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 (τ 2) and orthopositronium (o-Ps) lifetime (τ 3). From the lifetime parameters the hole size distribution, its mean value 〈v h〉 and dispersion σ h were calculated. From a comparison of 〈v 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.

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-volumetemperature experiments (PVT, P ) 0.1-200 MPa) and positron annihilation lifetime spectroscopy (PALS, P ) 0.1-448 MPa, all at 22.5 °C). Employing the Simha-Somcynsky equation of state the excess (hole) free volume fraction h and the specific free and occupied volumes, Vf ) hV and Vocc ) (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 Vocc exhibits a remarkable compressibility, κocc ≈ 2.4 × 10 -4 MPa -1 , while it shows (for T > Tg) almost no thermal expansion. The PALS spectra were analyzed using the new routine LT9.0 assuming dispersion in both the positron (τ2) and orthopositronium (o-Ps) lifetimes (τ3). 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 〈vh〉, and dispersion σh were calculated. From a comparison of 〈vh〉 with Vf the specific hole number Nh′ was estimated. Nh′ 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 Vocc the discrepancy in V vs 〈vh〉 plots between compression and thermal expansion experiments discussed in the literature disappears.

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Recent work at 3M has focused on the development of solvent cast proton exchange membranes (PEM's) for use in PEM fuel cells. These new membranes are a perfluorinated sulfonic acids based on a low molecular weight perfluorinated monomer and they exhibit excellent mechanical properties and chemical stability and high ionic conductivity. The low molecular weight of the monomer allows membranes with equivalent weight as low as 800 g/mole to have good mechanical properties when hydrated. Stabilizing additives in these membranes have been shown to improve the oxidative stability in Fenton's tests. Physical property, conductivity and fuel cell tests have been performed. When incorporated into membrane electrode assemblies, these new membranes have provided excellent performance and a greater than 15-fold increase in durability under accelerated fuel cell test conditions, compared with similar commercial PEM's.