Fluctuation effects on viscoelastic properties of diblock copolymer solutions in disordered state

Viscoelastic properties of three poly(styrene-d₈-Woc&- isoprene)s in dioctyl phthalate are measured under oscillatory and steady shear flows in the disordered state, to examine the fluctuation effects on zero shear viscosity n° and steady state compliance J_e based on the effective interaction parameter (x^N)_eff- Flow-induced microphase separation is not observed when (x^N)_eff is lower than 9.9. Enhancement of n° and J_e due to the fluctuation effects, which is more obvious for J_e, are observed in oscillatory flow, while suppression of the fluctuation effects are observed in steady shear flow, which is confirmed by small angle neutron scattering measurements. Comparing these results with those for poly(styrene-block-2-vinylpyridine)s (SP), it is concluded that the viscosity enhancement due to the fluctuation effects slightly depend on component viscosities. When viscosity ratio of components is close to 1 (i.e., SP), the viscosity enhancement is negligible. On the other hand, enhancement of reduced steady state compliance J_eR seems to be insensitive to the viscosity ratio of components.