  Effect of small additive molecules on the structural relaxation of polymer
melts is investigated via molecular dynamics simulations. At a constant
external pressure and a fixed number concentration of added molecules, the
variation of particle diameter leads to a non-monotonic change of the
relaxation dynamics of the polymer melt. For non-entangled chains, this effect
is rationalized in terms of an enhanced added-particle-dynamics which competes
with a weaker coupling strength upon decreasing the particle size.
Interestingly, cooling simulations reveal a non-monotonic effect on the glass
transition temperature also for entangled chains, where the effect of additives
on polymer dynamics is more intricate. This observation underlines the
importance of monomer-scale packing effects on the glass transition in
polymers. In view of this fact, size-adaptive thermosensitive core-shell
colloids would be a promising candidates route to explore this phenomenon
experimentally.
