the corresponding elastic moduli of PLLA fibers of similar diameter.

Mesophase formation

MWCNT-PLLA composite fibers also display an increase in the elastic modulus with increasing fiber diameter, a trend which was not observed for neat PLLA fibers. This difference is explained by a simple core-shell model where the carbon nanotube is aligned along the fiber axis. This model also predicts the formation of a mesophase introduced by the alignment of molecular chains in the shell region of the polymer-carbon nanotubes composites.

Distinguishing between dielectric constants

The researchers show that EFM measurements can be used to clearly distinguish between the dielectric constants for the two systems. MWCNT-PLLA composites display larger dielectric constants compared to PLLA fibers. This is consistent with a higher polarization of these composites. These results show that quantitative, AFM-based techniques are extremely useful. They provide high-resolution, non-destructive tools for measuring the mechanical and electrical properties of polymer nanocomposites with a broad range of applications in nanotechnology.

More information about this research can be found in the journal Nanotechnology 26 105702.

Further reading

Multifrequency atomic force microscopy: material properties on the nanoscale (Nov 2014)
Understanding the signal in electrochemical strain microscopy (Oct 2014)
Seeing atoms under viscous conditions (Apr 2014)