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

Abstract

In this work, the nucleation and crystallization of immiscible blends of poly(propylene) (PP) with ethylene/α-olefin copolymers of different branching contents (Linear Low Density Poly(ethylene), LLDPE, Very Low Density Poly(ethylene), VLDPE, and Ultra Low Density Poly(ethylene), ULDPE) were studied. Melt-mixed blends of LLDPE/PP, VLDPE/PP and ULDPE/PP were prepared covering a wide composition range by means of twin screw extrusion. Additionally, blends containing 0.1% of the nucleating agent Sorbitol were prepared. The thermal behaviour of the blends was studied by means of Differential Scanning Calorimetry (DSC) whereas the morphology was investigated by means of Scanning Electron Microsopy. When PP droplets constituted the dispersed phase of the blends, fractionated crystallization of the PP component lead to coincident crystallization with the ethylene/α-olefin matrix, regardless of the branch content of the copolymer. This complex crystallization process occurred when a small fraction of PP droplets initiated the crystallization upon cooling, causing nucleation of the ethylene/α-olefin copolymer matrix and subsequently simultaneous crystallization of the rest of the PP droplets and the matrix. The fractionated crystallization process disappears if nuclei capable of being active at low supercooling are injected in any of the blends. The injection of nuclei was achieved by the use of sorbitol (a powerful nucleating agent for α-PP) or by self-nucleation of the PP component. When PP constituted the matrix in the blends, a nucleation process of the ethylene/α-olefin copolymers was observed. The nucleation of the ethylene/α-olefin copolymers by PP, sorbitol or PP/sorbitol mixture was found to be particularly effective for the most linear chains within the wide chain branching distribution present in these copolymers. A peculiar synergistic nucleation effect 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 self-nucleation.