Temperature dependence of the free volume in amorphous teflon AF1600 and AF2400:: A pressure-volume-temperature and positron lifetime study

The microstructure of the free volume and its temperature dependence were studied for amorphous Teflon AFs using pressure-volume-temperature experiments (PVT) and positron annihilation lifetime spectroscopy (PALS). Two copolymers of tetrafluoroethylene and 2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole units with a composition of 35:65 (AF 1600, T-g = 160 degrees C) and 13:87 mol parts (AF 2400, Tg = 240 degrees C) were investigated after drying the as-received materials. The results are compared with those for CYTOP (T-g = 105 degrees C) and the perfluoroelastomer PFE (T-g = -2 degrees C) as well as for conventional (non-fluorinated) polymers. The PVT data were fitted by the Tait equation and the Simha-Somcynsky (S-S) equation of state (eos). From the latter one the hole fraction h and the specific hole free volume, V-f = hV, were determined. From the PALS data published recently by some of the authors (Rudel et a]. Macromolecules 2008, 41, 788) the free volume hole size distribution characterized by its mean, <nu(h)>, and standard deviation, sigma(h), was calculated. The identification of V-f = N-h'<nu(h)> from PALS with V-f = hV from S-S eos opens the way to estimate the specific hole density N-h'. We give evidence for the exceptional large local (<nu(h)>, sigma(h)) and fractional (h) hole free volume in the AF polymers already in the glassy state and relate this to the properties of the particular structure of the heterocyclic dioxole comonomer containing two exocyclic -CF3 groups and to the high dioxole fraction in these copolymers. Correlations between the volume parameters at T-g (<nu(hg)>, sigma(hg), and h(g)) and the value of T-g or the mass of the S-S mer, respectively, were analyzed. AF2400 shows in some aspects a particularly irregular behavior. Possible limits for the application of the S-S theory to our polymers and complications in interpretation of the PALS data are discussed.