Static and dynamic contributions to anomalous chain dynamics in polymer blends

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, which incorporate intramolecular barriers with bending and torsion terms, are semiflexible in the sense that static intrachain correlations are strongly non-Gaussian at all length scales. The dynamic asymmetry in the blend is strongly enhanced with 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 approximately exhibit 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 essentially of static nature. We discuss the implications of these observations for the applicability of theoretical approaches to chain dynamics in polymer blends.