Free-volume hole model for positronium formation in polymers: surface studies

Abstract

Positron annihilation lifetime and Doppler broadening of annihilation radiation experiments are performed in a polyurethane film using the mono-energetic slow positron probe as a function of positron energies. Significant variations of positron annihilation signals are observed at a short distance from the surface (<100 nm). The ortho-positronium lifetime in the polymer increases near the surface, while its intensity decreases. The intensity results are consistent with the description of a free-volume hole model for positronium formation proposed by Brandt, Berko and Walker. This study further confirms that positron annihilation spectroscopy is a sensitive probe for the characterization of physical properties of sub-nanometre defects, such as free volumes and holes for polymeric materials.

Studies on the Thin Films of Poly(di-n-hexylsilane) Using Slow Positron Beam Technique.

“…The positron/Ps diffusion lengths were found to be 19 and 25 nm respectively. These values are similar to that reported in polymer by other group [7]. The difference in the diffusion length further supports our observation that the microstructures in thin and thick samples are different.…”

“…1) that the S-parameter monotonically increases with increase in energy and reaches a maximum following which a decrease is seen. This is because the S-parameter directly reflects the intensity of Ps as reported by Cao et al [7]. On the surface, a considerable amount of positron/Ps escape to the vacuum and hence the intensity of Ps is low resulting in lower S-value.…”

Positronium at polymeric surface

“…surface structure is more soft. This is consistent with Cao et al [5] that because of the large fraction of defects (2 -20%) in polymers, the free volume holes become effective sites to trap positrons for Ps formation. The saturation in τ 3 at higher values of incident positron energy and mean implantation depth is due to the fact that most of the Ps formed inside the polymer is not able to escape to the vacuum.…”

“…Here a raw lifetime spectrum is resolved into three lifetime τ 1 , τ 2 and τ 3 . The longest lifetime τ 3 is on the scale of ns and it has been associated with the free volume hole radius [5]. The technique of variable mono-energetic positron beam with shallow implantation depth profiles has enabled us to probe surfaces and thin polymer films [6,7].…”

“…They observed that the DBES study on the surface of polystyrene shows a significant T g depression from 5 nm to about 50 nm from the surface. Cao et al [5] studied the positron annihilation lifetime and Doppler broadening annihilation radiation in a polyurethane film using the monoenergetic slow positron beam as a function of positron energy. The study confirmed that PAS is a sensitive probe for the study of near surface defects such as free volume holes in a polymeric material.…”