VACANCY SPECTROSCOPY OF RADIATION CROSS-LINKED AND DEGRADED POLYTETRAFLUOROETHYLENES

摘要

Polytetrafluoroethylene samples having different morphology were prepared by radiation induced cross-linking and degradation, and positron annihilation lifetimes were measured to extract information about the vacancy structure from the ortho-positronium (o-Ps) component. The o-Ps intensity I_3 was inversely correlated with the crystallinity confirming that this component is associated with the amorphous part. The o-Ps lifetime τ_3, was distinctly different depending on the phase of the polymer. Below the γ relaxation temperature, the vacancies produced by degradation could be seen from an increase in the τ_3 value, but at temperatures between the γ and β relaxations such vacancies became invisible due to the activated segment motion. And above the β relaxation temperature vacancies in the interface between crystalline and amorphous parts appear to affect τ_3. The o-Ps intensity did not show stable reproducible results. Accumulation of positive charge can be responsible for it. The results, not only unveiling the not yet well understood behavior of the o-Ps component, demonstrate the uniqueness of the vacancy spectroscopy using o-Ps as a probe.