|Ph.D Student||Granite Meirav|
|Subject||Interactions of Polymers with Carbon Nanotubes in Solution|
|Department||Department of Chemical Engineering||Supervisor||Professor Yachin Cohen|
|Full Thesis text|
Carbon nanotubes (CNTs) both single-wall (SWCNT) and multi-wall (MWCNT) are known by their remarkable unique properties. During production they tend to form heterogeneous bundles, which hinder their applicability and realization of their unique properties. Successful ways to disperse CNTs are based on polymers such as Pluronic?-(Ethylene oxide)n(Propylene oxide)m(Ethylene oxide)n (EOnPOmEOn)- an amphiphilic block copolymer which is known as an excellent dispersant for nanotubes.
Aqueous dispersions of SWCNT/Pluronic F108 (n=132,m=50) and SWCNT/Pluronic F127 (n=106,m=70) were prepared by sonication and light centrifugation to remove catalyst particles and amorphous carbon, yielding homogenous stable ink-like dispersions. Cryo-TEM images revealed isolated, very small bundles of carbon nanotubes, with diameters between 1 to 5 nm and approximately 500 nm length. Small-angle neutron scattering (SANS) with contrast variation experiments were conducted at different D2O/H2O content of the dispersing solvent. The data for both systems showed surprisingly minimal intensity values at 70% D2O solvent composition, much higher than the expected value of 17% D2O, based on the scattering length density (SLD) of PEO. At this near match point the data exhibited a q-1 power law relation of intensity to scattering vector (q) indicating rod-like entities. A core-shell-chains model was evaluated were the hydrophobic block is considered to form a continuous hydrated shell on the CNT surface, whereas the hydrophilic blocks emanate into the solvent. The model fit required special considerations of tight association of water hydrated shell around PEO chains with slight isotopic selectivity and found to reasonably fit the experimental data.
The unusual effect of a dense hydration layer around the polymer chains was further investigated with the homopolymer polyvinylpyrrolidone - PVP, and found again a minimal intensity values at much higher solvent composition (75% D2O) than the expected value of 29% D2O. The minimum scattering curve exhibited a nearly q-1 power law at small angles, an indication of rod-like entities.
It was explored whether the coverage of Pluronic? F127 around the CNTs depends on the polymer concentration, in the range of 1-6% (w/w). Observations indicated that at these concentrations the SWCNT surface is fully saturated at about 14 chains per unit length of 100 Å.
SAXS measurements of MWCNTs/MSM dispersions could not provide information on the adsorbed polymer's quantity or conformation. However, this lack of significant polymer scattering enabled the evaluation of MWCNTs' concentration in the dispersion after the centrifugation process. After data analysis, a hollow cylindrical model was best fitted.