Lateral Diffusion of Dispersing Molecules on Nanotubes As Probed by NMR

Abstract

Noncovalent dispersion of carbon nanotubes is essential to most applications but still poorly understood at the molecular level. The interaction of the dispersing molecule with the nanotube, wrapping or nonwrapping, still awaits consensus. Herein, we have studied by 1 H NMR diffusometry some features of molecular dynamics in the system of carbon nanotubes dispersed by triblock copolymer Pluronics F127 in water. The diffusional decays obtained at different diffusion times, Δ, are not single-exponential and have a complex Δdependent profile, ultimately implying that the polymer is observed in two states: free (in unimeric form) and nanotubebound. Fitting a two-site exchange model to the data indicates that at any instant, only a small fraction of polymers are adsorbed on the nanotubes, with polydisperse residence times in the range of 100−400 ms. Most significantly, we further provide an estimate of D = (3−8) × 10 −12 m 2 s −1 for the coefficient of lateral diffusion of the polymer along the nanotube surface, which is an order of magnitude slower than the corresponding self-diffusion coefficient in water. The emerging picture is that of a nonwrapping mode for the polymer−nanotube interaction.

Block Copolymers as Dispersants for Single-Walled Carbon Nanotubes: Modes of Surface Attachment and Role of Block Polydispersity

“…Namely, as shown in Figure , in neat solution, the PEO and PPO signals of F127 both exhibit single-exponential decays in NMR diffusion experiments as predicted in a homogeneous bulk solution characterized by a single diffusion coefficient, , yielding identical (as they must for blocks residing within the same diffusing molecule) diffusion coefficient of D = 5 × 10 –10 m 2 /s. In SWNT dispersions, although the two blocks still reside within the same molecule, we get a radically different behavior: the PPO block exhibits decay similar to that in bulk solution, but the PEO block presents a slowly decaying component associated with a much lower self-diffusion coefficient. ,, A similar slow component was also observed with other nonionic block copolymers . The sole possible explanation for this behavior is that the 1 H NMR signal of the PPO block that anchors a fraction of the F127 molecules to the SWNT surface is broadened beyond detectability in the spectrum (indeed, a strong NMR spectral broadening has been detected directly in polyacrylic acid adsorbed upon multiwalled CNTs).…”

“…Recently, the adsorption of PEG-ylated lipids (block copolymers with a large PEO block and a hydrophobic alkyl chain) in nanohorns (capped very short, ∼30 nm, single-walled CNTs, usually aggregated) has been studied by NMR . In striking contrast to previous findings with Pluronics-dispersed CNTs, ,, the adsorption of those lipids to the nanohorns resulted in a loss of the 1 H NMR signal, both for the hydrophobic alkyl and for the hydrophilic PEO blocks.…”

“…The sample preparation was carried out similarly to that in previous studies. ,, The SWNT powder (either 1.5, 3.0, or 4.5 mg, see also Table below) was weighed into a tube with a spherical bottom, followed by the addition of 3 mL of F127 solution (with copolymer concentration c F127‑init = 3.0 or 6.0 mg/mL), hence yielding initial SWNT concentrations c SWNT‑init = 0.5, 1.0, or 1.5 mg/mL. The sample was sonicated with a 3.2 mm tip (Qsonica Q500) during 10 min, transferring to the liquid an energy density of approx.…”

“…In order to use low sample volumes (∼100 μL) and provide a homogeneous gradient, the samples were placed in 5 mm Shigemi tubes. Additional NMR details were performed as in our previous works. ,,, …”

“…Yet, this wrapping picture has not been verified for other dispersants. For example, some nonionic block copolymers (Pluronics) have instead been surmised to have their hydrophobic block (polypropylene oxide, PPO) attached to the CNTs, whereas the hydrophilic blocks (polyethylene oxide, PEO) extend into the surrounding good solvent, water, ,,− thereby exerting steric repulsion between the CNTs. This mode of dispersant adsorption, in line with the experimental observations in systems exhibiting other surfaces and also supported by simulations, , permits both rather fast surface diffusion and a short residence time on the CNT that have been revealed by NMR diffusion experiments. , These particular experiments were, indeed, evaluated under the tacit assumption that the hydrophilic blocks have a very large degree of freedom for fast segmental motions that sets their relevant NMR relaxation times long enough to allow such experiments.…”