https://doi.org/10.1140/epjd/e2005-00015-y
Insights into positron annihilation lifetime spectroscopy by molecular dynamics simulations
Free-volume calculations for liquid and glassy glycerol
1
Polymer Institute of SAS, 842 36 Bratislava, Slovak Republic
2
European Laboratory for Non-linear Spectroscopy (LENS), Via Nello Carrara 1, 50019 Sesto Fiorentino, Italy
3
Dipartimento di Chimica, Università di Firenze, Via della Lastruccia 3, 50019 Sesto Fiorentino, Italy
Corresponding author: a chelli@chim.unifi.it
Received:
10
August
2004
Revised:
17
November
2004
Published online:
1
February
2005
The relationship between free-volume properties measured from positron
annihilation lifetime spectroscopy (PALS) and calculated from molecular
dynamics simulations has been investigated for glassy and liquid glycerol in
the temperature range 150–400 K. A virtual probing procedure has been
developed to retrieve information on the basic free-volume properties of the
simulated microstructures, i.e. mean cavity volume and free-volume cavity
fractions. Our data leads us to infer on the occurrence of experimentally
non-detectable small cavities with mean equivalent radius of 1.8–1.9 Å
between 250 and 275 K. The size of these limiting cavities is found to be
temperature dependent, being smaller at low temperatures. At high
temperatures, above a characteristic PALS temperature
, the formation of very large cavities is
predicted. This finding suggests that, when the dimension of the holes in
the system exceeds a given value, the PALS measurements become unable to
catch the complete structural information and phenomena of dynamical origin
enter into play in the PALS signal decay. The calculated number of cavities
is found to be almost independent on the temperature from the glassy up to
the liquid phase, thus furnishing a certain support to theoretical models
proposed to evaluate the free-volume cavity fractions.
PACS: 61.20.Ja – Computer simulation of liquid structure / 78.70.Bj – Positron annihilation
© EDP Sciences, Società Italiana di Fisica, Springer-Verlag, 2005