SPHERULITE NUCLEATION IN BLENDS OF ISOTACTIC POLYPROPYLENE WITH ISOTACTIC POLY(BUTENE-1)

The phase morphology and the influence of composition on the primary n ucleation of isotactic polypropylene in isotactic polypropylene/isotac tic poly(butene-1) (iPP/iPB) blends were investigated by electron and light microscopy and small-angle light scattering. It was found that i PP and iPB are miscible but the thermal treatment induces partial phas e separation of components and the formation of iPP-rich and iPB-rich phases. The complete phase separation needs high temperatures and/or a long time of melt annealing. In samples crystallized isothermally at low undercooling the heterogeneous primary nucleation in blends is dep ressed as compared to plain iPP. In blends the less active heterogenei ties lose their activity because of an increase of the energy barrier for critical size nucleus formation due to phase separation of blend c omponents during crystallization. For the same reason the rate of homo geneous nucleation in blends decreases, as observed in samples crystal lized at very high undercooling. At very high undercooling IPP and iPB are able to crystallize with similar rates, which results in the form ation of a fraction of iPB spherulites in addition to iPP spherulites. Consequently the number of spherulites in the blend is larger than th at in plain iPP, in spite of the decrease in the homogeneous nucleatio n rate of iPP in the blend. (C) 1994 John Wiley and Sons, Inc.