Strain Hardening in Extensional Rheology of Polyolefin Multilayer Films

The interfacial strength between isotactic polypropylene (iPP) and polyethylene (PE) depends on commercial grades, i.e., detailed molecular structures of the polymers.¹ We utilized extensional rheology to understand molten iPP/PE interfaces. We produced iPP/PE multilayer films prepared from Ziegler-Natta catalyzed high-density PE (zHDPE) and isotactic PP (ziPP) as well as metallocene catalyzed high-density PE (mHDPE), linear low-density PE (mLLDPE), and iPP (miPP). Films with 2, 160, and 640 alternating layers of ziPP/zHDPE, miPP/mHDPE, and miPP/mLLDPE were fabricated via multilayer coextrusion. Extensional rheology of the 160-layer miPP/mLLDPE system revealed a strain hardening behavior, which became more pronounced when the number of layers increased to 640, effectively quadrupling the interfacial area in multilayer films. The miPP/mHDPE system showed strain hardening only when the number of layers reached to 640. Strain hardening was not observed in individual mHDPE, mLLDPE, and miPP films, nor in their bilayers. We quantitatively model strain hardening in the multilayer films by adding the stress contribution from the equilibrium melt interfacial tension to the measured extensional contribution from the components.
¹ Jordan et al., Macromolecules 2018, 51, 2506.