(721b) Interfacial Crystallization of Polyolefins: An Improved Outlook for Polymer Blends
Although isotactic polypropylene (iPP) and polyethylene (PE) are the two highest production volume polymers in the world, interfacial adhesion between these chemically similar, yet immiscible, polymers is poorly understood. Past work has highlighted the consequences of catalyst selection (i.e. multi-site Ziegler-Natta vs. site-specific metallocene) on interfacial adhesion in laminated films. Metallocene catalyzed PE/iPP pairs often possess interfacial weld strength considerably greater than the very poor adhesion obtained with Ziegler-Natta catalyzed PE/iPP pairs. This result has been attributed to the dispersity in molecular weight and branch content obtained with Ziegler-Natta catalysts leading to amorphous build-up at the PE/iPP interface which thwarts interfacial crystallization. Here, we utilize multiple manufacturing techniques to evaluate the role of processing parameters, catalyst selection, and chain architecture on PE/iPP adhesion. First, we demonstrate a dual-bore capillary rheometer adaptation to investigate interfacial properties in melt-coextruded multilayer structures using small quantities of material (~10 g), a technique that mimics traditional laboratory-scale (> 2 kg) multilayer coextrusion. We demonstrate a 3-fold increase in interfacial adhesion between metallocene high-density polyethylene (mHDPE) and metallocene iPP (miPP) when the interface is rapidly quenched compared with slow cooling. By altering the PE chain architecture and substituting a metallocene linear low density polyethylene (mLLDPE), adhesion between mLLDPE/miPP is improved more than 12x, highlighting the role of chain architecture in PE/iPP adhesion. Finally, PE/iPP blends were fabricated to investigate the role of interfacial adhesion on mechanical properties. In the case of weakly adhering pairs, blend mechanical properties were worse than those of the individual components, while blends fabricated from strongly adhering pairs possessed superior mechanical properties than the constituent polyolefins. Understanding how molecular parameters control polymer-polymer interfacial adhesion has enabled us to produce blends of commercial polymers with desirable end-use properties that inform product design.