Ternary polymer blends, comprising two homopolymers and the corresponding diblock copolymer, have been examined by small-angle neutron scattering ͑SANS͒ and dynamic light scattering ͑DLS͒. Two chemical systems have been employed: one consisting of polyethylethylene, polydimethylsiloxane, and poly͑ethylethylene-b-dimethylsiloxane͒, and another containing polyethylenepropylene, polyethyleneoxide, and poly͑ethylenepropylene-b-ethylene oxide͒. The molecular weights and compositions were chosen to emphasize the region of the phase prism dominated by the bicontinuous microemulsion ͑BE͒ phase; the homopolymer molecular weights and volume fractions were approximately equal. The SANS intensity was compared quantitatively with the Teubner-Strey structure factor, and interpreted via the amphiphilicity factor f a . The transition from a fully disordered mixture at higher temperatures to a well-developed BE upon cooling did not correlate well with either the disorder line ( f a ϭϩ1) or the total monomer Lifshitz line ( f a ϭ0). However, DLS provided a clear signature of this transition, via a distinct maximum in the temperature dependence of the dynamic correlation length. We hypothesize that this maximum is closely correlated to the homopolymer/homopolymer Lifshitz line. The structure of the interfaces in the BE was further examined in terms of the curvature and the copolymer coverage, as functions of copolymer concentration and temperature.

SynopsisWe have investigated the effects of shear flow on a polymeric bicontinuous microemulsion using neutron scattering, light scattering, optical microscopy, and rheology. The microemulsion consists of a ternary blend of poly͑ethyl ethylene͒ ͑PEE͒, poly͑dimethyl siloxane͒ ͑PDMS͒, and a PEE-PDMS diblock copolymer. At equilibrium, the microemulsion contains two percolating microphases, one PEE rich and the other PDMS rich, separated by a copolymer-laden interface; the characteristic length scale of this structure is 80 nm. Low strain amplitude oscillatory shear measurements reveal behavior similar to that of block copolymer lamellar phases just above the order-disorder transition. Steady shear experiments expose four distinct regimes of response as a function of the shear rate. At low shear rates ͑regime I͒ Newtonian behavior is observed, whereas at intermediate shear rates ͑regime II͒ development of anisotropy in the morphology leads to shear thinning. When the shear rate is further increased, there is an abrupt breakdown of the bicontinuous structure, resulting in flow-induced phase separation ͑regime III͒. Rheological measurements indicate that the shear stress is almost independent of the shear rate in this regime. Light scattering reveals a streak-like pattern, and correspondingly a string-like morphology with micron dimensions is observed with video microscopy. Upon a further increase of the shear rate ͑regime IV͒, the sample resembles an immiscible binary polymer blend with the block copolymer playing no significant role; the stress increases strongly with the shear rate. In some respects these results resemble those from other weakly structured complex fluids ͑sponge phases, liquid crystals, worm-like micelles, a͒ Current address:

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Ternary polymer blends of immiscible homopolymers and the corresponding diblock copolymer are useful models for studying equilibrium and nonequilibrium behavior of self-assembled fluids. We report linear viscoelastic data for a polymeric bicontinuous microemulsion, experiments that are possible due to the comparatively high viscosities of the principal components. After subtracting a viscous background contribution from the pure constituents, the microemulsion exhibits "excess" viscoelastic behavior similar in character to that resulting from the Rouse model of polymer dynamics. The data are compared to the predictions of a time-dependent Landau-Ginzburg model developed by Pä tzold and Dawson, using structural parameters derived from neutron scattering as input. This model captures the essential characteristics of the viscoelastic behavior very well. However, using independent dynamic light scattering measurements of the Onsager coefficient for these blends, it appears that the model fails to predict either the magnitude or the temperature dependence of the zero shear viscosity and relaxation time of the microemulsion accurately. Possible origins for these discrepancies are discussed.