Linear and nonlinear rheology of dendritic star polymers: Experiment

The stress relaxation dynamics of a series of second and third generation dendritic star polymers have been investigated experimentally in the linear regime with small amplitude oscillatory shear and also by nonlinear step shear deformations. In linear rheology, the relaxation dynamics of dendritic star melts agree with the expected relaxation hierarchy, where the characteristic tan(δ) minima that appear at high frequency correspond to the faster relaxing parts of the given dendritic topology. On the other hand, the degree of dilution, estimated from the ratio of the second plateau modulus G _II to the rubbery plateau modulus G _N, was much less than the expectation of the hierarchical theory, which implies a relatively slower rate of dilution due to multiple arms and generations. The nonlinear damping behavior of the second generation dendritic architecture was similar to that of H-shaped/multiarm architectures, featuring a novel damping transition from, less strain softening to agreement with the Doi - Edwards damping function as the branch-point withdrawal motion. The third generation dendritic polymers exhibited much weaker strain dependence than second generation samples, suggesting a stronger stretching effect due to multiple branching generations even with arms of a few entanglements at each generation. Such a stretching effect even with small arms at each generation induced extensional hardening in the linear polymer matrix maintaining the shear viscosity.