We discuss simulations of a simple model for polymer blends in the framework of the Rouse model. At odds with standard predictions, large dynamic asymmetry between the two components induces strong non-exponentiality of the Rouse modes for the fast component. Despite chains being much shorter than the entanglement length, it also induces dynamic features resembling a crossover to entangled-like chain dynamics. This unusual behavior is associated to strong memory effects which break down the assumption of time uncorrelation of the external forces acting on the tagged chain.

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