SANS study of polymer-containing microemulsions

Abstract

The water soluble polymer polyethylene oxide (PEO) was introduced into dilute water-in-oil microemulsions stabilised by the surfactant Aerosol-OT (AOT). Since PEO is insoluble in the oil (n-heptane), the polymer is confined within the aqueous microphase. The phase stability of the resulting microemulsions is significantly affected by the concentration and molecular weight of the polymer. At fixed mass concentration of the polymer relative to the aqueous phase, any structural or phase behaviour changes as a function of molecular weight are primarily entropic. We have used small-angle neutron scattering (SANS) and conductivity measurements to show that the polymer (PEO) can induce droplet aggregation, with increased polydispersity at higher volume fractions of the aqueous microphase, and critical-like scattering on approaching the upper-temperature phase boundary. At R = [Water]/[AOT] = 40, the polymer-free microemulsion shows critical scattering, with the correlation length, ζ, diverging as the cloudpoint, Tc, is approached. It appears that when the polymer radius of gyration, Rg, is large compared to the water droplet radius, r, the polymer chains are only solubilised when the temperature is sufficiently high so that ζ ≈ Rg. The critical exponents extracted from the data (v = 0.67 ± 0.08 and γ = 1.25 ± 0.16) are consistent with the 3-dimensional Ising model. Thus, the presence of polymer in these microemulsions does not affect their universality class.