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

Dynamic light scattering has been used to examine the order parameter fluctuations in ternary homopolymer/homopolymer/block copolymer blends. The chemical system consists of poly(dimethylsiloxane) (PDMS) and poly(ethylethylene) (PEE) homopolymers of nearly equal molecular volumes, and a nearly symmetric PDMS-PEE diblock copolymer. The phase diagram along the isopleth (i.e., equal volumes of each homopolymer) includes a disordered region at high temperatures that at low temperatures evolves into (a) a swollen lamellar phase in the copolymer-rich region; (b) a phase-separated state when there are only modest amounts of copolymer; (c) a narrow channel of bicontinuous microemulsion for copolymer compositions near where mean-field theory anticipates an isotropic Lifshitz point. Intensity autocorrelation functions for the binary blend are single exponential decays, and the associated correlation 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 increases the magnitude of nu. For compositions within the microemulsion channel xi exhibits a clear maximum with decreasing temperature, near the Lifshitz line obtained from the static structure factor. For one particular composition there is a "re-entrant" microemulsion, as the system passes into and then out of the phase-separated region upon cooling. Below the Lifshitz line there is consistent evidence of one or two small amplitude, faster modes in the correlation functions. These modes are also apparent in blends with compositions corresponding to the swollen lamellar phase, and are tentatively attributed to translational diffusion of the copolymer and undulations of the layers.