The influence of phase separation on the linear viscoelastic response has been studied in miscible blends of polystyrene and poly(viny1 methyl ether) with a lower critical solution temperature near 110 "C. At temperatures between 25 and 155 'C, and for compositions in the range 20% to 60% polystyrene, the complex moduli G' and G" were measured at frequencies in the range of 0.01 to 100 rad/s. Timetemperature superposition was applied in the single phase region to obtain the complex modulus over eight decades of frequency. Increasing the polystyrene content resulted in an increase in the zero-shear viscosity and a shift of the terminal behavior to lower frequencies or longer times. The phase separation above the lower critical solution temperature was measured as a sudden increase in the fluorescence intensity of an anthracene-labeled polystyrene (approximately 1 wt % in the blend), using an optical probe in the rheometer fixture. For the 20/80 and 4W60 PWPVME samples, the terminal zone was in the accessible frequency window and phase separation was accompanied with a large increase in G' and G". In contrast, the complex modulus of the 60/40 PS/PVME blend could not be measured near the terminal zone and the G' and G" did not exhibit any significant changes near the phase transition temperature.

In miscible blends of polystyrene with poly(viny1 methyl ether), a shear-induced demixing occurs at temperatures as much as 40 K below the lower critical solution temperature. Demixing occurs at relatively low shear rates ranging from 1.5 to 4 s-l depending on the temperature and composition; the corresponding shear stresses and first normal stress differences range from 20 to 35 kPa and 30 to 120 kPa, respectively. There appears to be no threshold value of the shear rate or the shear stress for the onset, but a certain value of rate of work done on the sample roughly describes the steady-state demixing at a fixed temperature. When shear does induce demixing, some of the concentration differences are stable under shear for as long as 24 h. However, in other cases, a demixing appears for a shorter time, along with relatively high stresses, shortly after the inception of shear but disappears as the stresses drop at longer times. The time scale for the development of demixing ranges from hundreds to thousands of seconds and remixing after cessation of the flow typically occurs in thousands of seconds. Particularly at temperatures near the lower critical solution temperature, a substantial time is required for the stress development, even when there is no measurable demixing as indicated by fluorescence. Transmission electron microscopy on sheared samples confirms the presence of a shear-induced structure.

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Emulsion polymerized interpenetratingpolymer networks (IPN) of polyacrylate and polystyrene exhibit a power law relaxatio~over a wide frequency range. The response of the material to oscillatory shear, step shea~r strain and a constant stress can be described with a two parameter constitutive equation. The power law behavior was previously found in polymers at their critical state where molecular motions were correlated over large distances without intrinsic size or time scale.The effect of composition and crosslink density on the behavior of the material is studied. The behavior might be explained with the granular structure of the material.

In this paper we describe the linear viscoelastic properties of copper phthalocyanine (CuPCN) dispersions that are used in the manufacturing of offset lithographic printing inks. Transmission electron microscopy shows that the primary pigment particles are rod-like and have sizes in the range of 10 to 300 nm. Steady shear measurements show that the dispersions are Newtonian at a pigment volume fraction of 0.073 and become increasingly shear thinning as the pigment volume fraction is increased. The strong shear-thinning nature of these dispersions can be attributed to the highly flocculated nature of the dispersions, which is due to interparticle attractions. The structural complexity of the dispersions also results in an unexpected linear viscoelastic response. While at low frequencies (0.1 and 1.0 Hz) the ex-tent of the linear region decreases with increasing pigment concentration, at a higher frequency (10 Hz) the extent of the linear region increases with increasing pigment concentration. This increase in the Iinear region with increasing pigment concentration suggests that at higher frequencies the dispersion is less brittle, and that the rheological behavior is dominated by intra-aggregate associations. In addition, frequency sweeps show that the dispersions behave like a viscoelastic liquid at low pigment concentrations. However, at higher pigment concentrations (yet significantly lower than the maximum packing fraction) the dispersions behave like a cross-linking polymer at its gel point.