Linear Rheology of Entangled Six-Arm and Eight-Arm Polybutadienes

Abstract

Relaxation dynamics of various model entangled six-arm (A3-A-A3) and eight-arm (A3-A-A2-A-A3) polybutadiene melts are investigated using low-amplitude oscillatory shear and time-dependent step strain measurements. The frequency (time) and temperature range covered in these experiments are sufficiently broad to characterize the entire liquid-state relaxation spectrum of the materials. Several new findings about multiarm polymer dynamics are reported. First, the mean segmental relaxation time of multiarm polymers is a function of crossbar (A) molecular weight and polymer architecture. Second, for polymers with fixed arm (A) molecular weight, but variable crossbar molecular weight, terminal relaxation time (λ) and limiting shear viscosity (η0) scale quite strongly with crossbar molecular weight Mb (λ ∼ Mb∼6.8-7, η0 ∼ Mb∼8). When the crossbar tube length is renormalized by dilution of relaxed arms and the relaxation time and viscosity are rescaled to remove the inherent Mb dependence of segmental scale properties, these scaling exponents become closer to values expected for crossbar reptation in a dilated tube. Finally, relaxation dynamics of eight-arm A3-A-A2-A-A3 polybutadienes are found to be quite different from those of six-arm A3-A-A3 polymers with comparable arm molecular weight. Specifically, the slowest relaxation mode in well-entangled eight-arm polymers appears to be dominated by Rouse-like fluctuation effects, which blur the transition from high-frequency arm to terminal backbone relaxation.