Distribution of an antioxidant in polypropylene/ethylene-octene copolymer blends studied by atomic force microscopy-infrared

In this work, the microscopic distribution of antioxidant 1010 ( AT1010 ) in blends of isotactic polypropylene ( iPP ) with an ethylene-octene copolymer ( POE ) was investigated in situ using the atomic force microscopy-infrared (AFM-IR) technique. Pellets of an iPP / POE blend containing AT1010 at a mass ratio of 79.5 : 20 : 0.5 were extruded at different screw speeds, and were then injection-molded into plates. The domain size of the POE disperse phase in the pellets was about 1 μm, regardless of the screw speed, and remained unchanged in the injection molding. AFM-IR analyses revealed that AT1010 preferred to stay in the POE disperse phase rather than in the iPP matrix, with a concentration ratio of ∼1.2 in the extruded pellets independent of the screw speed, which was further increased to ∼1.5 in the molded plates. The preferred concentration of AT1010 in the POE was in line with its higher solubility in rubber than in iPP , and the enhanced partition of AT1010 in the molded plates was attributed to a longer processing time in the molten state than that for the extruded pellets, which was verified by AFM-IR analyses of pellets extruded with different residence times. Small molecule additives prefer to stay in the elastomer phase, and the partition is enhanced with a longer mixing time in the molten state.