Tube dynamics in binary polymer blends

Stress relaxation dynamics of entangled linear/linear and short star/linear 1,4-polybutadiene and 1,4-polyisoprene blends are studied using a combination of mechanical rheometry and theory. In binary blends comprised of long linear chains and entangled short star or short linear molecules, we find that relaxation of the faster relaxing "short" molecules leads to a reduction in shear modulus consistent with expectations for solventlike dilution of the entanglement network in which the slower relaxing "long" chains diffuse. Terminal relaxation dynamics of the long chains are nonetheless found to be substantially different in blends with short star and short linear molecules. Specifically, while terminal relaxation in linear/linear blends is consistent with expectations for reptation diffusion of the long linear molecules in a uniformly dilated tube, terminal relaxation in the short star/linear blends occur by reptation diffusion of the long chains in an undilated tube. Thus, in the short star/linear blends the longer linear chains appear unable to take advantage of the more dilated network produced by relaxed stars, even when relaxation times of the two species differ by more than 2 orders of magnitude. This finding is inconsistent with expectations from dynamic tube dilation models for polymer blend relaxation. It indicates that architecturally complex molecules exert a longer-lived influence on tube equilibration in blends than anticipated by these models.