Interfacial in situ polymerization of single wall carbon nanotube/nylon 6,6 nanocomposites
Introduction
Single wall carbon nanotubes (SWNT) are considered promising fillers in nanocomposites due to their exceptional mechanical, electrical, and thermal properties and their large aspect ratio, all of which can lead to significantly improved composite performance. The fabrication of SWNT/polymer nanocomposites has been achieved by the use of several different fabrication methods that combine various polymer matrix systems with carbon nanotubes. Solution processing methods are available if the polymer is soluble in a solvent that can suspend nanotubes, enabling the mixing of polymer and nanotubes in the solvent [1], [2], [3], [4]. Melt compounding incorporates the nanotubes into a molten thermoplastic polymer that is mechanically sheared in a compounder [5], [6], [7], [8], [9]. Nanotubes can be added to this polymer melt in the compounder dry or suspended in a solvent to achieve good dispersion [10], [11]. In situ polymerization methods offer the possibility to incorporate SWNT into polymer matrixes while preserving the nanotube dispersion initially found in the reaction medium containing the monomers [12], [13], [14], [15], [16].
In any of these fabrication methods, SWNT that are well dispersed in solvents (including monomers) prior to composite fabrication facilitate good SWNT dispersion in the subsequent composites. This can be achieved with the aid of surfactants or functional groups that are self-assembled or covalently attached to the nanotube surface, respectively [17], [18], [19], [20]. Well-dispersed SWNT exist as small bundles or individual nanotubes. Incorporation of functionalized nanotubes is preferably done by the use of a solvent processing method or an in situ polymerization to preserve the superior nanotube dispersion.
Nylon 6,6, a commercially important thermoplastic, cannot be readily solvent processed with nanotubes because nylon 6,6 is soluble in only a few solvents that either do not suspend nanotubes, (e.g. formic acid) or may even damage nanotubes, (e.g. sulfuric acid). Melt compounding can be used, but the melt viscosity of nylon 6,6 is rather low, resulting in small shear forces and poor SWNT dispersion when dry nanotubes are added. We previously used a melt compounder to combine SWNT suspensions using HDPE [11], but this process cannot be applied to nylon 6,6 because the processing temperature (∼270 °C) is well above the boiling temperature of suitable solvents to suspend SWNT.
Here we present an interfacial in situ polymerization method for SWNT/nylon 6,6 nanocomposites that can be used with a variety of SWNT types. Based on the familiar ‘nylon rope trick’, this step growth polymerization method incorporates both an organic and an aqueous phase, each carrying one of the two highly reactive monomers. The polymerization takes place at the interface between the two immiscible organic and aqueous phases where the monomers meet and rapidly react. Thus, nanotubes can be suspended in either phase, allowing the use of functionalized nanotubes that prefer either an aqueous or an organic solvent environment. The initial dispersion of the nanotubes in suspension is preserved in the resulting nanocomposites. Here, SWNT were incorporated into nylon 6,6 nanocomposites from suspensions of purified SWNT, surfactant-assisted suspensions, or suspensions of SWNT functionalized with short alkyl chains, to study the effect of the nanotube dispersion method. The electrical and mechanical properties of the SWNT/nylon 6,6 nanocomposites are also recorded.
Section snippets
SWNT purification and functionalization
SWNT were synthesized by the high-pressure carbon monoxide method (HiPco, Rice University) [21]. Nanotube suspensions were made with purified, functionalized and surfactant suspended SWNT. Purified nanotubes were obtained after a soft-bake at 250 °C for 24 h followed by sonicating in concentrated HCl at 80 °C for 20 min, and washing with water [22]. Functionalization [19], [20] was initiated by refluxing the SWNT at 115 °C in 2.6 M nitric acid for 12 or 48 h while stirring to decorate the SWNT with
Interfacial polymerization of nylon 6,6
The reaction product of the interfacial polymerization is a white powder. Fourier-transform IR confirmed the chemical structure of the nylon 6,6, showing absorptions for all required chemical groups: N–H stretch at 3304 cm−1, C–H stretch at 2860–2940 cm−1, amide-I at 1632 cm−1, and amide-II at 1540 cm−1 [25]. The Mark–Houwink equation ( using Kv=3.53×10−4 and a=0.786, for nylon 6,6 in 90% HCOOH at 25 °C [25]) was used to determine the viscosity averaged molecular weight from the
Conclusions
We have adapted an interfacial in situ polymerization method to the fabrication of SWNT/nylon 6,6 nanocomposites. This versatile fabrication method incorporates SWNT suspended in either water or toluene, and can be readily extended to a variety of nanofillers with different surface properties and solvent preferences. The quality of the nanofiller suspension prior to the in situ polymerization determines to a large extent the nanofiller dispersion in the resulting nanocomposite. In the case of
Acknowledgements
Funding for this research was provided by the National Science Foundation (DMR–MRSEC 05-20020 and DMR–IMR) and the Office of Naval Research (N00014-03-1-0890 and DURINT N00014-00-1-0720).
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