Dispersions of isotactic polypropylene (PP) particles
in polystyrene (PS) were produced by interfacially
driven breakup of nanolayers in multilayered systems that
were fabricated by means of layer-multiplying coextrusion.
The droplet size was controlled by the individual PP layer
thickness ranging from 12 to 200 nm. In addition, PP was
melt blended with PS to produce PP droplets larger than
those formed by breakup of nanolayers. The dispersions of
PP particles in the PS matrix were melted and annealed
under high pressure of 200 MPa. Only the largest PP droplets,
with average sizes of 170 μm, crystallized predominantly
in the γ form. In the 42-μm droplets obtained by
breakup of 200 nm layers, a minor content of the γ form was
found whereas the smaller droplets obtained by breakup of
the thinner nanolayers contained the α form and/or the
mesophase. The results showed that the γ phase formed
only in the droplets sufficiently large to contain the most
active heterogeneities nucleating PP crystallization under
atmospheric pressure. It is concluded that the presence of
nucleating heterogeneities is necessary for crystallization of
PP in the γ form under high pressure.