Fuyong Cheng

Supramolecular complexes of single walled carbon nanotubes with conjugated polymers

We have synthesized a series of conjugated polymers, including poly[(2,7-(9,9-dioctylfluorene)-alt-2,7-(9,9-didodecylfluorene)] (PF), poly[(2,7-(9,9-dioctylfluorene)-alt-2,5-(3-dodecylthiophene)] (PFT), and poly(3-dodecylthiophene) (PT). Non-covalent functionalization of single walled carbon nanotubes (SWNTs) with these polymers can impart good solubility to nanotubes in a number of organic solvents, including THF, dichlorobenzene, chloroform, and toluene. Solution and solid-state characterization of the resulting polymer–SWNT composites are described, including UV-Vis absorption and Raman spectroscopy. It was found that the UV-Vis absorption maximum of the polymers was red-shifted in their corresponding composites due to the planarization of the polymer backbone following adsorption upon the SWNT surface. Polymer–SWNT complexes also exhibited good solution stability at elevated temperature in THF and dichlorobenzene, with no significant SWNT sedimentation observed at elevated temperatures. Both UV-Vis absorption and Raman spectroscopy results indicated that the interaction of PT with the nanotubes was different from those of PF and PFT, suggesting that the choice of aromatic ring in the polymer structures plays an important role in the supramolecular complex formation with carbon nanotubes.

The effect of molecular weight on the supramolecular interaction between a conjugated polymer and single-walled carbon nanotubes

The synthesis and fractionation of poly[2,7-(9,9-dioctylfluorene)-alt-2,5-(3-dodecylthiophene)] resulted in the isolation of eight different molecular weight (Mw) samples ranging from Mn of 5 to 85 kg mol−1. These individual polymer samples were fully characterized by Gel Permeation Chromatography, Nuclear Magnetic Resonance, as well as absorption and fluorescence spectroscopy. Each sample was separately mixed and ultrasonicated with single-walled carbon nanotubes (SWNTs) in tetrahydrofuran, and the nanotube concentration within the resulting solutions was measured. It was found that the solubility of the polymer–SWNT complexes strongly depends on the Mw of the conjugated polymer, with a maximum concentration reached when Mw ranged between 10 and 35 kg mol−1. Higher and lower Mws resulted in substantially reduced nanotube concentrations.

Supramolecular interactions of conjugated Zn- and protonated porphyrin polymer with carbon nanotubes

The interaction of a highly soluble conjugated Zn-porphyrin containing polymer with a surface of single-walled carbon nanotubes, producing a soluble polymer-nanotube complex, is reported. We found that successful complexation required the addition of trifluoroacetic acid to the solvent tetrahydrofuran in order to disrupt solvent-porphyrin coordination that prohibits close interaction of the polymer with the nanotube surface. In the presence of trifluoroacetic acid, the complex remained soluble even after excess free polymer was removed from solution, and could be centrifuged at high speed with no observable sedimentation. Furthermore, the polymer-nanotube assembly resulted in enhanced planarization and conjugation within the porphyrin polymer, which was observed via a 127 nm bathochromic shift of the Q-band absorption in the UV-vis spectrum. In addition, removal of the Zn atoms and protonation of the porphyrin repeat units under acidic conditions resulted in a polycationic polymer that also forms strong interactions with carbon nanotubes which result in soluble supramolecular complexes. Control experiments with the Zn-porphyrin monomer indicated that homogeneous solutions could be prepared by sonication, but the monomer-nanotube interactions were weak and resulted in nanotube precipitation within minutes. Atomic force microscopy and transmission electron microscopy studies indicated that the polymer is capable of exfoliating large nanotube bundles into individual tubes and small bundles that are coated in polymer.