  Polydispersity is inevitable in industrially produced polymers. Established
theories of polymer dynamics and rheology, however, were mostly built on
monodisperse linear polymers. Dynamics of polydisperse polymers is yet to be
fully explored -- especially how chains of different lengths affect the
dynamics of one another in a mixture. This study explored the dynamics of
bidisperse polymer melts using molecular dynamics and a bead-spring chain
model. Binary mixtures between a moderately entangled long-chain species and an
unentangled or marginally entangled short-chain species were investigated. We
found that adding short chains can significantly accelerate the dynamics of the
long chains by substantially lessening their extent of entanglement. Meanwhile,
although introducing long chains also hinders the motion of the short chains,
it does not qualitatively alter the nature of their dynamics -- unentangled
short chains still follow classical Rouse dynamics even in a matrix containing
entangled chains. Detailed Rouse mode analysis was used to reveal the effects
of entanglement at chain segments of different scales. Stress relaxation
following a step shear strain was also studied and semi-empirical mixing rules
that predict the linear viscoelasticity of polydisperse polymers based on that
of monodisperse systems were evaluated with simulation results.
