Positron annihilation in syndiotactic polystyrene containing α and β crystalline forms

• Published in: Polymer (Guilford) Vol. 43; no. 25; pp. 6775 - 6784
• Main Authors: Olson, B.G., Prodpran, T., Jamieson, A.M., Nazarenko, S.
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 01.01.2002; Elsevier
• Subjects: Applied sciences; Crystalline structure; Exact sciences and technology; Organic polymers; Physicochemistry of polymers; Positron annihilation; Properties and characterization; Structure, morphology and analysis; Syndiotactic polystyrene; Positron annihilation; Syndiotactic polystyrene; Crystalline structure; Beta form; Styrene polymer; Property structure relationship; Crystallinity; Semicrystalline polymer; Experimental study; Syndiotactic polymer; Comparative study; Alpha form
• ISSN: 0032-3861; 1873-2291
• DOI: 10.1016/S0032-3861(02)00604-3

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 (τ 4=2–3 ns) exhibits thermal expansion behavior typical of amorphous polymers, while the second longest lifetime (τ 3∼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.