The microstructure of the free volume and its temperature dependence were studied for amorphous Teflon AF 1600 (T-g = 160 degrees C) and AF 2400 (T = 240 degrees C). These copolymers consist of tetrafluoroethylene and 2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole units and are used as highly permeable membrane materials. We employed positron annihilation lifetime spectroscopy (PALS) to investigate the free volume. From the lifetime spectra analyzed with the routine LT9.0 the hole size distribution, its mean size and width were calculated. Results obtained from a three and a four component analysis of lifetime spectra are compared. Both polymers show very large orthopositronium lifetimes tau(3) (corresponding to large hole sizes) and glass transitions in tau(3) near the expected values. AF 2400 shows an unusual behavior characterized by a nonlinear temperature dependence of the orthopositronium lifetime, extraordinarily large mean values of tau(3) and the widths of the lifetime distributions sigma(3). Possible explanations for this will be discussed in relation to the microstructure of the copolymer.