A Definition of Dynamically Accessible Volume for Thermal Systems

Abstract

[EN] The glass transition and physical aging processes of a polymeric material have been simulated using the bond fluctuation model. Two potentials that represent intra- and inter-molecular interactions have been employed. Simulations of different thermal histories that include cooling from equilibrium have been performed. The evolution of the system and the structure attained at low temperature are analyzed as a function of the assumed weight of inter- and intra-molecular potentials in the total energy of the system. A new way of characterizing the free volume of the system and its evolution with the temperature or time is proposed. It is based on the concept of dynamically accessible volume but modified in the sense of considering the probability of an empty site to be accessed according to a Metropolis criterion. The results obtained show that the thermal redefinition of the dynamically accessible volume, TDAV, offers a better representation of the real mobility of the polymeric systems. The use of information on the structure of the system coming from the pair-correlation function and the molecular mobility in the glassy state characterized by the time evolution of TDAV allows to reach the conclusion that a combination of inter- and intra-molecular potentials produces the vitrification of the polymer system on cooling.