High-pressure dependence of the free volume in fluoroelastomers from positron lifetime and PVT experiments

The microstructure of the free volume and its high-pressure dependence in fluoroelastomeric copolymers of tetrafluoroethylene (TFE) and perfluoro(methyl vinyl ether) (PMVE), PFE, as well as vinylidene fluoride (VDF) and hexafluoropropylene (HFP), VDF/HFP(22), was studied by pressure-volume-temperature experiments (PVT, P = 0.1-200 MPa) and positron annihilation lifetime spectroscopy (PALS, P = 0.1-448 MPa, all at 22.5 degreesC). Employing the Simha-Somcynsky equation of state the excess (hole) free volume fraction h and the specific free and occupied volumes, V(f) = hV and V(occ) = (1- h)V, were estimated from the specific total volume V. The pressure variation of these volumes and their compressibilities are discussed. We found that the occupied volume V(occ) exhibits a remarkable compressibilitY7 K(occ) approximate to 2.4 x 10(-4) MPa(-1), while it shows (for T > T(g)) almost no thermal expansion. The PALS spectra were analyzed using the new routine LT9.0 assuming dispersion in both the positron (tau(2)) and orthopositronium (o-Ps) lifetimes (tau(3)). We speculate that positrons (e(+)) may show Anderson localization at sites of the entire empty volume possibly assisted by an affinity to fluorine atoms. From the o-Ps lifetime parameters the (excess) free volume hole size distribution, its mean value <nu(h)>, and dispersion sigma(h) were calculated. From a comparison of <nu(h)> with V(f) the specific hole number N(h)' was estimated. N(h)' was determined to be constant and the same in compression and thermal expansion experiments. When taking correctly into account the compressibility of the occupied volume V(occ) the discrepancy in V vs <nu(h)> plots between compression and thermal expansion experiments discussed in the literature disappears.