Role of Crystallization on Polyolefin Interfaces: An Improved Outlook for Polyolefin Blends

Polyolefins including linear low density polyethylene (LLDPE), high density polyethylene (HDPE), and isotactic polypropylene (iPP) account for nearly 2/3 of the worldwide plastics market. With wide-ranging applications, often short term in nature such as packaging, recycling of polyolefins is becoming increasingly important in developing a sustainable worldwide plastics market. However, it is difficult to separate polyolefins in mixed recycle streams; it would be advantageous to melt blend them, but their immiscibility leads to blends with poor properties. Here we demonstrate the role of synthetic history (i.e., site specific metallocene vs heterogeneous Ziegler-Natta catalyzed) on the oligomer content of HDPE, LLDPE, and iPP and its influence on adhesion between PE and iPP. Using a range of polymers and processing conditions, we identify four classes of such interfaces with a wide range of interfacial adhesion strengths (G_IC): excess oligomer (G_IC < 30 N/m), easy chain pullout (G_IC ≅ 100 N/m), kinetically trapped entanglements (G_IC ≅ 600 N/m), and crystallization across the interface (G_IC > 1200 N/m). Using molecular weight distribution data, we identified a critical oligomer content where the interfacial failure mechanism transitions from cohesive failure (G_IC > 1200 N/m) to adhesive failure (G_IC ≅ 100 N/m). Transmission electron microscopy (TEM) and atomic force microscopy (AFM) highlight distinct interfacial semicrystalline morphologies for each class of polyolefin interface which are defined by molecular parameters and processing conditions. Polyolefin blends were compression molded to highlight the role of interfacial strength in blends formed from mixed polyolefin streams; weak interfaces resulting from excess oligomer buildup yielded brittle failure while superior interfacial adhesion resulted in ductile blend failure.