Free volume and tacticity in polystyrenes

Measurements of the free volume hole size distributions obtained from posit ron annihilation lifetime spectroscopy (PALS) in relation to the chain tact icity in polystyrene and poly(p-methylstyrene), and in relation to the stat ic free volume distribution in modeled amorphous cells of these polymers ar e reported. The free volume distribution was estimated using the Voronoi te ssellation of space and direct calculations of unoccupied space in modeled amorphous cells of the polystyrenes. The results indicate that the syndiota ctic polystyrene and the syndiotactic poly(p-methylstyrene) have broader ho le distributions than the more atactic samples. The polystyrenes have a max imum in the free volume hole size distribution at volumes smaller than for the poly(p-methylstyrenes). The calculations indicate the presence of a lar ge portion of holes too small to be detected with positron annihilation. Th e small holes are related to structural features of the polymer chain itsel f. The calculated distribution of large holes in the polymers agree satisfa ctorily with the results from the PALS measurements. The difference is due to the irregular shapes of the holes in the calculated models as compared t o the spherical potential wells assumed in the interpretation of results fr om PALS measurements.