Importance of capillary forces in the assembly of carbon nanotubes in a polymer colloid lattice.

We highlight the significance of capillary pressure in the directed assembly of nanorods in ordered arrays of colloidal particles. Specifically, we discuss mechanisms for the assembly of carbon nanotubes at the interstitial sites between latex polymer particles during composite film formation. Our study points to general design rules to be considered to optimize the ordering of nanostructures within such polymer matrices. In particular, gaining an understanding of the role of capillary forces is critical. Using a combination of electron microscopy and atomic force microscopy, we show that the capillary forces acting on the latex particles during the drying process are sufficient to bend carbon nanotubes. The extent of bending depends on the flexural rigidity of the carbon nanotubes and whether or not they are present as bundled ensembles. We also show that in order to achieve long-range ordering of the nanotubes templated by the polymer matrix, it is necessary for the polymer to be sufficiently mobile to ensure that the nanotubes are frozen into the ordered network when the film is formed and the capillary forces are no longer dominant. In our system, the polymer is plasticized by the addition of surfactant, so that it is sufficiently mobile at room temperature. Interestingly, the carbon nanotubes effectively act as localized pressure sensors, and as such, the study agrees well with previous theoretical predictions calculating the magnitude of capillary forces during latex film formation.