We have carried out conventional theological measurements to explore the well-known stress overshoot behavior in startup shear of eight entangled polybutadiene solutions. In the elastic deformation regime with (gamma)over dot tau(R)> 1, we have identified universal scaling features associated with the stress maximum sigma(max) for samples of different levels of entanglements per chain ranging from 27, 40 to 64. Specifically, at the moment t(max) of the peak stress sigma(max) where the applied strain is gamma(max) = (gamma)over dott(max): (a) sigma(max) varies linearly with gamma(max), (b) sigma(max) scales with t(max) as sigma(max)similar to(t(max))-1/2. The combination of (a) and (b) yields a striking "prediction" that gamma(max) similar to(gamma)over dot(1/3). Remarkably, these scaling laws form master curves when the peak stress, strain rate, and peak time are all normalized with the crossover modulus G(c,) reciprocal Rouse time, and Rouse time tau(R), respectively. The dependence of sigma(max) on t(max) and gamma on (gamma)over dot is weaker in the crossover regime with (gamma)over dot tau(R) < 1. Equally noteworthy is the emergence of a super-master curve for the normalized stress sigma-(t)/sigma(max) as a function of the normalized strain (gamma)over dott/gamma(max) at various applied rates in these solutions. The solution with only 13 entanglements per chain exhibits behavior deviating appreciably from the well entangled systems. Strain recovery experiments revealed irreversible deformation (i.e., flow) when the sample is sheared beyond the stress maximum (for (gamma)over dot tau(R)> 1) or when sheared with (gamma)over dot tau(R)< 1 for a period longer than the Rouse relaxation time tau(R). (C) 2008 The Society of Rheology.
Available at: http://works.bepress.com/shi-qing_wang/14/