Composition\nDependence of Flow-Induced Crystallization\nin High-Density Polyethylene/Isotactic Polypropylene Blends

Polyolefins, including high-density polyethylene (HDPE)\nand isotactic\npolypropylene (iPP), account for over half of the worldwide plastics\nmarket and have wide-ranging applications. Recycling of these materials\nis hindered due to separation difficulties as co-mingled blends of\nHDPE and iPP often exhibit brittle mechanical behavior because phase\nseparated domains detach under stress due to low interfacial adhesion.\nMotivated to improve mechanical properties of mixed recyclates during\nprocessing, this work examines the effect of shear on the crystallization\nkinetics and rheological properties of HDPE–iPP blends utilizing\na combination of differential scanning calorimetry (DSC), rheo-Raman\nspectroscopy, polarized optical microscopy, and scanning electron\nmicroscopy (SEM). In the quiescent experiments, the crystallization\ntemperature as a function of blend composition exhibits a distinct\ndecrease when the iPP forms the droplet phase, as expected, due to\nfractionated crystallization. In the presence of shear, we find elongated\ndomains due to high capillary number. Unexpectedly, we find a compositional\ndependence to the flow-induced crystallization (FIC) of iPP: stronger\nFIC is observed in all blends compared to the pure iPP. Moreover,\nthe flow completely counteracts the reduced crystallization arising\nfrom fractionated crystallization, indicating that the flow is able\nto induce nucleation in droplets to an extent such that it offsets\nthe reduction in active nucleating agents in finite size droplets.\nWe attribute these effects to differing microflow fields in the various\nmorphologies as the iPP domains deform under shear.