The Couette configuration of the Los Alamos Neutron Science Center Neutron Rheometer for the investigation of polymers in the bulk via small-angle neutron scattering

Abstract

A neutron rheometer in the Couette geometry has been built at the Los Alamos Neutron Science Center to examine the molecular steady-state and dynamic responses of entangled polymeric materials in the bulk under the application of shear stress via small-angle neutron scattering. Although similar neutron rheometers have been fabricated elsewhere, this new design operates under the extreme conditions required for measuring the structure and behavior of high molecular weight polymer melts. Specifically, the rheometer achieves high torques (200 N m) and shear rates (865 s−1) simultaneously, never before attainable with other neutron rheometers at temperatures up to 240 °C under an inert gas environment. The design of the instrument is such that relatively small sample sizes are required. The testing of the Los Alamos Neutron Science Center Neutron Rheometer in the Couette design both as a rheometer and in the small-angle neutron optical configuration on highly viscous polystyrene is presented. The observed anisotropic neutron scattering pattern of the polystyrene melt at a molecular weight above entanglement provides evidence that the conformation of the polymer chains are elongated in the direction of the melt flow, in agreement with the current theories concerning linear polymers in the bulk.