Pressure dependence of sound velocity in Viton and its carbon-filled composite*

Viton A (a dipolymer composed of hexafluoropropylene and vinylidene fluoride) is used as a binder for high explosives, and its carbon-filled composite is widely used in high vacuum sealing O-rings over a broad temperature range. However, Viton shows variations in mechanical strength from lot to lot, which has caused concern about its performance in extreme conditions. Binder crystallinity has been recognized as a potential factor which may influence the mechanical strength of Viton, but this has not yet been fully explored. We carried out ultrasonic and x-ray diffraction measurements of Viton under pressure to 7 GPa in the Paris-Edinburgh cell to investigate the elastic and crystalline properties of Viton and its carbon filled composite. We observed the first broad diffraction peak in addition to crystalline diffraction peaks from both Viton and the composite. For comparison, we also investigated the acoustic properties and first diffraction peak of Sylgard polymer (another explosive binder), which shows a broad amorphous peak but does not exhibit any crystalline diffraction.