Density dependence of structural and dynamical heterogeneity in a linear polymer melt system

We perform molecular dynamics simulations of a glass-forming linear polymer melt system at two different densities near its glass transition temperature Tc. Radial distribution function and nearest neighbour distribution of monomers in the linear polymer melt system at two densities show that the system at relatively lower density shows phase separation at low temperature, which is compared with the system at higher density and same temperature. The denser regions of both systems show similar packing, while lower density system shows loosely packed and gaseous domains also. These loosely packed monomers have larger squared displacements in comparison to the densely packed monomers, which creates a large disparity in their squared displacements. Disparity in the squared displacements leads to more heterogeneous dynamics in the lower density system that is supported by the monomer non-Gaussian parameter as its peak value is thrice the peak in the higher density system. This study reveals that dynamic heterogeneity in the linear polymer melt system is not only depend on the dynamics but also on the structural inhomogeneity and consequently it gives more light on dynamic heterogeneity during formation of porous glasses.