Determination of free-volume and hole number density in polycarbonates by positron lifetime spectroscopy

Positron lifetime measurements at room temperature have been carried out to determine the average interchain volume in a series of polycarbonates. App lying different hydrostatic pressures of Ar and CO2 and dissolving the corr esponding molecules in the polymer allows one to determine the change of th e nonoccupied holes of the free-volume by positronium (Ps). A semiempirical relation between the average Ps lifetime and the average free-volume size is used to determine quantitatively the fraction of free volume. Assuming a distribution of hole volumes and a preferential occupancy of large holes b y the dissolved molecules gives rise to a decrease of the Ps lifetime in ag reement with experiments at low pressures. At higher concentrations, swelli ng of the polymer occurs and the microstructure changes irreversibly leadin g to a larger free-volume and therefore to an increase of the Ps lifetime. Three different approaches are used in order to calculate the number densit y of holes. The results favor an uniform occupancy of all holes by Ps inste ad of a preferential occupancy of large holes. Depending on the chosen dist ribution the hole density varies between 2.3 x 10(21) and 6.6 x 10(21) cm(- 3).