Dynamic light scattering from ternary polymer blends:: Critical behavior and bicontinuous microemulsions

We have used dynamic light scattering to study the dynamics of ternary polymer blends consisting of poly(dimelhylsiloxane) (PDMS) and poly(ethylethylene) (PEE) homopolymers and a PDMS-PEE diblock copolymer nearly symmetric in composition. The intensity autocorrelation functions for the binary blend are single-exponential, and the associated correlation length xi scales with reduced temperature epsilon in accordance with the Ising universality class (i.e., xi similar to epsilon(-v) with nu = 0.63) An addition of copolymer depresses the critical temperature, but also increases the magnitude of nu. For compositions within the microemulsion channel, xi exhibits a distinct maximum with decreasing temperature, near the Lifshitz line obtained from the static structure factor. For a particular composition, there is a "re-entrant" microemulsion, as the system passes into and then out of the phase-separated region upon cooling.