Modified and normal Arrhenius equation to characterize glass-rubber transition shifts in filled HTPB-IPDI formulations

The glass-rubber transition (GRT) in elastomer bonded energetic material as composite rocket propellants and high explosives is an important property for the use of these materials. The temperature range of application of such rubber materials is always above the GRT, in contrast to thermoplastic elastomers, which are used below the GRT. The GRT is strongly determined by intermolecular interactions. An important behaviour is the deformation rate dependence of the GRT. With increasing deformation rate, means measurement frequency in dynamic mechanical measurements, the GRT is shifted to higher temperatures, means the in-service temperature range changes under vibrational loads. The interaction between the binder polymer chains surely changes with the type of solid filler, as RDX or AP (ammonium perchlorate). Based on recent work using HTPB-IPDI (hydroxyl-terminated polybutadiene-isophorone diisocyanate) polyurethane binder, the determined data on GRT shifts are re-characterized by the so-called modified Arrhenius equation (MAE), which has proven to be equivalent to the WLF (Williams-Landel-Ferry) equation. The MAE has some advantages against WLF: no data are lost by set-ting a reference point and the WLF invariant C1-C2 is expressed with an activation energy, which is used to interpret interaction in the material. A comparison with the activation energies obtained with the normal Arrhenius equation (NAE) is made.