(346c) Characterization of Pristine and Functionalized Multi-Walled Carbon Nanotubes in Unsaturated Polyester Resin

Most carbon nanotube – thermoset nanocomposite studies have focused in epoxy based resins, but the global unsaturated polyester resin market comprises more than 80 % of all thermoset resins.  Therefore, it is important to develop an understanding of the dispersion of carbon nanotubes in UPR and characterization of the resulting systems.  UPR research has however been hindered due to styrene evaporation and developing functionalization schemes which are compatible with the resin.  In this research, dispersions of functionalized and pristine MWNT in UPR were prepared using high shear mixing for three consecutive days.  The MWNTs had a length of ~ 5000 nm and a diameter of 8.5 nm.  The structure of the suspensions was studied using optical microscopy and rheological analysis.  The experimental setup allowed prolonged rheological measurements with minimal solvent evaporation during testing.  A mechanical percolation threshold of 0.097 vol % was obtained for unmodified MWNTs, which is very close to the theoretical value of 0.085 vol % calculated for individually dispersed MWNTs.  Data previously reported by us for SWNTs in UPR yielded an experimental threshold of 0.098 %; close to the theoretical value of 0.1 vol % for individually dispersed SWNTs with L/D = 500.  The fractal structure of the dispersions was described based on plots of shear storage modulus (G’) and critical strain for linear viscoelasticity (γ_c) vs. vol. %.  A fractal dimension d_f = 1.28 was obtained for MWNTs, while for SWNTs d_f = 1.88; the higher d_f for SWNTs reflect a denser network consistent with a lower aspect ratio.  The rigidity of the flocs was determined by the parameter m, which has values of 0.23 – 0.29 for rigid flocs while for soft flocs 0.4 < m < 0.5.  For the MWNTs, m was 0.34, while for the SWNTs it was 0.31 indicating a higher rigidity for SWNTs than for MWNTs.                 

Chemical modification of the MWNTs was achieved by simultaneous polymerization and grafting of polystyrene to the sidewall of the nanotubes using a modified Billips reaction.  The polymerization was confirmed by Fourier Transform Infrared (FTIR) spectroscopy. Thermogravimetric analysis was used quantify the weight percentage of polystyrene on the surface, and Raman spectroscopy confirmed the covalent attachment of the polymeric chains to the nanotubes.  Testing the rheological properties of the modified MWNTs in UPR yielded a lower G’ than the unmodified nanotubes, which may be an indication of better compatibility with the resin.  The weaker attractive interactions of the modified nanotubes result in a weaker network yielding a lower G’.

The results of this research demonstrate the dispersion of  MWNTs in UPR by high shear mixing and the effects of sidewall functionalization on the dispersion state.  They provide a foundation for incorporating CNT into systems where solvent evaporation and surface chemistry