Molecular dynamics simulations of the diffusion coefficient of systems of p
olydisperse chains are presented. Each system consists of two lengths of ch
ain of chemically identical flexible polymers. The mean square displacement
of the center of mass of each species is measured as a function its length
and volume fraction in the blend. The polymer lengths range from N = 10 mo
nomers per chain to N = 90, about three times the entanglement length. The
polymer species that comprises the bulk of the melt shows little change in
behavior regardless of the length of polymer which makes up the remainder.
By contrast, when a species is the minority component, its motion is signif
icantly affected by the length of the matrix chains. When a chain is immers
ed in a matrix of longer chains, its diffusion coefficient is smaller than
its monodisperse value; conversely when a chain is in a blend of shorter ch
ain its diffusion coefficient increases compared to a monodisperse melt. Fo
r chains shorter than the entanglement length, the diffusion coefficient co
mpares well to theoretical predictions. The scaling exponent of the mean sq
uare displacement of the longest polymer is found to be sublinear, unless b
lended with very short polymers. The scaling exponent seems to be a measure
ment of the entanglements that the long polymers experience. (C) 2000 Ameri
can Institute of Physics. [S0021-9606(00)52106-0].