Entanglements in Binary Blends of Monodisperse Polystyrenes

Abstract

Linear viscoelastic properties of binary blends of monodisperse polystyrenes (PS) with low (M₁) and high (M₂) molecular weights were examined. The relaxation modes of the longer chain component were strongly affected by its content ω₂ as well as by the molecular weights of the components. In the blends with sufficiently small ω₂, the longer chain (2-chain) entangled only with the shorter ones (1-chains). If M₂ is sufficiently larger than M₁, the 2-chain in such a dilute blend relaxed by Rouse-like modes : In the dilute blends with M₁ less than the entanglement spacing M_e⁰, the 2-chain relaxed by its intrinsic Rouse mode with the characteristic time τ₂,Rouse∝M₁⁰ M₂². In those with M₂>>₁>M₁⁰, the retarded Rouse-like modes of the 2-chain were observed and the characteristic time was proportional to M₁³M₂².
In the blends with large ω₂, the 2-chain entangled with both 1- and other 2-chains. In such a concentrated blend with M₂>>>M₁>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 M₁ and M₂), which in turn is longer than the intrinsic time τ₂, Rouse.