Composition dependence of in high-vinyl polybutadiene/ cis-polyisoprene blends studied by means of positron annihilation lifetime spectroscopy

Abstract

The positron annihilation lifetime spectroscopy (PALS) and differential scanning calorimetry (DSC) techniques were utilized to study high-vinyl polybutadiene (HVBD)/ cis-polyisoprene (CPI) blends. A single DSC glass transition temperature is observed, whose composition dependence strongly deviates from additivity, showing an apparent cusp when the weight fraction of HVBD . The orthopositronium (o-Ps) intensities, , and lifetimes, , obtained from PALS were used to determine the free-volume hole size, , and the scaled fractional free volume, . In the glass, and are smaller for CPI than for HVBD, but the thermal expansion coefficient for the hole volume, , is larger for CPI than for HVBD in the melt; thus, an iso-hole-volume temperature occurs in these blends at . Using free-volume quantities obtained from PALS, a quantitative interpretation of the cusp in the composition dependence of can be obtained, via a modified version of the analysis of Kovacs, from which we assume that the free volume of the high- component becomes constant. Specific volume analysis was also performed on the pure materials, from which the fractional free volume at room temperature, and hence the values of the scaling constants and , are obtained. This enables us to determine that the free volume of the high- component `freezes in' at a particular value intermediate between those of the pure components at their respective s