Stress Relaxation of Star/Linear Polymer Blends

Abstract

Stress relaxation dynamics in a series of star/linear 1,4-polybutadiene blends with fixed star-arm molecular weight Ma, variable linear polymer molecular weight ML, and variable star polymer volume fraction φs are investigated. Storage and loss moduli, G‘(ω) and G‘ ‘(ω), obtained from small amplitude oscillatory shear experiments are compared with predictions of a parameter-free molecular theory for star/linear blend dynamics. For star/linear blends with moderate star polymer concentrations φs ≈ 0.28, theoretical G‘(ω) and G‘ ‘(ω) are found to be in good to excellent accord with experimental results over the entire range of frequencies and ML values studied. The quality of the predictions worsen as star polymer concentration is varied in either direction (i.e., higher or lower). At low φs, the greatest discrepancies between theory and experiment are observed at oscillation frequencies ω ≤ ωd, where ωd-1 ∼ ML3.4±0.25 is approximately the terminal time of linear polymer molecules in the blends. A theoretical analysis based on the idea that constraint release progresses by Rouse motion in a narrow exploring tube confined in a larger super tube yields a theory with better predictive properties for blends with low star polymer concentration. A new criterion is proposed for determining whether relaxation in star/linear polymer blends goes to completion after constraint release.