Shear-Induced Crystallization in Polyethylene/Polypropylene Blends

The two most abundant single-use plastics are polyethylene (PE) and polypropylene (PP), contributing to the largest share of the plastics waste stream. PE/PP blends, created as a solution to separation issues in recycling, are often brittle due to their immiscibility and the resulting micro-separated domains that crystallize separately upon cooling. The crystallization behavior of PE/PP blends during processing controls the overall mechanical properties but is poorly understood. In this work, the crystallization and rheological properties of high-density polyethylene and isotactic polypropylene blends are studied using a combination of the rheo-Raman microscope, differential scanning calorimetry, and electron microscopy. In particular, we examine the impact of shear, blend morphology and composition on the crystallization kinetics, and how the resulting semicrystalline microstructure influences the rheological properties of the blend. We find unexpected composition dependence to the shear-induced crystallization behavior of the polypropylene which we attribute to domain structure and micro-flow fields.