This article summarizes our recent studies on structures and viscoelastic properties of polymer solutions and multicomponent-multiphase polymer systems, particularly polyelectrolyte solutions and polymer blends. Viscoelastic properties of polyelectrolyte solutions can be understood by classifying the solutions at least into dilute, semidilute and concentrated regions almost similarly to non-ionic polymer solutions, and they can be explained by a reptation model with electrostatic interactions in the semidilute region. Viscoelastic properties of polymer blends are different in miscible and immiscible regions, and regions close to phase separation temperature. In the miscible region polymer blends behave like polymer solutions when one of two components is not entangled. In the immiscible region first normal stress difference N
1 as well as shear stress are proportional to shear rate in accordance with the theory of textured materials presented by Doi and Ohta. In the two-phase region close to phase separation temperature, the shear rate dependence of N
1 changes with shear rate owing to shear-induced homogenization. The similar shear rate dependence of N
1 is also observed for block copolymers in the ordered region close to order-disorder transition, but it is due to the orientation of lamellar domains.
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