Optical properties of the three polymorphic forms of isotactic polypropylene

Abstract

A review of the results obtained for the birefringence of different supermolecular structures of three polymorphic forms of polypropylene is presented. All known lamellar super-molecular formations of the monoclinic, hexagonal and triclinic phases are included. The observed morphology varies among negative hedrite and oval structures with axial symmetry, spherulites and typical dendrites. An attempt to explain the observed variety of the various structures' birefringence is made in terms of the mutual orientation of the polarizability ellipsoid and crystallographic axes in the lamellar surfaces. For the • phase, the orientation of the axes of the polarizability ellipsoid relative to the orientation of the unit cell is determined. The axis Nm coincides with the a axis, the fastest growth direction. The hedritic structures, grown by branching homoepitaxy, have negative birefringence. The spherulites differ in intensity and sign of birefringence because of cross-hatching. For the • phase, the crystallographic axis b coincides with Ne, the short axis of the rotational polarizability ellipsoid. The hedritic structures can grow by the screw dislocation mechanism. Spherulites grow by an epitaxy mechanism. Both structures show negative birefringence. The long axis Ng of the polarizability ellipsoid of the • phase coincides with the b axis. The short molecules of the • phase are inclined 40° towards the lamellar surface. The a and b axes do not lie in the lamellar surface. The axes of the polarizability ellipsoid are also inclined towards the lamellar surface. The preferred fast growth direction of the branching lamellae is the b∗ direction which coincides with Ng', the longest axis in the projection of the optical indicatrix on the lamellar surface. Thus the positive birefringence of the branching lamellae in the mixed •• spherulites increases more than in the monoclinic positive spherulites.