  We address the general question of how the molecular weight dependence of
chain dynamics in unentangled polymers is modified by blending. By dielectric
spectroscopy we measure the normal mode relaxation of polyisoprene in blends
with a slow matrix of poly(ter-butylstyrene). Unentangled polyisoprene in the
blend exhibits strong deviations from Rouse scaling, approaching
'entangled-like' behavior at low temperatures in concomitance with the increase
of the dynamic asymmetry in the blend. The obtained results are discussed in
the framework of the generalized Langevin equation formalism. On this basis, a
non trivial relationship between the molecular weight dependence of the longest
chain relaxation time and the nonexponentiality of the corresponding Rouse
correlator is found. This result is confirmed by molecular dynamics
simulations.
