Dispersions of multiwalled carbon nanotubes in liquid crystals: new challenges to molecular theories of anisotropic soft matter
Longin Lisetski
Recently, great attention has been attracted by the anisotropic soft matter systems including particles of different size, shape and aspect ratio. Among the most interesting are dispersions of multi-walled carbon nanotubes (NT) in liquid crystalline (LC) media displaying many interesting and unexpected features and presenting stimulating challenges to molecular theories of anisotropic liquids.
For theoretical consideration, two different types of LC+NT dispersions can be assumed. At low NT concentrations, each NT aligns several molecular layers of nematic molecules on a short-range scale. On the long-range scale, nematic as an anisotropic fluid orients the NTs formally considered as non-mesogenic dopants. In real systems, NTs show a strong tendency for aggregation in a LC medium. Above a certain concentration of NTs (“percolation threshold”), the spanning network arises and grows with time through incorporation of individual NTs from the quasi-homogeneous dispersion. The aggregates in the vicinity of percolation threshold are of fractal nature. These ramified aggregates capture surrounding LC molecules, and their volume becomes by 2-2.5 orders greater than the total volume of the NTs involved. It was shown by methods of singular optics that such anisotropic microsized LC cladding initiates strong speckled scattering with induced optical singularities. One can conceive of these aggregates as of large quasi-macroscopic particles in the nematic matrix.
We studied dynamics of NT aggregate formation in freshly prepared (by ultrasonication) dispersions of 5CB + multi-walled carbon NTs (0.01-0.1 wt %). Several different methods were used: (a) microscopic observation of microstructure, (b) light transmission changes at the point of nematic-isotropic phase transition (Ti), (c) differential scanning calorimetry of phase transition peaks, (d) temperature and concentration dependences of electrical conductivity, and conductivity vs. applied voltage behavior in the Freedericks transition geometry, (e) measurement of birefringent structure of LC cladding around NT aggregates and induced optical singularities.
The obtained results were in reasonable agreement and could be explained by the proposed tentative model. The individual NTs are gradually assembled into fractal aggregates with 5CB molecules incorporated into the “micropores” of the “skeleton” formed by a loose NT arrangement. For example, at 0.1% wt of NTs the aggregates formed after several hours of incubation effectively behave like micron-sized small particles occupying up to 20-40% of the bulk nematic volume. Higher concentration and higher aspect ratio of NTs strongly favor the aggregation, while in general the behavior was similar. The obtained results were compared with other systems involving anisotropic micro- and nanoparticles dispersed in a LC matrix (e.g., plate-like particles of organomodified montmorillonite, self-assembling aggregates of anthraquinone dyes in 5CB, etc.).