Free Volume and Density Gradients of Amorphous Polymer Surfaces As Determined by Use of a Pulsed Low-Energy Positron Lifetime Beam and PVT Data

Algers, John; Suzuki, Ryoichi; Ohdaira, Toshiyuki; Maurer, Frans H.J.

doi:10.1021/ma0359335

Cited by 29 publications

(32 citation statements)

References 77 publications

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“…For example, structural defects and chemical modifications generated by irradiation modification of materials with highly energetic ions (170)(171)(172) or chemical (173)(174)(175) treatments can be investigated. Other fields include the study of interfaces between bilayers (176)(177)(178)(179) and the glass transition as a function of thickness of very thin films (180).…”

Section: Application To Polymers Monoenergetic Positron Beams Can Bementioning

confidence: 99%

Positron Annihilation Spectroscopy

Dlubek

2008

Encyclopedia of Polymer Science and Technology

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FundamentalsThe positron is the antiparticle of the electron, ie, it has the opposite charge but otherwise the same properties. In matter, a positron may be created either by the β + decay of an unstable nucleus or from a high-energy γ-photon via e + -e − pair production. If the positron encounters an electron, both particles annihilate, ie, they are transformed into photons of an energy, which is given by the famous Einstein equation, E = 2mc 2 . Here, m is the mass of the electron, c the speed of light in vacuum, and for particles at rest E = 2 × 0.511 MeV. The selection rules for the γ-photon emission are derived from the conservation laws of energy and momentum and parity considerations. Free positrons annihilate mainly into two γ-photons of energy 0.511 MeV emitted in almost collinear directions, or with much less probability into three photons of an overall energy of 1.022 MeV (16,24,25).In nonconducting materials, such as polymers, positrons can exist as free particles (e + ) or as positronium (Ps, e + e − ), which is a positron-electron bound state. Ps was experimentally discovered by DeBenedetti and Richings in 1952 (26). The "chemical" symbol Ps for positronium appears to have been introduced by McGervey and DeBenedetti in 1959 (27). Its properties and interaction with matter were reviewed in several articles (25-30). Ps is a particle of the size of a hydrogen atom, r Ps = 0.53Å, but only the mass of two electrons, and a binding energy of 6.8 eV, that is half of the ionization energy of hydrogen. The lifetime of a Ps atom depends on the relative spin orientation of positron and electron. The para-Ps (p-Ps, singlet state, spins antiparallel) decays into two photons with a mean intrinsic lifetime (self-annihilation in a vacuum) of τ pPs 0 = 0.125 ns. The ortho-Ps (o-Ps, triplet state, spins parallel) can decay only into three photons, a much slower process than two-photon emission. Therefore, o-Ps has an intrinsic lifetime of τ oPs 0 = 142 ns. The formation probabilities of the p-Ps and o-Ps states are 0.25 and 0.75 of the total Ps yield (16,25).In condensed matter studies, 22 Na is usually used as the positron source. This isotope has a half-life of 2.6 years and emits positrons with an energy distribution that has its maximum at 0.20 MeV and ranges up to 0.54 MeV (10,16). The stopping profile of positrons is a decreasing exponential function. In materials with a density of 1 g/cm 3 50%, 90%, and 99% of positrons are stopped within a thickness of 0.170, 0.60, and 1.20 mm, respectively. The energetic positrons lose their energy rapidly via ionization and excitation of electrons and finally phonons. After a few picoseconds, the positrons are in thermal equilibrium with the ambient material. In defect-free molecular crystals, positrons migrate over a mean distance of typically 100 nm before annihilating. In defected crystals and in amorphous matter, positrons may annihilate from local free volumes. The mean positron lifetimes are typically τ e+ = 200-400 ps.

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Section: Application To Polymers Monoenergetic Positron Beams Can Bementioning

confidence: 99%

Positron Annihilation Spectroscopy

Dlubek

2008

Encyclopedia of Polymer Science and Technology

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“…The WLF equation acquires physical significance when examined in the light of the theory of free volume [28]. The constants then become:…”

Section: Quantitative Treatment Of Experimental Data To Facilitate Inmentioning

confidence: 99%

A fundamental approach for the estimation of the mechanical glass transition temperature in gelatin

Kasapis

Sablani

2005

International Journal of Biological Macromolecules

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“…Glass transition in the sub-surface regions of polymers was studied by several groups [25][26][27][28][39][40][41]. Glass transition appears as an inflection in the temperature dependence of the free volume size, so for the mere determination of the glass transition temperature (T g ) DBAR can be used as well as PALS (see Fig.…”

Section: Studies Of Free Volume and Poresmentioning

confidence: 99%

“…However, Algers et al reported that the enhancement is observable only at depths shallower than a few nm for polystyrene, PMMA, SAN and poly(styrene-co-maleic anhydride) (SMA) [39]. The discrepancy may partly arise from the different method to account for the very long lifetimes due to o-Ps annihilating in vacuum [25].…”

Section: Studies Of Free Volume and Poresmentioning

confidence: 99%