Positron annihilation in syndiotactic polystyrene containing α and β crystalline forms

Positron annihilation lifetime spectroscopy (PALS) measurements were performed on a series of syndiotactic polystyrene (s-PS) samples crystallized from the melt state. These s-PS samples contained either pure α or β forms with various crystallinities. The α and β crystalline forms are known to have densities very similar to the amorphous phase. The low-density is believed to be associated with nanovoids or nanochannels, existing between the polymer chains in crystalline s-PS. A question can be raised whether positronium can form and annihilate in these low-density crystalline structures. In analyzing the PALS spectra, it was found that four distinct lifetime components gave the best fit for all studied samples. The longest lifetime component (τ4=2–3 ns) exhibits thermal expansion behavior typical of amorphous polymers, while the second longest lifetime (τ3∼0.8 ns) is constant, independent of temperature, crystallinity and tacticity. For all semicrystalline and amorphous s-PS samples, the PALS spectra were found to be essentially indistinguishable, with remarkably similar lifetimes and intensities. This behavior implies that ortho-positronium can indeed form in the low-density crystal structures of s-PS. Two possibilities for ortho-positronium annihilation in the semicrystalline polymer were proposed and analyzed: (a) ortho-positronium annihilates in the crystal phase with a lifetime characteristic of the nanochannel dimensions; (b) ortho-positronium rapidly diffuses into the amorphous phase and annihilates with a lifetime typical of that phase. No evidence for the existence of a lifetime component characteristic of the crystal nanochannels was found, indicating that the second annihilation mechanism is dominant.