Lamellar thickness of the polypropylene matrix determines surface induced nucleation of polyethylene droplets in immiscible blends

Abstract

Polyethylene (PE)/polypropylene (PP) blends have attracted much attention since they offer interesting physical properties, including intriguing crystallization behaviour. When PE minority phase is dispersed as droplet in PP matrix, we have recently proved that PP can induce surface induced nucleation of polyethylene. This occurs due to epitaxy, and is detected by measuring an increase of PE crystallization temperature upon varying the crystallization temperature of PP via self-nucleation. However, so far the role of the microstructure of PP substrate on this peculiar nucleation process has not been addressed. To gain insights on this issue PP with varying content of 2,1- erythro regiodefects or ethylene comonomer has been employed to tune the microstructure of the blend’s matrix. Surface induced nucleation is investigated upon modifying the PP matrix morphology via self-nucleation. The modification of the nanoscale morphology of the matrix has been analysed with Small Angle X-ray Scattering (SAXS), allowing to determine the crystalline lamellar thickness. The increase in the crystallization temperature of PP matrix results in a thickening of the lamellae, which in turns enhances the surface induced nucleation of PE phase, as reflected by an increase in the crystallization temperature of PE. A clear correlation between the crystallization temperature of PE and the crystalline lamellar thickness of PP is found. In general, nucleation of PE is promoted by thicker PP lamellae, but surprisingly it can also occur (with lower efficiency) on PP lamellae that are thinner than the average lamellar thickness of the nucleating PE.