Molecular simulation of the glass transition of polyphosphazenes

Molecular dynamics simulation (NPT ensemble) has been used to obtain specific volume as a function of temperature for four polyphosphazenes. From these results, the volumetric glass transition temperature has been determined as the temperature marking the discontinuity in slope of the V-T simulation data. The molecular mechanics force field used in this study was COMPASS (Condensed-phase Optimized Molecular Potentials for Atomistic Simulation Studies) which has been extensively parameterized and validated for phosphazenes in a previous communication. The polyphosphazenes include poly[bis(2,2,2-trifluoroethoxy)phosphazene] (PTFEP) and three isomers of poly(dibutoxyphosphazene) -poly[bis(n-butoxy)phosphazene] (PnBuP), poly[bis(iso-butoxy)phosphazene)] (PiBuP), and poly [bis(sec-butoxy)phosphazene] (PsBuP). In all cases, there was reasonable agreement between experimental results and values of density and T-g obtained from the simulations. In the case of PnBuP, two different methods for amorphous cell building are compared to explore the relationship between the state of equilibration and the T-g obtained from simulation. (C) 1999 Elsevier Science Ltd. All rights reserved.