Complex fluids including polymer solutions and blends, surfactants, liquid crystals, block copolymers, and colloids often exhibit fascinating rheological properties, and can undergo a variety of morphological transformations under flow. The complex internal microstructure of these fluids leads to behavior that is vastly different from that of conventional Newtonian or non-Newtonian fluids. In many cases shear flow is found to disrupt the equilibrium morphology and to induce apparent shifts in phase boundaries.

Bicontinuous microemulsions, formed by mixing appropriate amounts of oil, water, and a surfactant [Scriven (1976)] form an intriguing class of complex fluids. They possess a characteristic nanostructure consisting of undulating surfaces with vanishingly small interfacial curvature. Other properties of these fluids include a very large amount of interfacial area and negligible surface tension due to the presence of the amphiphile. With their complicated and delicately balanced internal structure, one can expect bicontinuous microemulsions to exhibit rich rheological properties. However, there have been relatively few reports of the rheological behavior of oil–water–surfactant (o/w/s) bicontinuous microemulsions [Chen and Warr (1992); Anklam et al. (1995); Warr (1995)]; such studies have been hindered by rapid microstructural dynamics, which necessitate the use of extremely high shear rates.

Recently we have demonstrated the utility of ternary polymer blends as model surfactant systems [Washburn et al. (2000)]. In particular, bicontinuous microemulsions can be generated in polymers by mixing appropriate amounts of two homopolymers and the corresponding diblock copolymer [Bates et al. (1997); Fredrickson and Bates (1997); Hillmyer et al. (1999); Matsen (1999); Kielhorn and Muthukumar (1997)]. One expects the dynamics of polymeric bicontinuous microemulsions to be much slower than those of o/w/s bicontinuous microemulsions due to higher molecular weights and larger viscosities. Here we present a systematic experimental investigation of the dynamic response of a model polymeric bicontinuous microemulsion.

Flow-induced effects on soft materials have been described in numerous publications. Sponge phases consist of a three-dimensional network of a bilayer of surfactant molecules that separates the solvent into two subvolumes, and closely resemble bicontinuous microemulsions. Mahjoub et al. (1998) and Yamamoto and Tanaka (1996) have reported the shear-induced transition of the sponge phase to a lamellar state. Berret et al. (1994, 1998) have observed a shear-induced isotropic to nematic transition in worm-like micellar solutions. Warr (1995) and Anklam et al. (1995) described shear thinning in ternary o/w/s bicontinuous microemulsions at very high shear rates (10³–10⁴ s^–1). A shear-induced increase in the isotropic to lamellar transition temperature was predicted by Cates and Milner (1989), and documented in diblock copolymers by Koppi et al. (1993). In contrast, lamellae-forming triblock and pentablock copolymers exhibit shear-induced disordering [Tepe et al. (1997); Vigild et al. (2001)]. Shear alignment of block copolymer lamellae can result in parallel or perpendicular orientations, depending on the shear frequency and the temperature [Koppi et al. (1992); Fredrickson (1994); Gupta et al. (1995)]. Nakatani et al. (1997) investigated block copolymer-modified homopolymer blends and found that shear flow has a destabilizing effect on the disordered phase above a threshold block-copolymer concentration. Pätzold and Dawson (1996) made predictions of the rheological properties of a bicontinuous microemulsion using a Landau–Ginzburg model.

Flow effects on polymer solutions and blends also have attracted significant interest. Semidilute polymer solutions near the coexistence curve are found to exhibit enhanced concentration fluctuations or phase separation under shear flow [Menasveta and Hoagland (1991); Moses et al. (1994); Migler et al. (1996); Kume et al. (1997)]. A common feature of these studies is the appearance of so-called "butterfly" light scattering patterns. Flow in polymer blends often causes apparent shifts in phase boundaries [Hashimoto et al. (1988); Nakatani et al. (1990)], leading to either flow-induced mixing [Nakatani et al. (1990); Kim et al. (1997); Hindawi et al. (1992)] or demixing [Fernandez et al. (1995); Gerard et al. (1999); Chopra et al. (1998)]. Recently Martys and Douglas (2001) reported calculations for flow-induced phase separation in lattice-Boltzmann fluid mixtures. In most of these cases, phase-separated blends are found to exhibit a string-like morphology aligned along the flow direction.

The equilibrium phase behavior of a ternary polymeric system A/B/A–B can be represented in the form of a phase prism as shown in Fig. 1. Simpler phase diagrams can be obtained by cutting the prism with horizontal or vertical planes. The former gives an isothermal ternary phase diagram, while the latter yields a phase diagram at a constant ratio of the homopolymer concentrations. A schematic of the isothermal phase diagram is shown in Fig. 1(a) for the case of a symmetric copolymer and equal homopolymer molecular weights. Ordered morphologies including lamellae, hexagonally packed cylinders, and cubic (gyroid and spherical) phases are prevalent at high block copolymer concentrations [Washburn et al. (2000)], while macroscopic phase separation into two or three phases occurs at low diblock concentrations. The gray triangle in Fig. 1(a) represents the three-phase window, with coexistence between two homopolymer-rich phases and a middle bicontinuous microemulsion phase. This three-phase triangle is surrounded by two-phase regions, and the bicontinuous microemulsion (bµE) phase is present over a small composition range at the tip of this triangle.

Figure 1.

The phase diagram along a vertical plane of the prism is an isopleth, and the example shown in Fig. 1(b) corresponds to nearly equal volume fractions of the homopolymers. The total homopolymer volume fraction is plotted along the abscissa and the temperature along the ordinate. At high temperatures, only a disordered phase exists, whereas at low temperatures, there are three regimes. At low homopolymer concentrations (i.e., high block copolymer concentrations), this symmetric mixture produces a lamellar phase. With only small amounts of the amphiphile (< 10 vol %), two-phase and three-phase regions occur as anticipated by Fig. 1(a). In between the lamellar and phase-separated regions, there is a narrow channel of bicontinuous microemulsion. Typically this corresponds to a block copolymer concentration of 8–12 vol % [Hillmyer et al. (1999)]. We have characterized this bicontinuous microemulsion phase for various polymeric systems in previous studies [Bates et al. (1997); Hillmyer et al. (1999); Morkved et al. (1999)], and recently we noted the intriguing rheological behavior of a polymeric bicontinuous microemulsion under shear [Krishnan et al. (2001)]. In this article we explore details of the rheological and structural properties under steady shear flow at different shear rates and temperatures that extend deep into the microemulsion channel.