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 perfluorinated polymer (Tg = 378 K). To this aim we employed pressure–volume–temperature 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 = hV and Vocc = (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 compressibility, while the compressibility of Vf is rather low, although h (300 K) = 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, 〈vh〉, and mean dispersion, σh, were calculated. From a comparison of 〈vh〉 with Vf a constant hole density of Nh′ = 0.25 × 1021 g−1 was estimated. The volume of the smallest representative freely fluctuating subsystem, 〈VSV〉 ∝ 1/σh2, is unusually small. This was explained by an inherent topologic disorder of this polymer. 〈vh〉 and σh show an exponential‐like decrease with increasing pressure P at 298 K. The hole density, calculated from Nh′ = Vf/〈vh〉, 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. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2519–2534, 2007

Positron Annihilation Spectroscopy

“…Examples for the distribution g n (v h ) are shown in Figure 3 (75), whereas Figure 4 displays the temperature dependency of the hole free volume for a larger series of polymers (75)(76)(77)(78)(79)(80)(81)(82). The mean and the width of the hole volume distribution show a weak expansion below T g followed by a much stronger above.…”

Comparison of the free volume sizes and shapes determined from crystallographic and PALS data

“…It is correlated with positronium formation probability and trapping. In the simplest case, it is connected to the free volume amount in the material, sometimes to their number [7,8]. In complicated inner structures, a few components ascribed to o-Ps decay in a few cavity types can be found in the spectrum.…”

Gas Pressure Induced Porosity of CYTOP Polymer. A~Positron Study

“…[7,8], the CYTOP spectra without gas intrusion were analyzed assuming the presence of one component belonging to o-Ps but with the lifetime distribution of logGaussian shape. However, the MELT processing shows that the o-Ps lifetimes in the samples without previous treatment by high pressure represent not a single broad component, but two distinctly dierent, quasi-discrete ones, ≈ 1.5 ns and ≈ 4 ns.…”

“…The PALS spectra of CYTOP taken as a function of temperature or pressure with no gas exposure [8] show only a monotonous decrease of the lifetime and intensity with an increase of pressure. Like in many other polymers, swelling of free volumes after the exposure to CO 2 at the pressure up to 5 MPa was observed in the whole range of temperatures below the glass transition point [7].…”