**Understanding Dynamics in Binary Mixtures of Entangled
cis-1,4-Polybutadiene Melts at the Level of Primitive Path Segments by
Mapping Atomistic Simulation Data onto the Tube Model**

C Baig and PS Stephanou and G Tsolou and VG Mavrantzas and M Kroger, MACROMOLECULES, 43, 8239-8250 (2010).

DOI: 10.1021/ma101211b

We study dynamics in bidisperse melts of linear cis-1,4-polybutadiene
composed of probe and matrix chains at the level of the segment survival
probability function psi(s,t) which is computed directly in the course
of long atomistic molecular dynamics simulations **Stephanou et al. J.
Chem. Phys. 2010, 132, 124904**. By controlling precisely the matrix
chain length and composition, the effect of contour length fluctuations
(CLFs) and constraint release (CR) on melt dynamics is quantified. Our
study shows that (a) the values of the static topological properties of
the probe chains (e.g., the average value of their primitive path (PP)
contour length and its fluctuation) remain unaltered in the different
matrices, but (b) their dynamical properties (including psi(s,t) and its
average over all seaments s, psi(t), the time autocorrclation function
of the PP contour length, and the time autocorrelation function of the
chain end-to-end vector) vary significantly from matrix to matrix. As
the length of the matrix chains decreases, the functions psi(s,t) and
Psi(t) describing the rcptation relaxation of the probe chains are found
to decrease more rapidly. Furthermore, the relaxation of longer probe
chains is seen to be delayed as the concentration of shorter matrix
chains decreases. Overall, our direct computational study proves that CR
is the dominant relaxation mechanism in melts of long and short
cis-1,4-polybutadiene chains accounting for the majority of differences
observed in their relaxation dynamics in different environments (since
CLFs appear to be unaffected by compositional differences); as a result,
it has a profound effect on the linear viscoelastic properties of the
melt, such as the spectra of storaue and loss moduli. By further
analyzing the mean-square displacement of a tomistic segments in the
different matrices, we find that while the tube diameter is constant in
the mixtures with M(S) >= M(e) where M(S) is the molecular weight of
short chains and M(e) the entanglement molecular weight, it gradually,
increases in the mixtures with M(S) < M(e). How the simulation results
compare with laboratory measurements on melts of bidisperse polymers
reported in the literature is also discussed.

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