On the Limits of Sensitivity of PALS to Incorporated Free Volume Holes in Polymeric Membranes and Sorbents: Effect of <i>sc</i>-CO₂ in Polyhexafluoropropylene

Abstract

Positron annihilation lifetime spectroscopy (PALS) is a recognized instrument for the studies of size-distribution of nanopores (intrinsic free volume holes FVH) in polymers, particularly membrane materials. The limits of this application in the case of “alien” pores, produced by swelling of poly (hexafluoro propylene) PHFP in the gas (CO2) in super-critical (sc-) state are discussed. The obtained conclusions are controlled by measurements of low temperature gas (N2) sorption (LTGS) and by comparison of the data with the results on permeation of various gases through the PHFP membrane. Attention is payed to relaxation with time of the newly created FVH in the PHFP membrane.

Nanoporosity of Polymer Sorbents and Membrane Materials as Seen by PALS and Low Temperature Gas Sorption

“…On the other hand, the apparent density of mesopores is too low for positronium trapping and, correspondingly, for the pore characterization using PALS. The only exclusion, when LTGS could be used, was found [2,3] in poly(hexafluoropropylene) PHFP, where mesopores, directly open to atmosphere, were generated by sample swelling in Sc-CO 2 (Table III, Figs. 5, 6).…”

“…Barrett-Joyner-Halenda methods used [2][3][4] to analyze low-temperature gas (N 2 ) sorption-desorption (LTGS) curves. Inscription Sc-CO 2 for membrane mesopores (4) means LTGS experiments on mesopores in polymers swelled by CO 2 in super-critical state (7.38 MPa, 31.1…”