Extensional Flow Resistance of 3D Fiber Networks in Plasticized Nanocomposites

The presence of a three-dimensional network of high-aspect-ratio CO 2 -philic fibers increases the extensional viscosity of melts but decreases the shear viscosity under a CO 2 atmosphere. The generation of such a rheological response is rare in literature because the addition of solid-state fibers typically increases both extensional and shear viscosities. These results are supported by the high-pressure rheological characterization of polypropylene (PP) containing various loadings of polytetrafluoroethylene (PTFE) fibers. While the shear viscosity decreases with an increase in fiber loading due to the higher uptake of CO 2 from the CO 2 -philicity of PTFE, a strikingly different behavior is seen in extensional flows: below the fiber percolation threshold, the extensional viscosity decreases; however, above the percolation threshold, the extensional viscosity jumps by an order of magnitude. The enhancement in extensional viscosity is attributed to the large resistance offered by the network against extensional deformations. These results have important consequences in CO 2 -based polymer processes.