Dynamics of ternary polymer blends: Disordered, ordered and bicontinuous microemulsion phases

Dynamic light scattering has been used to examine the order parameter fluct uations in ternary homopolymer/homopolymer/block copolymer blends. The chem ical system consists of poly(dimethylsiloxane) (PDMS) and poly(ethylethylen e) (PEE) homopolymers of nearly equal molecular volumes, and a nearly symme tric PDMS-PEE diblock copolymer. The phase diagram along the isopleth (i.e. , equal volumes of each homopolymer) includes a disordered region at high t emperatures that at low temperatures evolves into (a) a swollen lamellar ph ase in the copolymer-rich region; (b) a phase-separated state when there ar e only modest amounts of copolymer; (c) a narrow channel of bicontinuous mi croemulsion for copolymer compositions near where mean-field theory anticip ates an isotropic Lifshitz point. Intensity autocorrelation functions for t he binary blend are single exponential decays, and the associated correlati on length xi scales with reduced temperature epsilon in accordance with the Ising universality class (i.e., xi similar to epsilon(-nu), with nu = 0.63 ). The addition of copolymer depresses the critical temperature, but also i ncreases the magnitude of nu. For compositions within the microemulsion cha nnel xi exhibits a clear maximum with decreasing temperature, near the Lifs hitz line obtained from the static structure factor. For one particular com position there is a "re-entrant" microemulsion, as the system passes into a nd then out of the phase-separated region upon cooling. Below the Lifshitz line there is consistent evidence of one or two small amplitude, faster mod es in the correlation functions. These modes are also apparent in blends wi th compositions corresponding to the swollen lamellar phase, and are tentat ively attributed to translational diffusion of the copolymer and undulation s of the layers.