The temperature dependence of the mean size of nanoscale free-volume holes, < V-h >, in polymer blend system consisting of polar and nonpolar polymers has been investigated. The positron lifetime spectra were measured for a series of poly-mer blends between polyethylene (PE) and nitrile butadiene rubber (NBR) as a function of temperature from 100 to 300 K. The glass transition temperatures (T-g) for blends were determined from the ortho-positronium (o-Ps) lifetime tau(3) and the mean size of free-volume holes < V-h > versus temperature as a function of wt % of NBR. The T(g)s estimated from the PALS data agree very well with those estimated from DSC in view of different time scales involved in the two measurements. Both DSC and PALS results for the blends showed two clear T(g)s of a two-phase system. Furthermore, from the variation of thermal expansivity of the nanoscale free-volume holes, the thermal expansion coefficients of glass and amorphous phases were estimated. Variations of the o-Ps formation probability I-3 versus temperature for pure PE and blends with low wt % of NBR were interpreted on the basis of the spur reaction model of Ps formation with reference to the effects of localized electrons and trapping centers produced by positron irradiation. (C) 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 227-238, 2009