A spectroscopic study of polymer : Carbon nanotube composites

Abstract: Since the identification of carbon nanotubes (CNTs) by Ijima in 1991, this material has become a subject of great interest and effort in science because of the outstanding physical properties it exhibits. CNTs can be thought of as graphene sheets rolled into seamless cylinders of various diameters and in principle infinite length. Depending on the number of concentrically arranged tubes, CNTs are termed single‐walled (SWCNT), double‐walled (DWCNT), and multi‐walled (MWCNT) CNTs. Moreover SWCNTs exist as semiconducting or metallic types, depending on the orientation of the hexagonal lattice relative to the tube axis, as classified by the chiral indices (n, m). Their extraordinary mechanical, electrical, thermal, and optical properties render them very attractive for a wide range of applications including advanced composite materials. However synthesis of CNT‐based composite materials still remains a big challenge. In particular it remains to overcome the difficulties in achieving good nanotubes dispersion within the matrix material. The fact that present synthesis routes produce SWCNTs in a bundled state due to van der Waals intertube interaction is another serious hurdle, as SWCNT bundles do not exhibit the excellent properties of their individual components. Thus special treatment has to be applied in order to break these bundles. In an ideal composite material, the individual SWCNTs would be homogeneously dispersed in the matrix. A second issue is the interaction between the CNTs and the host: to improve the load transfer between host and filler covalent linking between the two components is desirable. One approach to solve these problems is functionalization of the CNT source material prior to its incorporation into the polymer matrix. Optimization is required to maximize the transfer from the polymer to the CNTs but minimize the number of wall defects created by the covalent grafting of the functional groups on the CNT sidewalls. Moreover appropriate functional groups have to be chosen to assure compatibility with the polymer being used. Synthesis of the polymethyl methacrylate (PMMA) composite material used here, based on functionalized SWCNTs, was reported recently and its study revealed inhomogeneities in the CNT distribution within the polymer and associated degradation in the mechanical properties suggested as being attributed to the presence of CNT agglomerates. Since Raman spectroscopy, as a mostly non‐destructive analysis method, has proven to be a powerful tool for studying both pure CNT materials and CNT‐based composites, it was used in this work along with supporting methods (scanning electron microscopy (SEM) and focused ion beam (FIB)) for extended characterization of the composite material, including analysis of the source SWCNT material before and after functionalization. Employment of different laser excitation energies (1.96eV and 2.33eV) allowed to separately probe metallic and semiconducting CNTs in the composite samples. The CNT distribution in the samples was illustrated by Raman spectral mapping of the G+‐ peak intensity as a function of position, thus elucidating the presence of CNT agglomerates of different size and shape. At both photon energies, spectral line scans across the boundary regions were performed revealing a substantial drop in intensity of G+ CNT Raman mode and an increase of the D/G+‐intensity ratio. Examination of the D/G+‐ intensity ratio of the SWCNT material before incorporation into the composite showed a higher value for functionalized than for the raw SWCNTs. Furthermore, the metallic nanotubes exhibited a higher degree of functionalization. Raman spectral imaging revealed some inhomogeneities of the CNT distribution in the composite material: the spectra of the areas with good CNT dispersion in the composite exhibit a higher D/G+‐ intensity ratio than in areas with CNT agglomerates indicating that functionalized CNTs are preferentially dispersed in the polymer matrix while non functionalized ones tend to group together in agglomerates. Furthermore significant laser heating of the SWCNTs in composites has been revealed resulting in a downshift of the G+‐ peak position which was much more pronounced in agglomerates than in the areas with dispersed CNTs and detected at the very lowest laser irradiances. SEM/FIB dual beam technique was employed as a supplementary analysis tool. The composites microstructure in CNT agglomerates as well as in the dispersed area was investigated by acquisition of SEM crossectional images confirming the different local CNT concentrations.