MOLECULAR-DYNAMICS SIMULATION OF POLYMER LIQUID AND GLASS .4. FREE-VOLUME DISTRIBUTION

Molecular dynamics simulation has been performed with dense systems of alkane-like chain molecules subject to potentials restricting bond lengths, bond angles, and trans-gauche torsional angles and interacting with neighboring chains according to a truncated Lennard-Jones potential. Distribution of occupied and unoccupied space in the system was then evaluated by the technique of Voronoi tessellation of space and by enumeration of the cavities formed when hard spheres of diameter D were assumed placed on atomic centers. The distribution of volumes of Voronoi polyhedra is broad at high temperatures but becomes narrower as the temperature is lowered through the T(g). The polyhedra associated with chain and segments are generally much larger than those associated with internal segments. The distribution of cavity volumes is extremely broad and often exhibits a bimodal or trimodal character. The overall volume fraction of cavities in the system decreases when the temperature is lowered toward the T(g), below which the rate of decrease is greatly reduced; the overall behavior thus parallels the change in the total volume with temperature. The number density of cavities either increases or decreases on cooling, depending on the value of the diameter D, but remains fairly constant at temperatures below the T(g). These cavities change shape and size with time; even at temperatures much below the T(g), persistent low-frequency, large-amplitude molecular motions produce waxing and waning of the cavities. The reappearance of a cavity at the position from which it disappeared earlier suggests the existence of spatial inhomogeneity in density, which does not decay on the time scale of our simulation.