Effect of hydrophilically modified graft polystyrene on AOT oil-continuous microemulsions: Viscosifying effects of P(S-g-PEO) as a function of graft chain length and graft density

The effect of hydrophilically modified polystyrene, poly(styrene-g-poly(ethylene oxide)), P(S-g-PEO), on the viscosity of AOT oil-continuous microemulsions has been investigated as a function of PS and PEO chain lengths and PEO chain density. A series of polymers, P (S-g-PEO) with PEO chain lengths of 23 or 9, and different graft densities were synthesized by living radical polymerization. Thus, a range of polymers with different PS and PEO chain lengths were prepared. Five different microemulsion compositions were studied in an attempt to determine the effect of the microemulsion droplet concentration and droplet size on the viscosity enhancement by the different copolymers. Viscosities were determined at temperatures in the range 25−40 °C. The presence of polymer in the microemulsions always led to increased viscosity relative to the polymer-free microemulsion, the extent of which depended mainly on the polymer composition. Comparisons between the polymers were made in terms of the molar concentration of PEO to take into account differences in the polymer molecular weights and graft densities. A clear correlation was found between the length of the PS chain between grafts and the relative viscosity, with longer PS chains leading to higher viscosities. Increasing the microemulsion droplet concentration resulted in increasing relative viscosity with each of the graft copolymers, because of the shorter interdroplet distances facilitating the formation of active chains. Increasing temperature resulted in a decrease in relative viscosity. Finally, direct comparisons are made between the viscosifying behavior of the graft copolymers and the compositionally similar PEO−PS−PEO triblock copolymers studied previously.