Viscoelastic properties of bidisperse homopolymer blends

Abstract

Linear and nonlinear stress relaxation dynamics of well entangled polymer liquids were studied using a series of entangled, bidisperse 1,4-polybutadiene blends. Blend systems comprising high-(M[L]) and low-(M[S]) molecular weight components with M[L]>>M[e] and M[S]>M[e] were formulated. The linear rheology was used to characterize the effect of the short polymer on the long one based on its limiting shear viscosity ([]₀) and terminal relaxation time ([]) dependencies on the M[S]. The blend systems of M[L]>>M[e] and M[e]<M[S]<M[S]* were found such that the short polymer component function as an ideal, non-volatile solvent for the long polymer chains. For example, in blends with M[L] = 5.15x10⁵ g/mol, a critical value M[S]*=4.8x10⁴ g/mol was identified, below which terminal viscoelastic properties of the blends ceased to depend on M[S], and varied with volume fraction of the long polymer component, [][L], in a manner consistent with expectations for an entangled polymer solution that is suitable for studying nonlinear rheology in well to highly entangled systems. In agreement with previous nonlinear step strain studies using well entangled polystyrene solutions, an unusual short-time (t<[]k) crossing pattern in shifted nonlinear relaxation moduli, G(t,y)h(y)¹̄, was observed in blends with M[L] = 5.15x10⁵ g/mol and M[S]*-1.0x10⁴ g/mol, beginning at []L = 0.1 (N /N [e]= 13). The unusual short-time G(t,y)h(y)¹̄ dynamics were accompanied by a continuous transition from type A to type C damping at long time, t>[]k. Our results appear to rule out possible roles for shear-induced phase segregation and incipient glassy dynamics in previous observations of type C damping in entangled polymeric liquids. Our findings also lend support to the idea that type C damping is a characteristic feature of well entangled polymer systems, and that type A damping is a special case of type C, applicable only in marginally entangled polymer liquids.