Measurements of the free volume hole size distributions obtained from positron annihilation
lifetime spectroscopy (PALS) in relation to the chain tacticity in polystyrene and poly(p-methylstyrene),
and in relation to the static free volume distribution in modeled amorphous cells of these polymers are
reported. The free volume distribution was estimated using the Voronoi tessellation of space and direct
calculations of unoccupied space in modeled amorphous cells of the polystyrenes. The results indicate
that the syndiotactic polystyrene and the syndiotactic poly(p-methylstyrene) have broader hole distributions than the more atactic samples. The polystyrenes have a maximum in the free volume hole size
distribution at volumes smaller than for the poly(p-methylstyrenes). The calculations indicate the presence
of a large portion of holes too small to be detected with positron annihilation. The small holes are related
to structural features of the polymer chain itself. The calculated distribution of large holes in the polymers
agree satisfactorily with the results from the PALS measurements. The difference is due to the irregular
shapes of the holes in the calculated models as compared to the spherical potential wells assumed in the
interpretation of results from PALS measurements.
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