Atomic force microscopy characterization of ultrathin polystyrene films formed by admicellar polymerization on silica disks

摘要

Atomic force microscopy was used to study the characteristics of polymer films formed via admicellar polymerization (the polymerization of monomers solubilized in adsorbed surfactant aggregates). The investigated system included cetyltrimethylammonium bromide (C(16)TAB) as a cationic surfactant, styrene, 2,2'-azobisisobutyrilnitrile as an initiator, and polished silica disk substrates. Our goal was to examine changes in the properties and morphology of the formed polymer films due to changes in the surfactant and monomer feed levels. Normal tapping and phase-contrast modes in air were used to image the nanoscopic and microscopic morphologies of the polystyrene-modified silica. The root-mean-square roughness of the surface before and after modification was statistically analyzed and compared. The images were captured with loading-force set-point ratios of 0.2-0.9, and this allowed us to examine the stability of the polystyrene films. In the first series, for which the feed ratio of C(16)TAB to styrene was kept constant and the total feed concentration was varied, a uniform layer of a polystyrene film was observed along with some nanometersize aggregates at high feed concentrations of both C(16)TAB and styrene. These droplets eventually agglomerated with the film beneath and formed larger macrodroplets in a ring arrangement. At lower concentrations, droplets and holes were observed that eventually agglomerated to form a bi-continuous thin film. In the second experimental series, the concentration of C(16)TAB was kept constant, and the feed ratio of C(16)TAB to styrene was varied. A smooth thin film was observed at high concentrations of styrene. This film could be deformed and/or removed to expose the silica surface beneath. At lower styrene loadings, the polystyrene film became unstable and formed dropletlike aggregates, possibly because of either the uneven adsolubilization of the styrene monomer within the admicelle or the dewetting effect during washing and drying. The structure of the polystyrene film formed on a smooth silica disk was very dependent on the amount of the surfactant fed to the system; this contrasted with the results on precipitated silica. (C) 2003 Wiley Periodicals, Inc. [References: 24]