Calculation of enthalpies of formation and band gaps of polymeric binders

Abstract

Enthalpies of formation and band gaps of polymeric binders are important parameters to consider when designing compositions of explosives and propellants. We have used computational methods to determine enthalpies of formation and band gaps of polymeric binders. Initially, we computed the enthalpy of formation of the known non-energetic binder hydroxy terminated polybutadiene (HTPB), and the value obtained is close to that of its experimentally determined enthalpy of formation. The applied computational methodology has also been validated by the calculation of enthalpies of formation for the known energetic binder glycidyl azide polymer GAP. Furthermore, enthalpy of formation of azido HTPB (AHTPB) was calculated, which indicates that the addition of the azido group helps produce an energetic, insensitive and compatible polymeric binder. In this study, enthalpy of formation of the polymer was determined by extrapolating the results of the calculations carried out on monomers and oligomers, whereas periodic boundary condition (PBC) computations were carried out on the dimers to obtain band gap values of polymers. Highly positive enthalpies of formation of oligomers of AHTPB compared to those of HTPB and GAP suggests that AHTPB can be a potentially energetic binder in explosive compositions. Analysis of periodic boundary condition (PBC) – frontier molecular orbital band gap studies suggests that stabilities or insensitivities of these polymeric binders are in the order: HTPB > GAP > AHTPB. A superior curable nature of AHTPB over HTPB is indicated by the calculation of interaction energies. These results are vital in the quest for molecular-based predictions of polymer properties in general, but they are in particular encouraging for the study of polymers with monomers of relatively large molecular weight.