Temperature dependence of the free volume in fluoroelastomers from positron lifetime and PVT experiments

The microstructure of the free volume and its temperature dependence in fluoroelastomeric copolymers of tetrafluoroethylene (TFE) and perfluoro(methyl vinyl ether) (PMVE), PFE, as well as vinylidene fluoride (VDF) and hexafluoropropylene (HFP), VDF78/HFP22, were studied by pressure-volume-temperature experiments (PVT, T = 300-485 K, P = 0-200 MPa) and positron annihilation lifetime spectroscopy (PALS, T = 100-473 K, P = 0 MPa). Employing the Simha-Somcynsky equation of state (S-S eos), the excess free (hole) 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 temperature and pressure variation of these volumes and their expansivity and compressibility will be discussed. The PALS spectra were analyzed using the routine LT9.0 assuming a dispersion in both the positron (tau(2)) and ortho-positronium (o-Ps) lifetime (tau(3)). From the lifetime parameters the hole size distribution, its mean value <upsilon(h)> and dispersion sigma(h) were calculated. From a comparison of <upsilon(h)> with V and V-f the specific hole number N-h' was estimated. N-h' was determined to be independent of the temperature. Indications were found that o-Ps may prefer larger holes with a weight approximately proportional to the hole volume. Apparent discrepancies between S-S modeling and the conclusions from PALS are discussed. From the comparison of the hole size distribution with the theory of thermal fluctuation a fluctuation volume <V> is estimated which decreases above T-g with increasing temperature. Attempts are made to interpret the PALS results in terms of the theory of structural and dynamic heterogeneity of glass-forming liquids.