Xs. Li et Mc. Boyce, ON THE MEASUREMENT OF STRUCTURAL RELAXATION IN POLYMERS USING POSITRON-ANNIHILATION LIFETIME SPECTROSCOPY, Journal of polymer science. Part B, Polymer physics, 31(7), 1993, pp. 869-873
Positron annihilation lifetime spectroscopy has been identified as an
effective means of characterizing the free volume content of amorphous
polymers. The lifetime and intensity of the ortho-positronium (o-Ps)
pick-off annihilation has been found to correlate with the average siz
e and density of free volume sites, respectively. Recently, PALS has b
een used to evaluate and monitor the physical aging and structural rel
axation of polymers in terms of both initial state and evolution in st
ate with time. However, during extended PALS measurements in insulatin
g materials, an electric field can build up due to positron-electron a
nnihilation and can effectively reduce the probability of positronium
formation. In this paper, an observed decrease in intensity associated
with the o-Ps annihilation component in the glassy polymers polycarbo
nate and polystyrene is found to be unrelated to structural relaxation
of the materials over the time periods examined as reported earlier b
y others, and, instead, to be more likely a result of electric charge
build-up.