Static and dynamic density effects due to the finite length of polymer chains: a molecular-dynamics investigation

The finite length of polymer chains affects both the static and the relaxation properties of the density of the melt state. These have been investigated by molecular-dynamics simulations of a Lennard-Jones model with fixed bond length. Under isothermal-isobaric conditions the density increases with the molecular weight. A suitable Voronoi tessellation reveals the extra free volume around the chain ends and shows that it is strongly localized within the first end monomer. Simple arguments are given for interpreting the main changes of the monomer radial distribution function and the corresponding static structure factor when the chain length is increased. As to the relaxation aspects of the density, it is found that the structural relaxation time increases with the molecular weight, which is interpreted as a signature of the well-known corresponding increase of the glass transition temperature.