Linear viscoelastic properties of binary blends of monodisperse polystyrenes (PS) with low (M
1) and high (M
2) molecular weights were examined. The relaxation modes of the longer chain component were strongly affected by its content ω
2 as well as by the molecular weights of the components. In the blends with sufficiently small ω
2, the longer chain (2-chain) entangled only with the shorter ones (1-chains). If M
2 is sufficiently larger than M
1, the 2-chain in such a dilute blend relaxed by Rouse-like modes : In the dilute blends with M
1 less than the entanglement spacing M
e0, the 2-chain relaxed by its intrinsic Rouse mode with the characteristic time τ
2,Rouse∝M
10 M
22. In those with M
2>>
1>M
10, the retarded Rouse-like modes of the 2-chain were observed and the characteristic time was proportional to M
13M
22.
In the blends with large ω
2, the 2-chain entangled with both 1- and other 2-chains. In such a concentrated blend with M
2>>>M
1>M
e0, the 2-chain behaved in the long time region as if in a usual concentrated solution in a low molecular weight solvent. This indicates that the 1-2 entanglements become ineffective at intermediate time scales before the relaxation of the 2-chain completes. A detailed analysis on the storage and loss moduli of the 2-chain in the blends revealed that the constraints due to 1-2 entanglements are released at time scales being close to the characteristic time for the corresponding dilute blends (with the same M
1 and M
2), which in turn is longer than the intrinsic time τ
2, Rouse.
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