Model bicontinuous microemulsions in ternary homopolymer block copolymer blends

We have identified a channel of bicontinuous microemulsion in three chemica lly 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 tempe rature. A symmetric condition was achieved through the use of homopolymers with matched degrees of polymerization (N-A approximate to N-B = N-H) and c ompositionally symmetric diblock copolymers (f approximate to 0.5) where al pha = N-AB/N-H approximate to 0.2 We explored PE-PEP/PE/PEP (EP), PEE-PDMS/ PEE/PDMS (EED), and PE-PEO/PE/PEO (EO) ternary systems differing in molecul ar weight by nearly 2 orders of magnitude. Using a combination of small-ang le neutron scattering (SANS), rheology, and cloud point measurements, we ma pped 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-disord er transitions was observed. On the homopolymer-rich side of the phase diag rams 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 sys tems and that the general phase behavior is independent of polymer molecula r weight. The low molecular weight of the EO system is ideal for fundamenta l phase behavior studies in polymeric blends, since the kinetic limitations that plague high-molecular-weight mixtures are avoided. Furthermore, the E O system utilizes polyethylene-poly(ethylene oxide) block copolymers that a re chemically very similar to well-known nonionic surfactants, and thus, co nnections to surfactancy can readily be made.