Positron annihilation in polyaniline

Abstract

Compressed pellets of amorphous, chemically synthesized, polyemeraldine hydrochloride (PEHCl) and its neutral (dedoped) form, polyemeraldine base (PEB), were studied by positron annihilation lifetime spectroscopy in the temperature range 77–400 and 77–450 K, respectively. The positron lifetime spectra of PEB were resolved, without any constraint, into three components; those of PEHCl showed only one component. The corresponding lifetimes and intensities at 300 K for PEB are: τ 1 = 0.212±0.010 ns, τ 2 = 0.378±0.007 ns, I 2 = 69.5±1%, τ 3 = 2.45±0.50 ns and I 3 = 0.3±0.05%. The lifetime of the sole PEHCl component at 300 K is τ = 0.354±0.001. The τ 1 lifetime is attributed to free-positron annihilation events and the τ 2 lifetime component to positrons annihilating from trapped states. Most positrons annihilate in PEB with the τ 2 lifetime component from trapped states in vacancies or in voids. The high packing coefficient of PEB indicates a structure free of cavities or porosities, preventing the usual assignment of the τ 3 lifetime component to o-positronium ( o-Ps) trapped at cavities of diameters in the range 3–10 Å. The component might result from o-Ps bound in the cavities remaining in the boundary region of the raw powder material. The positron annihilation lifetime in PEHCl is determined by the density of chlorine anions, as in inorganic salt crystals.