Spherulite nucleation in blends of isotactic polypropylene with high-density polyethylene

Abstract

Primary nucleation of spherulites in blends of isotactic polypropylene (iPP) with high-density polyethylene (HDPE) has been investigated by means of differential scanning calorimetry and optical microscopy. The number of iPP nuclei in blends crystallized isothermally at temperatures greater than 127°C decreases with increasing HDPE concentration to a much greater extent than follows from the decreasing amount of iPP, whereas in blends crystallized below 127°C this number increases strongly. The shapes of the spherulite size distributions indicate that athermal (heterogeneous) nucleation is disturbed. Experiments with nucleating agents demonstrate that heterogeneous nuclei migrate acrossinterphase boundaries from iPP melt to HDPE melt during mixing due to the difference in interfacial energy between the nuclei and the molten components of the blend. At temperatures below 127°C the HDPE crystals growing in the blend cause additional heterogeneous nucleation of iPP spherulites. This nucleating activity of HDPE crystals is responsible for the increase in nucleation density of the blend in spite of migration.