Effect of Electrolytes and Temperature on the Structure of a Poly(ethylene oxide)−Poly(propylene oxide)− Poly(ethylene oxide) Block Copolymer Adsorbed to a Perfluorocarbon Emulsion

Abstract

We have used small-angle neutron scattering to study the detailed structure of the poly(ethylene oxide)−poly(propylene oxide)−poly(ethylene oxide) (PEO−PPO−PEO) block copolymer Poloxamer 188 adsorbed at the interface in a perfluorodecalin in water emulsion and specifically to investigate the effect of electrolytes and of temperature. Low (<0.5 M) concentrations of 1:1 electrolyte caused the polymer layer to extend from the interface, apparently by desorption of loop contacts between the PEO chains and the perfluorocarbon−water interface. The extension of the polymer layer is accompanied by a reduction in the fraction of bound polymer segments. A similar effect was found with increasing temperatures. Higher concentrations of electrolyte caused the PEO chains to collapse slightly toward the surface, and it is suspected that this was due to the water becoming a poorer solvent for PEO as the ionic strength of the dispersion medium was increased. No corresponding collapse of the polymer chains was observed at higher temperatures, in the temperature range studied (25−55 °C), but this does not rule out collapse at higher temperatures; the cloud point of the polymer in 1% solution is 112 °C.