Macroscopic volume changes versus changes of free volume as determined by positron annihilation spectroscopy for polycarbonate and polystyrene

Positronium lifetimes in bisphenol A-polycarbonate (PC) and atactic polysty rene (PS) were measured as a function of temperature and hydrostatic pressu re, and the results were compared with the macroscopic volume change of the polymer samples. A simple relation via the thermal expansion coefficient a nd the compressibility of the bulk material is applied and leads to direct information on the fractional free volume of the polymer in the investigate d state and furthermore to the mean volume size of intermolecular holes. By plotting macroscopic volume changes vs changes of the hole volume as probe d by the positronium, a linear relationship is obtained independent of whet her the measurements were made above or below the glass transition temperat ure. The slope of the corresponding straight lines is steeper for samples e xposed to hydrostatic pressure which is interpreted as a shrinkage of holes induced by pressure.