Model bicontinuous microemulsions in ternary homopolymer/block copolymer blends

We have identified a channel of bicontinuous microemulsion in three chemically distinct hompolymer/ hompolymer/block copolymer (A/B/A-B) ternary bends. Experiments were conducted along the isopleth, defined by equal volumes of homopolymer and varying amounts of block copolymer, as a function of temperature. A symmetric condition was achieved through the use of homopolymers with matched degrees of polymerization (N_A ≈ N_B = N_H) and compositionally symmetric diblock copolymers (f ≈ 0.5) where α ≡ N_AB/N_H ≈ 0.2. We explored PE-PEP/PE/PEP (EP), PEE-PDMS/PEE/PDMS (EED), and PE-PEO/PE/ PEO (EO) ternary systems differing in molecular weight by nearly 2 orders of magnitude. Using a combination of small-angle neutron scattering (SANS), rheology, and cloud point measurements, we mapped the phase diagram along the isopleth for each of these systems. On the block-copolymer-rich side of the phase diagrams, a line of lamellar-disorder transitions was observed. On the homopolymer-rich side of the phase diagrams a line of transitions separating one-phase and phase-separated regions was found. A narrow channel of bicontinuous microemulsion separates these two regimes in all three systems. This bicontinuous microemulsion phase is similar to the analogous bicontinuous phases found in oil/water/surfactant mixtures. We have demonstrated that there is a common region in phase space over which the bicontinuous microemulsion is stable in these polymeric systems and that the general phase behavior is independent of polymer molecular weight. The low molecular weight of the EO system is ideal for fundamental phase behavior studies in polymeric blends, since the kinetic limitations that plague high-molecular-weight mixtures are avoided. Furthermore, the EO system utilizes polyethylene-poly(ethylene oxide) block copolymers that are chemically very similar to well-known nonionic surfactants, and thus, connections to surfactancy can readily be made.