The structure factor, viscosity, and diffusivity of four (styrene-b-isoprene-b-styrene-b-isoprene) tetrablock copolymers have been examined as functions of temperature (T). The copolymers have styrene compositions (f) of 23, 42, 60, and 80 vol % and total degrees of polymerization ca. 120; polystyrene and polyisoprene homopolymers with similar degrees of polymerization have been used for comparison. Small angle neutron scattering (SANS) measurements in the disordered state are well-described by the appropriate Leibler/RPA structure factors, and extrapolation of the inverse peak intensities to lower T yields estimates of the order−disorder transition temperatures, which are at or below −50 °C. Consequently, over the T range of interest (25−180 °C) and over length scales greater than the chain dimensions, the tetrablocks provide homogeneous matrices containing varying amounts of styrene and isoprene, in which the f and T dependence of segmental friction may be examined. The diffusivity (determined by pulsed-field-gradient NMR and forced Rayleigh scattering) and viscosity provide estimates of the effective monomeric friction factor ζeff(f,T) via the Rouse model; the two dynamic properties yield equivalent values of ζeff. The T dependence of ζeff is well-described by the WLF function, with the f dependence almost entirely contained in the composition dependence of the glass transition temperature (T g). Thus, when compared at constant T − T g, ζeff(f) is only slightly larger than ζPS° or ζPI°, in marked contrast to the results for miscible blends such as PS/PVME and PS/PPO. Prediction of ζeff(f,T) on the basis of the homopolymer values alone, i.e., ζPS° (T) and ζPI°(T), is only successful when T g(f) is incorporated explicitly. An approach using equation of state estimates of free volume is significantly less successful, implying that the most important determinant of local friction in the mixture is the effective T g sensed by each chain; T g(f) does not represent an iso-free volume state.

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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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