Nucleation and crystallization of blends of poly(propylene) and ethylene/alpha-olefin copolymers

In this work, the nucleation and crystallization of immiscible blends of po ly(propylene) (PP) with ethylene/alpha-olefin copolymers of different branc hing contents (Linear Low Density Poly(ethylene), LLDPE, Very Low Density P oly(ethylene), VLDPE, and Ultra Low Density Poly(ethylene), ULDPE) were stu died. Melt-mixed blends of LLDPE/PP, VLDPE/PP and ULDPE/PP were prepared co vering a wide composition range by means of twin screw extrusion. Additiona lly, blends containing 0.1% of the nucleating agent Sorbitol were prepared. The thermal behaviour of the blends was studied by means of Differential S canning Calorimetry (DSC) whereas the morphology was investigated by means of Scanning Electron Microscopy. When PP droplets constituted the dispersed phase of the blends, fractionate d crystallization of the PP component lead to coincident crystallization wi th the ethylene/alpha-olefin matrix, regardless of the branch content of th e copolymer. This complex crystallization process occurred when a small fra ction of PP droplets initiated the crystallization upon cooling, causing nu cleation of the ethylene/alpha-olefin copolymer matrix and subsequently sim ultaneous crystallization of the rest of the PP droplets and the matrix. Th e fractionated crystallization process disappears if nuclei capable of bein g active at low supercooling are injected in any of the blends. The injecti on of nuclei was achieved by the use of sorbitol (a powerful nucleating age nt for alpha-PP) or by self-nucleation of the PP component. When PP constit uted the matrix in the blends, a nucleation process of the ethylene/alpha-o lefin copolymers was observed. The nucleation of the ethylene/alpha-olefin copolymers by PP, sorbitol or PP/sorbitol mixture was found to be particula rly effective for the most linear chains within the wide chain branching di stribution present in these copolymers. A peculiar synergistic nucleation e ffect on LLDPE was produced by the addition of a mixture of PP and sorbitol . Even the addition of 1% PP and 0.1% sorbitol was enough to produce a very strong nucleation effect on the LLDPE, only comparable to that caused by s elf-nucleation.