Linear and nonlinear rheology of bidisperse polymer blends

Abstract

We investigate linear viscoelastic properties and nonlinear stress relaxation dynamics in a series of bidisperse 1,4-polybutadiene blends. Blend systems comprising high- (ML) and low- (MS) molecular weight polymers with ML≫MS > Me are formulated in which the short polymer component functions as an ideal nonvolatile solvent for the long polymer chains. In blends with ML=5.15×105 g/(mol) a critical molecular weight MS*≈4.8×104 g/(mol) is identified below whose terminal viscoelastic properties are independent of MS and vary with volume fraction of the long polymer molecules, φL, in a manner consistent with expectations for entangled polymer solutions. In blends with MS > MS* several approximate scaling relationships between terminal rheological properties and MS can be determined from the experimental results, η0∼MS1.6, τd0=η0Je0∼MS0, and τd,step∼MS1.6. The scaling exponents observed are consistent with earlier experimental reports, but disagree with theoretical predictions of the constraint release time in bidisperse melts. In agreement with previous step strain studies using entangled polystyrene solutions, nonlinear step shear measurements using blends with MS < MS* reveal an unusual short-time (t < λk) crossing pattern in shifted nonlinear relaxation moduli G(t,γ)h(γ)−1. The unusual short-time G(t,γ)h(γ)−1 dynamics are first observed at φL=0.1 (N/Ne≈13) and are accompanied by a continuous transition from type A to type C damping at long times, t > λk. Our findings are consistent with the idea that type C damping is a characteristic feature of well entangled polymer systems and suggest that type A damping is just a special case of type C, applicable only in the limit of weakly entangled polymer chains.