Temperature Dependence Free Volume and Dynamic Heterogeneity in Polymer Melt*

Free volume and dynamic heterogeneity may be the most frequently used concepts in polymer physics, but they have never been observed directly in experiment. Here, by a combination of neutron scattering and molecular dynamic simulation, the temperature dependence of free volume and dynamic heterogeneity can be observed in a Polystyrene melt. Neutron scattering profiles shows that only the segment-segment peak moves to higher scattering vector with the decrease of temperature. It indicates that the volume of the polymer melt decreases with the decease amplitude of thermal fluctuation. When it is close to the glass transition temperature, polymer chains are confined in the cage from its neighboring chains. The fractional free volume can be defined by the volume outside the confinement in the cage over the cage volume. Glass formation happens when the cage size is small enough, either temperature or pressure changes, to freeze the out-of-cage motion. During solidification, dynamic heterogeneity develops over a long range of time and length scales, and string-like cooperative rearrangement regions are observed.