Mechanical behavior of polystyrene grafted carbon nanotubes/polystyrene nanocomposites

This study is concerned with the improvement of interfacial adhesion in polystyrene - carbon nanotubes composites by grafting polystyrene chains onto the nanotube surface. Based upon a previously described method (Fragneaud B, Masenelli-Varlot K, Gonzalez-Montiel A, Terrones M, Cavaille JY. Efficient coating of N-doped carbon nanotubes with polystyrene using atom transfer radical polymerization. Chem Phys Lett 2006;419(4-6):567-73), the polystyrene was covalently bond onto the nanotube surface with an average molecular weight of about 10⁴ g mol^-1 from which composites with different nanotube weight fraction were synthesized. The grafting efficiency, has been extrapolated from dynamic stress-train data in the linear viscoelastic domain (below and above T_g) as the same as using compressive tests below T_g. In this work, it is shown that short chains polymer grafting, (i) enhanced the dispersion of the nanotubes within the matrix, (ii) increased the Young modulus below T_g, and (iii) increased the stress breaking of the nanocomposites. We also note that the low molecular weight grafting plasticize the matrix near the nanotube surface. The linear viscoelastic behavior is compared with predictions from a mechanical coupling model. Finally, the role of the nanotube waviness, percolation, as well as the entanglement is discussed.