Positron Annihilation Lifetime Study of High and Low Free Volume Glassy Polymers:  Effects of Free Volume Sizes on the Permeability and Permselectivity

Abstract

Free volume in a big group of glassy polymers distinguished by a wide range of gas permeability (P in the range 1-10 4 Barrer) was studied using the positron annihilation lifetime (PAL) method. To take into account an additional channel of decay of positronium (Ps), caused by the reaction with sorbed O2, measurements were performed both under ambient conditions and under a nitrogen atmosphere. The finite-term and continuous lifetime analyses (PATFIT and CONTIN programs) were used for the treatment of the positron annihilation decay curves. It was shown that better statistical fitting parameters can be obtained for four component PAL spectra of a vast majority of the glassy polymers studied. The presence of two Ps lifetimes (τ 3 ) 1.7-3.3 ns and τ4 ) 2.7-10.9 ns) indicates that size distribution of free volume elements (FVE) in glassy polymers is typically bimodal. The presence of smaller and larger FVE (R3 ) 2.5-3.5 Å, R4 ) 3.5-7.0 Å) was proved by PATFIT analysis of the data. Continuous analysis clearly indicated two broad peaks, especially for more permeable polymers. As permeability and free volume of the polymers decrease, the two peaks start to approach and overlap each other, ending with one broader or asymmetric peak for the less permeable polymers. In the presence of oxygen both lifetimes τ 3 and τ4 are shifted to shorter values; this shift is more pronounced for the polymers with larger radii R4 of FVE. A novel correlation of diffusion selectivity of glassy polymers with the size of FVE is demonstrated.

Selective, Room-Temperature Transformation of Methane to C1 Oxygenates by Deep UV Photolysis over Zeolites

“…O 2 permeability decreases from 10,083˘936 Barrer at 298 K to 7024˘1050 Barrer at 333 K, while O 2 diffusivity diminishes from 5.47ˆ10´8˘1.36ˆ10´8 cm 2¨s´1 at 298 K to 3.46ˆ10´8˘7.72ˆ10´9 cm 2¨s´1 at 333 K. These values are in good agreement with those reported in the literature [17,[22][23][24][25]32,33]. However, in spite of the high oxygen permeability, the ideal O 2 /N 2 selectivity of pure PTMSP is very low, ranging from 1.32˘0.12 at 298 K to 0.94˘0.41 at 333 K. This is a characteristic behavior of PTMSP due to its high free volume and the fact that the weakly and rigid molecular sieving framework is more prone to changes in solubility than diffusivity [24,32].…”

Methods for the calculation of occupied volumes in glassy polymers: The lattice integration and the monte carlo methods

“…[8,9,10]). However, the application of these methods, especially for polymers with complicated chemical structures, requires the use of powerful computers and very long computation times.…”

Free Volume, Molecular Mobility and Polymer Structure: Towards the Rational Design of Multi-Functional Materials

“…Two distinct o-Ps lifetimes can be resolved in the spectra measured for the membranes, suggesting they have a bimodal free volume distribution [17]. The ZIF-8 nanoparticles possess pores with a diameter of 11.6 Å, connected by smaller apertures, 3.4 Å in diameter [18].…”