Atomistic simulation of the temperature dependence of density and van-der-Waals interactions of binders, plasticizers and mixtures of them

The intermolecular energetic interactions between binders, plasticizers and fillers in elastomer bonded composite rocket propellants (CRP) and elastomer bonded high explosives charges (HEC) (some of the PBX family) control the glass-to-rubber transition in these elastomer systems. Therefore they are of great interest to explain the glass to rubber transition and to help to elucidate the different binder fractions and their specialized hindrances in molecular mobility. In a first step the van-der-Waals and electrostatic interactions between binders and plasticizers will be considered. Using the program package Materials StudioTM (MS) version 6 of company Accelrys an atomistic simulation of such interactions is possible. The substances used are hydroxyl terminated polybutadiene (HTPB), dioctyl adipate (DOA) and polypropylene oxide (PPO), which serves as a first step towards GAP (glycidyl azide polymer). Uncured binders are considered first. For energetic equilibration of the molecules and of the configuration to each other and then for the calculation of all the energy terms, molecular dynamics simulations were performed with the program part Forcite of MS in NPT ensembles in periodic unit cell, a cube with isotropic liquid systems. The pressure was set to 1 bar by controlling with a Berendsen barostat and the temperature was maintained to the wished values in the range of +120°C to -150°C with an Andersen thermostat. From the obtained NPT structures of the molecular systems the cohesive energy densities were calculated, from which the intermolecular energetic parts can be extracted. The temperature dependent calculations are used to point the glass-to-liquid transition in the systems.