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

Abstract

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, 2,2Ј-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 setpoint 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 bicontinuous 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.

Two-dimensional phase transition of styrene adsolubilized in cetyltrimethylammonium bromide admicelles on mica

“…It has been shown that the level of adsolubilization impacts the physical and cure properties of admicellar polymerization modified silica-reinforced rubber compounds [7,[21][22][23]. More recently the impact of the levels of adsorbed surfactant and adsolubilized monomer on the polymer formed during admicellar polymerization has been examined on flat and amorphous silica surfaces [18,19]. Results on both surfaces show that the distribution, thickness, and morphology of the formed polymer are very strongly determined by the amount of added monomer, and somewhat determined by the amount of adsorbed surfactant [18,19].…”

“…The structure of the formed polymer thin film has been examined using ellipsometry [5], scanning electron microscopy [17], and tapping mode atomic force microscope at ambient condition [18,19]. However, without an understanding of the behavior of the multicomponent adsorbed layer, the structure and properties of the formed polymer film cannot be predicted or effectively controlled [20].…”

“…More recently the impact of the levels of adsorbed surfactant and adsolubilized monomer on the polymer formed during admicellar polymerization has been examined on flat and amorphous silica surfaces [18,19]. Results on both surfaces show that the distribution, thickness, and morphology of the formed polymer are very strongly determined by the amount of added monomer, and somewhat determined by the amount of adsorbed surfactant [18,19]. Although other effects such as the molecular structure of surfactant and monomer, counterion concentration and type, initiator type, washing solvent and temperature may also influence the morphology of the polymer film, the systematic analysis of those effects on the polymer film has yet to be studied.…”

“…The droplet sizes fall into two ranges, 100-200 nm and ≈20 nm. These droplets are thought to be responsible for the irregular patchy polymer film found after admicellar polymerization[4][5][6][7][18][19][20][21][22][23].…”

Atomic force microscopy and contact angle studies of polymerizable gemini surfactant admicelles on mica

“…[19][20][21] More recently, the examination of polymer formation via admicellar polymerization has been studied using atomic force microscopy (AFM) that allows the film to be studied at the nanometer scale. [29][30][31] AFM has been used to probe the nature of surfactant-modified mica surface morphologies. [31][32][33] The AFM tip can interact with the sample surface at the atomic level.…”