Effects of Droplet Size and Volume Fraction on Relaxation Modulus of Immiscible Polymer Blends: Inclusion of Interface Velocity Term

Stress relaxation behavior was investigated under large step shear strains γ for polyisobutylene/poly(dimethyl siloxane) blends with different average droplet-size. The interfacial contribution to the relaxation modulus, Gint(t,γ), was evaluated by subtraction of the matrix contribution from the relaxation modulus of the blend, assuming the linear additivity rule for the relaxation modulus. The interfacial modulus Gint(t,γ) for the blends with different average droplet-size obtained at the same strain can be superposed in a reduced form, Gint(t,γ)/(Γφ/rV) vs. t/τD, where Γ is the interfacial tension, φ the volume fraction of the droplet phase, rV the volume-averaged radius of droplets, and τD is the linear viscoelastic relaxation time of the droplets (interface). Superposition can be applied for the blends with φ = 0.108 and 0.214. The superposition is developed from consideration on the theoretical expression for the stress tensor which includes both contributions from the interface velocity term and Laplace pressure term. It is suggested from the good superposition that contribution from droplet-droplet interactions to the stress can be neglected in the present blends with φ ≤ 0.214 or that the contribution is also reduced by the same factors as (Γφ/rV) and t/τD.