Volume 16, Issue 3

We present a simple methodology for extending linear viscoelastic measurements of crystallizing polymers to the high-frequency regime, where usually crystallization kinetics represents an obstacle. The idea is to perform rapid quenches, below the melting temperature (which corresponds to the high-frequency region), account for the thermal equilibration and the very slow early stages of crystallization, and perform the measurements before really crystallization sets in, in the “quasi-equilibrium” state. Albeit tedious, when properly done, this method works remarkably well, as demonstrated for a particular polybutadiene mixture, and opens the route for obtaining rheological master curves in a wide variety of crystallizing polymers.

The rheological behaviour of PVC plastisols composed of a blend of suspension and micro-suspension resins in different proportions was investigated. The present work was mainly focused on the variation of the zero shear viscosity versus the concentration of the extender resin. Hence, the packing fraction F m was evaluated using the Krieger-Dougherty equation. These experiments confirmed the major influence of particle size distribution concerning the decrease of viscosity for concentrated suspensions. In addition, a model based on the porosity model developed by Ouchiyama et al. (Ouchiyama N, Tanaka T: Porosity estimation for random packings of spherical particle”, Ind. Eng. Chem. Fundam. 23 (1984) 490-493) to predict the packing fraction in the case of multimodal lattices was successfully applied to our PVC plastisols formulations.

In this work we investigate the rheological behaviour of macroscopic buoyant hard spheres dispersed in a shear-thinning suspending fluid. We focus on the phenomenological study of the influence of the shear-thinning behaviour of the suspending medium on the effective apparent suspension viscosity at different volume fractions. In the oil industry, the effective viscosity concept is widely used and very useful to quickly characterize a change of viscosity due to an increase of the solid content. Viscosity measurements are compared to the effective viscosity of a suspension of hard spheres in an Ostwald fluid. The power law index of the suspending fluid is shown, both experimentally and theoretically, to influence strongly the volume fraction dependence of the suspension effective viscosity. All experimental results are shown to be quite correctly plotted on a master curve, with only one adjustable parameter, the maximum packing fraction f m . The best fit is obtained for f m = 0.57, corresponding to the theoretical maximum random packing volume fraction.

Liquid crystalline polymer (LCP) and polyethylene terephthalate (PET) were blended in an elastic melt extruder to make samples having different volume fractions of constituent polymers. Shear stress, shear viscosity, first normal stress difference at different shear rates under steady state conditions of these blends were evaluated at two different temperatures 265 and 285˚C. The LCP was in solid state at 265˚C and in melt state at 285˚C and was dispersed in molten matrix of PET at both temperatures. Shear viscosity of blend increased with addition of LCP in PET matrix. A maxima was observed in viscosity versus composition plot. Blends containing more than 50 vol. % of LCP in the blend show higher viscosity as compared to the constituent polymers. First normal stress difference, N 1 , increased with LCP content in the blend at 285˚C when ploted against shear stress whereas at 265˚C this trend was opposite. The increased value of N 1 with shear rate was explained assuming a tendency of asymmetric particles to rotate under velocity gradient of suspending medium. At 285˚C N 1 varied with shear stress in two stages. First stage was characterized with high sensitivity of N 1 with shear stress, which reduced in second stage on plastic deformation of LCP droplets.