Microscopic relaxation processes in branched-linear polymer blends by rheo-SANS

The relaxation time spectrum in blends of architecturally different polymers with strongly disperse time scales has been investigated by their time-dependent small angle neutron scattering signal after a fast uniaxial step strain. Model-hyperbranched dendrimeric polymers of second generation, dilutely dispersed within linear homopolymer matrices, acted like sensitive probes for structurally, though not firmly, established features of the tube model for bidisperse melts. We showed that the equilibration time of the linear matrix determines the size of the fluctuations that the outer and inner arms experience. Within a random phase approximation (RPA) treatment, which accounts for the different degrees of freedom inherent to the broad time scales, the observed loss of anisotropy with time was described in terms of two parameters only, namely the tube diameter and the fraction of relaxed arms of the minority component. The scattering data reveal details of mechanisms, which cannot be extracted from but determine the macroscopic flow properties. At intermediate times, a tube relaxation process was detected. At long times, the dynamic dilution model is confirmed.