  By means of computer simulations, we investigate the relaxation of the Rouse
modes in a simple bead-spring model for non-entangled polymer blends. Two
different models are used for the fast component, namely fully-flexible and
semiflexible chains. The latter are semiflexible in the meaning that static
intrachain correlations are strongly non-gaussian at all length scales. The
dynamic asymmetry in the blend is strongly enhanced by decreasing temperature,
inducing confinement effects on the fast component. The dynamics of the Rouse
modes show very different trends for the two models of the fast component. For
the fully-flexible case, the relaxation times exhibit a progressive deviation
from Rouse scaling on increasing the dynamic asymmetry. This anomalous effect
has a dynamic origin. It is not related to particular static features of the
Rouse modes, which indeed are identical to those of the fully-flexible
homopolymer, and are not modified by the dynamic asymmetry in the blend. On the
contrary, in the semiflexible case the relaxation times exhibit approximately
the same scaling behaviour as the amplitudes of the modes. This suggests that
the origin of the anomalous dynamic scaling for semiflexible chains confined in
the blend is esentially of static nature. We discuss implications of these
observations for the applicability of theoretical approaches to chain dynamics
in polymer blends.
