Molecular dynamics simulation of cohesion within solid propellants

Composite rocket propellants and plastic bound explosives are both based on elastomeric binder matrices that contain either oxidizer particles or high explosives. The components in a formulation differ widely in terms of crystallinity, thermal expansion behaviour and thus mechanical properties. Therefore, composites can exhibit a combination of brittle and ductile behaviour, stemming from filler and binder polymer, respectively. The binder-filler interaction is the weakest in the system, and their interface is prone to detach irreversibly, when stretched beyond a critical separation distance. In experiments, the effect of adhesion can be monitored by dynamic mechanical analysis (shape of loss factors), surface tension measurements and solution calorimetry. On a microscopic level, the cohesive zone model serves as a separation law for mechanical failure. It mimics the irreversiblity of crack formation: While small loading and unloading is reversible, higher strain beyond a threshold value changes the given distanceforce function for further loading cycles (local damage). Quantitative parameters for this model are hardly measured, although they are of interest for finite element simulations, to explore the material on a larger space and time scale. Here, we present the complete model of a pull-off experiment in the framework of a molecular dynamics simulation. In a MD-simulation, single atoms are modelled by point masses that interact via force-fields. It can reproduce thermodynamic and mechanical properties of multi-component systems. In an earlier contribution, the thermodynamic work of adhesion (equal to a surface tension) has been presented. Now, work and force of separation including local deformation of the surfaces is presented. It includes irreversibility during pull-off: As external stimulus, the interface is stressed until mechanical failure. This procedure yields higher order mechanical parameters beyond the linear elasticity model, which are input parameters for finite elements simulations: energy/force of detachment, and interaction length of entangled polymer at the interface.