STUDY OF FREE-VOLUME IN HIGH-VINYL POLYBUTADIENE CIS-POLYISOPRENE BLENDS USING POSITRON-ANNIHILATION SPECTROSCOPY/

High-Vinyl Polybutadiene (HVBD)/cis-Polyisoprene (CPI) blends were cha racterized by Differential Scanning Calorimetry (DSC) and Positron Ann ihilation Lifetime Spectroscopy (PALS). A single DSC glass transition temperature T-g is observed, whose composition dependence strongly dev iates from additivity, and shows an apparent cusp when the weight frac tion of HVBD approximate to 0.75. The free-volume hole size, V-h, and the scaled fractional free volume, h(ps)/C, = I3Vh were determined by PALS from the orthopositronium (o-Ps) intensities, I-3, and lifetimes, tau(3), over a temperature range encompassing T-g and the temperature at which ''positronium bubble'' formation occurs. In the glass, V-h a nd h(ps)/C are smaller for CPI than for HVBD, but the thermal expansio n coefficient for hole volume, alpha(f), is larger in the melt for CPI than for HVBD; thus, an iso-hole volume temperature occurs in these b lends at T-iso approximate to -34 degrees C. Above and below T-iso, V- h and h(ps)/C each show a negative departure from additivity. A quanti tative interpretation of the cusp in the composition dependence of T-g can be obtained, via a modified analysis of Kovacs, using free-volume quantities from PALS, with the ratio of scaling constants C-CPI/C-HVB D as an adjustable parameter. At high temperatures, the positron bubbl e size is smaller in CPI than in HVBD. This agrees with the observatio n that the thermal expansivity of hole volume, and, hence the internal pressure are larger in the equilibrium melt of CPI. The effect of e()-irradiation on the o-Ps intensity was investigated. I-3 decreases mo re rapidly in the melt as T --> T-g, and then more slowly in the glass , suggesting that the effect is due to trapping of radical or ionic sp ecies which inhibit o-Ps formation. (C) 1998 John Wiley & Sons, Inc.