Probing the Free Volume in Polymers by Means of Positron Annihilation Lifetime Spectroscopy

Abstract

Positron annihilation lifetime spectroscopy (PALS) is a valuable technique to investigate defects in solids, such as vacancy clusters and grain boundaries in metals and alloys, as well as lattice imperfections in semiconductors. Positron spectroscopy is able to reveal the size, structure and concentration of vacancies with a sensitivity of 10−7. In the field of porous and amorphous systems, PALS can probe cavities in the range from a few tenths up to several tens of nm. In the case of polymers, PALS is one of the few techniques able to give information on the holes forming the free volume. This quantity, which cannot be measured with macroscopic techniques, is correlated to important mechanical, thermal, and transport properties of polymers. It can be deduced theoretically by applying suitable equations of state derived by cell models, and PALS supplies a quantitative measure of the free volume by probing the corresponding sub-nanometric holes. The system used is positronium (Ps), an unstable atom formed by a positron and an electron, whose lifetime can be related to the typical size of the holes. When analyzed in terms of continuous lifetimes, the positron annihilation spectrum allows one to gain insight into the distribution of the free volume holes, an almost unique feature of this technique. The present paper is an overview of PALS, addressed in particular to readers not familiar with this technique, with emphasis on the experimental aspects. After a general introduction on free volume, positronium, and the experimental apparatus needed to acquire the corresponding lifetime, some of the recent results obtained by various groups will be shown, highlighting the connections between the free volume as probed by PALS and structural properties of the investigated materials.

Effect of Crosslinking Conditions on the Transport of Protons and Methanol in Crosslinked Polyvinyl Alcohol Membranes Containing the Phosphoric Acid Group

“…The annihilation lifetime of o-Ps serves as a significant indicator of the free volume size in the material. By precisely measuring the lifetime and relative strength of o-Ps, valuable information concerning the free volume size and free volume fraction of the material can be deduced [33][34][35].…”

Restricted Orientation Anisotropy Method for FVE Radii Characterization: Confirmed and Refined via the Study of Six Vibrational Probes

“…Due to the complicated amorphous nature of glassy polymers and the small physical size of the FVEs, the quantitative characterizations of the FVE sizes and radius probability distributions have proven difficult . Only a small number of experimental techniques have been used to make measurements of polymer FVEs, such as 129 Xe NMR, − inverse gas chromatography (IGC), and positronium annihilation lifetime spectroscopy (PALS). ,− PALS has been applied to study many different polymeric systems and has substantial efficacy for determining FVE sizes and, in some instances, size distributions. − PALS utilizes the ortho positronium ( o -Ps) lifetime, which becomes shorter when the positronium wave function overlaps with the electron cloud of FVE walls . Smaller FVEs give rise to more overlaps, resulting in shorter lifetimes.…”

Influence of Dilution on the Mechanical Properties and Microstructure of Polyurethane-Cement Based Composite Surface Coating

“…Therefore, the mechanical properties of a polymer are closely related to its free volume [ 29 ]. Positron annihilation lifetime spectroscopy (PALS) is an important method for measuring the free volume of a polymer [ 30 , 31 , 32 ], and the polymer free volume information can be comprehensively understood by analyzing the o -Ps annihilation lifetime ( τ 3 ) and intensity ( I 3 ) [ 33 , 34 , 35 ].…”