Direct Observation of Polymer Sheathing in Carbon Nanotube−Polycarbonate Composites

A series of observations of polymer sheathing in multiwalled carbon nanotube (MWCNT)−polycarbonate composites are presented. This sheathing was observed in images of the composite fracture surface and is consistent with diameter distributions of the as-received and embedded MWCNTs. A novel nanomanipulation experiment, where the sheathing balls up when contacted by an AFM tip, confirms this phenomenon. This sheathing layer is direct evidence of substantial MWCNT−polymer interaction and will influence the mechanical properties of MWCNTpolymer composites. Due to the outstanding physical properties of carbon nanotubes, intense activity is being devoted to the development of carbon nanotube-polymer composites.1 Specifically, carbon nanotube-reinforced polymer composites have demonstrated high strength and stiffness,2 which suggest their potential use as alternative materials for structural applications. Multifunctional nanotube-polymer composites are also under development, where in addition to improved mechanical properties, increases in electrical conductivity3 and improved thermal properties4 are obtained with small amounts of embedded nanotubes. One of the significant differences between micron-sized carbon fiber-filled polymers and nanotube-filled polymers is the large interfacial area of the nanotubes. This interfacial area provides an opportunity for altering the mobility and properties of a significant volume of polymer near the interface (i.e., the interphase region). Both the interface and interphase regions will play key roles in optimizing load transfer between the nanotube and the polymer matrix. While for traditional composites a variety of experimental techniques have been developed in an effort to quantify the fibermatrix interface,5 for nanotube-polymer composites these tests are exceedingly difficult because of the small size of the nanotubes. In the process of developing nanoscale pullout tests of individual multiwalled carbon nanotubes from a polymer matrix, we have found several forms of evidence that suggest multiple polymer layers sheath the embedded nanotubes. This polymer sheathing is consistent with models of a nonbulk polymer interphase region that has been identified in nanotube-polymer composite systems.6 The results presented in this paper are consistent with the findings of other researchers regarding the existence of intimate MWCNT-polymer interaction in nanotubepolymer composites. For example, strong polymer adherence has been reported in previous TEM studies of nanotubepolymer nanocomposite samples.7 Potschke et al. studied the rheological behavior of nanotube-polycarbonate composites, and their SEM observation of the fracture surface showed that the apparent nanotube diameters at the fracture surface were larger than the diameters of the original carbon nanotube material, indicating significant polymer wetting on the nanotube surface.8 However, to date a detailed study of this polymer sheathing phenomenon in carbon nanotubepolymer nanocomposites has not been undertaken. We present here a series of direct observations on MWCNTpolycarbonate samples that unambiguously support this polymer sheathing phenomenon, including (i) SEM images of the fracture surface showing an annular coating on the nanotube; (ii) a significant increase in the “apparent” diameter of nanotubes protruding from the fracture surface in comparison to the diameters of the as-received MWCNTs; (iii) a “balling up” effect that has been observed in an SEM * Corresponding author. E-mail: [email protected]. Phone: 847467-6596. Fax: 847-491-3915. † Northwestern University. ‡ Rensselaer Polytechnic Institute § University of Kentucky. NANO LETTERS 2003 Vol. 3, No. 11 1593-1597 10.1021/nl0345973 CCC: $25.00 © 2003 American Chemical Society Published on Web 10/25/2003 when this annular coating is touched by an AFM tip; and (iv) the presence of multiple layers of polymer sheathing adhered to a MWCNT pulled from the composite fracture surface. An understanding of this polymer sheathing phenomenon and its impact on the physical properties of the interphase region will be critical as methods to quantitatively characterize the nanotube-polymer interface in these systems