Nonlinear shear and elongational rheology study of MWCNTs enclosed multilayer systems

Abstract

The linear and nonlinear rheological behavior of filled polymers has been a research focus for decades. In this study, multi-walled carbon nanotubes (MWCNTs) were incorporated into a linear polypropylene (PPC) and co-extruded with a long-chain branched polypropylene (PPH) to form a multilayer system with a layered distribution of MWCNTs. The nonlinear shear and extensional rheological behaviors of the product films were then characterized in both machine direction (MD) and transverse direction (TD). Interestingly, the number of layers and layer thickness had a significant impact on rheological behavior. When fewer layers with thicker dimensions were present, strain hardening during extension was decreased in the filled system compared with the neat polymer multilayer system. Conversely, when the number of layers increased and the layer thickness decreased, strain hardening in the filled system was notably enhanced, particularly in the transverse direction (TD) during extensional rheology tests. This behavior is attributed to the PPC/MWCNTs layers were confined by the PPH layers effectively as the number of layers increased and layer thickness decreased close to or below the average length of the MWCNTs. In the multipliers, this confinement synergized with the extrusion flow, enhancing the orientation of MWCNTs in the machine direction (MD). In comparison with the multilayer systems composed of only LLDPE and MWCNTs, the neat LLDPE layer showed less impacts to the LLDPE/MWCNTs layer and the MWCNTs orientation. In addition, MWCNTs orientation effects to the elongational viscosities were more significant at lower Hencky strain rates. The enhancement of the MWCNTs orientation was further confirmed and studied by morphology analysis.

Graphical abstract