Computer simulation of the heterogeneity of segmental dynamics in amorphous polymers

Abstract

ElsevierMolina Mateo, J.; Torregrosa Cabanilles, C.; Sabater I Serra, R.; Meseguer Dueñas, JM.; Gómez Ribelles, JL. (2013) AbstractThe heterogeneity of local segmental dynamics in a polymer system was analysed by computer simulation with the Bond Fluctuation Model. In a previous work we showed the difficulties encountered in characterizing this heterogeneity by means of a distribution of relaxation times. In this work a different approach is proposed based on the concept of Dynamically Accessible Volume (DAV). A DAV value was assigned to each polymer chain as the fraction of cells in its surroundings that could be occupied in one Monte Carlo step. In this way it was possible to relate the mobility of a chain with the accessible volume around it, due to the relationship between DAV and diffusion coefficient. As temperature decreases in equilibrium the DAV distribution shifts towards lower values, its width decreases and the number of frozen molecules increases.The methodology proposed also provides a way of characterizing the evolution of segmental dynamics distribution in out of equilibrium states below glass transition temperatures.

Fluctuations of conformational mobility of macromolecules around the glass transition

“…Unlike other heterogeneity parameters, it is calculated from only a single "snapshot" or static structure of the system. The DAV distributions describes dynamic heterogeneity in simulations of polymeric materials [22] and show how distinct regions or chains The order of magnitude of dynamic heterogeneity, both in the liquid and the glassy state, seems to be determined by the average DAV (see Fig. 9).…”

“…All systems were formed by 400 polymeric chains with 10 molecular groups each. Previous simulations with the parameters of System 1 showed a glass transition in cooling ramps and structural relaxation in isothermal annealing below the glass transition interval [22]. The thermal history began with an initial equilibration period was simulated with specific isothermal annealing periods at temperatures around their glass transition interval for 10 7 MCS, from the states attained at the cooling ramp.…”

“…where L is the box simulation size in lattice units, n is the number of neighbors around a cell and ∆E i,j is the variation in the energy caused by the movement of a group from cell i to j. A DAV value is assigned to each polymer chain [22] as the average value of the DAV for each empty accessible cell that can be occupied in one MCS by a segment or group of that chain:…”

“…Both variables are related to the mobility of the material in the short term of one MCS [17,18]. There is a connection between the DAV and PSM distributions and the equilibrium or non-equilibrium state of the material [22]. Different DAV distributions obtained from simulations of equilibrium and non-equilibrium states around the glass transition and during structural relaxation showed the relationship between DAV fluctuations and the different states of the system [22].…”

“…The importance of free volume in controlling glass transition phenomena has recently been pointed out [20]. Monte Carlo simulations of DAV distributions show the dynamic heterogeneity that occurs around the glass transition in polymers [22]. The DAV distributions represent the mobility of the chains of a polymeric material in the short term of a Monte Carlo step (MCS).…”