<i>In situ</i> experimental investigation of fiber orientation kinetics and rheology of polymer composites in shear flow

In this study, we experimentally investigate the fiber orientation kinetics and rheology of fiber-filled polymer melts in shear flow. A novel setup is designed with custom-built bottom and top geometries that are mounted on a conventional rotational rheometer. Shear flow between parallel sliding plates is applied by vertical movement of the top geometry. The axial force measurement data of the rotational rheometer are used to determine the shear stress growth coefficient. The fiber orientation kinetics are measured in situ with this setup using small angle light scattering. We consider a non-Brownian experimental system with short glass fibers for the suspended phase (L/D=8–15) and different polyethylene based materials for the matrix phase. The fiber orientation kinetics are investigated as a function of fiber volume fraction (ϕ=1%, 5%, and 10%) and as a function of the shear rate (γ˙=0.03, 0.55, and 5s−1). Within the studied range, these parameters do not influence the fiber orientation kinetics, and a multiparticle model, based on Jeffery’s equation for single particles, can describe these kinetics. Our results show that, up to the concentrated regime (ϕ≈D/L), fiber-fiber interactions do not influence the fiber orientation in shear flow. Finally, we investigate the shear stress growth coefficient of these composites and demonstrate that a simple rheological model for fiber composites, which assumes a constant, isotropic orientation distribution of the fibers, is able to describe the shear stress growth coefficient of the short fiber composite samples.