Temperature and pressure dependence of the free volume in the perfluorinated polymer glass CYTOP: A positron lifetime and pressure-volume-temperature study

Abstract

The microstructure of the free volume was studied for an amorphous per- fluorinated polymer (Tg ¼ 378 K). To this aim we employed pressure-volume-temper- ature experiments (PVT) and positron annihilation lifetime spectroscopy (PALS). Using the Simha-Somcynsky equation of state the hole free volume fraction h and the specific free and occupied volumes, Vf ¼ hV and Vocc ¼ (1 � h)V, were determined. Their expansivities and compressibilities were calculated from fits of the Tait equation to the volume data. It was found that in the glass Vocc has a particular high compressi- bility, while the compressibility of Vf is rather low, although h (300 K) ¼ 0.108 is large. In the rubbery state the free volume dominates the total compressibility. From the PALS spectra the hole size distribution, its mean, hvhi, and mean dispersion, rh, were calculated. From a comparison of hvhi with Vf a constant hole density of Nh 0 ¼ 0.25 3 10 21 g � 1 was estimated. The volume of the smallest representative freely fluctuating subsystem, hVSVi ! 1/rh 2 , is unusually small. This was explained by an inherent topologic disorder of this polymer. hvhi and rh show an exponential-like decrease with increasing pressure P at 298 K. The hole density, calculated from Nh 0 ¼ Vf /hvhi, seems to show an increase with P which is unexpected. This was explained by the compression of holes in the glass in two, rather than three, dimensions. V C 2007 Wiley