Macroscale Experimental Evidence of a Reduced-mobility Non-bulk Polymer Phase in Nanotube-reinforced Polymers

Because of their outstanding physical properties, the use of carbon nanotubes within a polymer matrix has been proposed as a means to fabricate multifunctional composite materials with outstanding mechanical, electrical, and thermal properties. To date, experimental work studying the mechanical response of a nanotube reinforced polymer (NRP) has typically focused on the effective elastic modulus of the material, where recent experimental work has demonstrated significant modulus enhancement using small loadings of nanotubes. While these preliminary results are exciting, to date limited theoretical and experimental work has been done to investigate the impact of the nanotubes on the viscoelastic response of the polymer. Because the nanotubes are on the same length scale as the polymer chains, it has been suggested that the polymer segments in the vicinity of the nanotubes will be characterized by a molecular mobility that is different from that of the bulk polymer. This results in the significant differences between the viscoelastic behavior of the NRP, in comparison to the response of the pure polymer, seen experimentally. To further characterize these differences, we have studied the temperatureand frequency-dependent behavior of polycarbonate-nanotube systems with different weight fractions of multiwalled carbon nanotubes (MWNTs). Our experimental results show that the effective viscoelastic behavior of the nanotubereinforced samples are consistent with the presence of a non-bulk polymer phase with restricted molecular mobility within the NRP. These results suggest that analysis of the effective viscoelastic behavior of nanotube-reinforced polymers provided by macroscale mechanical experiments can provide important information concerning the nanoscale interaction between the nanotubes and the polymer.