Amphiphilic block copolymers consisting of a hydrophobic block of
hydrogenated polybutadiene and a partly sulfonated polyelectrolyte block of
poly(styrenesulfonate) are effective stabilizers in
emulsion polymerization. At low relative amounts of block
copolymer (0.4 wt % as a fraction of monomer
weight) and salt-free conditions, well-defined and stable latices with
particle diameters of ca. 100 nm
and solid contents of 20 wt % are obtained. This high
stabilization efficiency of optimized block copolymer
systems enables the formulation of latex systems with a relatively low
remaining polarity in solid films
and offers new interesting model systems with exclusively electrosteric
stabilization. A comparison of
polymerization in high or low ionic strength solution and the variation
of the degree of sulfonation of the
poly(styrenesulfonate) block shows an optimum stabilization of the
latices at low ionic strength during
polymerization. Fully sulfonated polymers systems presumably show
a molecular orientation perpendicular to the particle surface, whereas 50% sulfonated species take on
a traillike conformation along
the surface, which is explained by remaining hydrophobic interactions.
Due to this multiple surface
particle contacts, the application of partly sulfonated polymers leads
to more effective stabilization. At
higher stabilizer concentrations, aggregates are found, which can be
redispersed by ultrasonification or
addition of low molecular weight surfactant solution.
In this letter, we testify the feasibility of using freestanding foam films as a thin liquid gas separation membrane. Diminishing bubble method was used as a tool to measure the permeability of pure gases like argon, nitrogen, and oxygen in addition to atmospheric air. All components of the foam film including the nature of the tail (fluorocarbon vs hydrocarbon), charge on the headgroup (anionic, cationic, and nonionic) and the thickness of the water core (Newton black film vs Common black film) were systematically varied to understand the permeation phenomena of pure gases. Overall results indicate that the permeability values for different gases are in accordance with magnitude of their molecular diameter. A smaller gaseous molecule permeates faster than the larger ones, indicating a new realm of application for foam films as size selective separation membranes.
Gas permeability and thin-film interferometry are used as a tool to elucidate the orientation of polymeric headgroups in free-standing foam films. Nonionic polyoxyethylene (EO) surfactants were used to stabilize the foam films, keeping the size of the hydrophobic part constant (C12) and varying the size of the hydrophilic (EO numbers) part. The effect of headgroup size on the gas permeability of Newton black foam films was studied. Thickness, contact angle, and surface tension were measured to understand the permeation mechanism. Increase of film thickness and surface tension was observed while increasing the headgroup size, but the contact angle remains small and constant. Upon increasing the headgroup size, the permeability decreases showing that the headgroups provide a resistance to permeation. For smaller headgroups, the permeability follows a linear dependence on the film thickness, whereas for larger headgroups, the permeability essentially deviates from linearity. We use the conventional "coil model" of the EO chains to explain the observed results providing a detailed picture of the orientation of this important molecule in a confined volume of foam films.
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