Free Volume Microstructure and Its Relationship to the Chain Dynamics incis-1,4-Poly(butadiene) As Seen by Positron Annihilation Lifetime Spectroscopy

Abstract

The annihilation characteristics of the orthopositronium (o-Ps) in the structurally simplest elastomer cis-1,4-poly(butadiene) has been measured in the temperature range from 15 to 313 K and interpreted in the framework of the free volume concept. The temperature dependence of the mean lifetime of the o-Ps, τ 3 , and its relative intensity, 13, reflect a fine interplay between the free volume and microscopic chain dynamics. In addition to a pronounced break on the τ 3 -T plot at the glass transition temperature, Tg, a further two bends exist. The first, in glassy state, is relatively slight. The other one, in an elastic state, can be correlated with the melting temperature T m of the crystalline phase. The order agreement between the temperature coefficients of free volume hole expansion α h and the mean square displacement of chain atoms α (u 2 ) from a recent neutron scattering study indicates a microscopic cause for the hole size growth with temperature. The decrease of I 3 with temperature T above 120 K correlates with the onset temperature of the so-called fast motion from neutron scattering measurements as well as with the so-called Vogel temperature T o for the primary segmental dynamics. The N-like shape course above Tg correlates with the cold crystallization from DSC measurements. In the melt region above 265 K, the slightly dependent τ 3 and I 3 quantities correspond to a formation of the bubble states of the o-Ps in the soft matrix. Finally, the temperature dependence of free volume hole distributions exhibits the essential changes from relative narrow distributions in the glassy state to wide distributions above Tg and ones that do not change much in the melt region of the polymer matrix.