Nanocomposites from carbon nanotubes (CNTs) and anisotropic, nanostructured block copolymer melts are expected to have anisotropic mechanical and electrical properties. However, the effective incorporation of CNTs into the polymer matrix is challenging because of the strong attractive van der Waals interaction between CNTs which results in poor dispersability and the formation of large agglomerates. This is usually overcome by strong shear of the nanocomposite during mixing of the CNTs and the polymer, which is costly.
The aim of the present project is (i) to improve the dispersability of CNTs in solvents and (ii) to incorporate the CNTs in an aligned way into the block copolymer matrix via solvent processing. The modification of the CNTs with polymer grafts is expected to drastically increase the dispersability in solvents and to improve the load transfer between the polymer matrix and the CNTs.
We have applied direct chemical modification of the CNTs with polystyrene (PS) grafts by means of self-initiated photografting photopolymerization (SIPGP). The chemical nature of the modification and the kinetics of the process were studied by Raman spectroscopy and thermo-gravimetric analysis. Varying the polymerization time, it was found that, initially, direct grafting onto the CNTs’ framework takes place, whereas the later stages are dominated by growth and branching of the chain.
The agglomerates formed by the PS-modified CNTs in toluene as a function of the level of modification were investigated using small-angle X-ray and neutron scattering (SAXS, SANS). The latter allowed us to highlight either the CNTs themselves or the PS coatings and thus to gain detailed informations on structure of the modified CNTs. It was found that the radius of the modified single-walled CNTs increases with polymerization time. For modified multi-walled CNTs, in addition to a growth of a polymer layer on the outside, polymerization also takes in the core of the CNT and between the graphene layers. Moreover, it was found that the agglomerates formed by the modified CNTs are significantly less compact than those formed by the non-modified CNTs, which is a key factor for the formation of the nanocomposites with polymers.
Nanocomposites of the modified CNTs with poly(styrene-b-isoprene) (P(S-b-I)) diblock copolymers were prepared by solution processing in tetrahydrofurane, a good solvent for all components. The block copolymer forms PS cylinders in a PI matrix. Scanning electron microscopy indicates the homogeneous distribution of the modified CNTs within the matrix due to the selective accumulation of the modified CNTs in the PS cylinders. Thus, the block copolymer morphology serves as a template for the directed assembly of the modified CNTs.