Abstract
Different definitions of the free volumes are discussed and related to the total specific volume and to the local free (hole) volume. Positron lifetime measurements are reported for polyethylene, polytetrafluoroethylene, and CR39-copolymers. Two long-lived components appear in polyethylene and polytetrafluoroethylene, which were attributed to o-Ps annihilations in crystalline regions and in holes of the amorphous phase. From a relation between the coefficients of the thermal expansion of macroscopic and hole volume, the fractional hole volume h and from this the number of holes, N, is estimated. Values of Tg = 195 K, hg = 4.5% (polyethylene) and 5.7% (polytetrafluoroethylene) and Ng = 0.73 nm -3 (polyethylene) and 0.36 nm -3 (polytetrafluoroethylene) were obtained. In semicrystalline polymers these values agree with estimates obtained from the densities of the crystalline and amorphous phases. The effect of cross-linking on the free-volume properties of CR39-copolymer networks was studied. The comparison of the hole volume with the specific volume allowed us to estimate the number density of holes to = 1 nm -3 . PACS numbers: 61.41.+e, 65.70.+y, 78.70.—g
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