Papers by Author: Dušan Račko

Abstract: In the present contribution the dynamics of the intermolecular free volume is investigated. For this purpose a new computational algorithm has been proposed and programmed. The free volume is analyzed along the atomistic trajectory obtained by molecular dynamics simulations and a picture on time averaged free volume structure is obtained. The picture showed two distinct regimes of the free volume dynamics at pico-second and nano-second time-scale in liquid. The analysis has shown that at the time scale of the ortho-positronium lifetime and in the liquid phase the free volume microstructure ceases to exist in rigid cavities. This picture is consistent with the common considerations on the o-Ps existence, which suppose the o-Ps atom digging its own holes/cavities in liquids around the temperature of the o-Ps lifetime saturates.

Abstract: In this contribution a generalized picture of the cavity number behavior is obtained by means of molecular dynamics simulations and consequent free volume analysis. The cavity number obtained for free volume cavities defined by different probe radii shows a complex behavior with the temperature. The number of cavities determined by small probes drops with the temperature. The number of cavities determined by large probes increases. This complex behavior of the cavity number can be discussed with the distinct behavior of the relative intensity, as observed in the experiment, and a support for the most basic of assumptions, that the cavity number relates to the relative intensity, is given.

Abstract: In the Present Contribution the Atomistic Structure of the Polymer Melt at 300 K Is Simulated by Means of Molecular Dynamics. the Agreement with an Experimental Density Is Obtained with a Deviation Lower than 1%. the Free Volume Is Analyzed in 1,000 Structures and 6.5 X 108 Cubic Å of Molecular Space. a Model for the Free Volume Cavities Is Proposed. in the Model the Size and Number of the Cavities Can Be Scaled by Three Parameters: Probe Radius, Cavity Depth and Cavity Threshold Volume. the Experimental Values of the Nano-Sized Cavity Volumes as Well as Ortho-Positronium Lifetimes Are Obtained, as Compared to Models with Cylindrical and Spherical Geometry. a Typical Value of the Number Density of Free Volume Cavities at 0.001 Å-3 Is Obtained. the Cavities Have Typically Elongated Shape with a Side-to-Length Ratio 1:2.