Linear rheology of entangled six-arm and eight-arm polybutadienes

Relaxation dynamics of various model entangled six-arm (A(3)-A-A(3)) and ei ght-arm (A(3)-A-A(2)-A-A(3)) polybutadiene melts are investigated using low -amplitude oscillatory shear and time-dependent step strain measurements. T he frequency (time) and temperature range covered in these experiments are sufficiently broad to characterize the entire liquid-state relaxation spect rum of the materials. Several new findings about multiarm polymer dynamics are reported. First, the mean segmental relaxation time of multiarm. polyme rs is a function of crossbar (A) molecular weight and polymer architecture. Second, for polymers with fixed arm (A) molecular weight, but variable cro ssbar molecular weight, terminal relaxation time (lambda) and limiting shea r viscosity (eta (0))scale quite strongly with crossbar molecular weight M- b (lambda similar to M-b(similar to6.8-7), eta (0) similar to M-b(similar t o8)). When the crossbar tube length is renormalized by dilution of relaxed arms and the relaxation time and viscosity are rescaled to remove the inher ent M-b dependence of segmental scale properties, these scaling exponents b ecome closer to values expected for crossbar reptation in a dilated tube. F inally, relaxation dynamics of eight-arm A(3)-A-A(2)-A-A(3) polybutadienes are found to be quite different from those of six-arm A(3)-A-A(3) 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.