Free volume and tacticity in polystyrenes

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.