ABSTRACT
The in situ molecular scale response of end-grafted polystyrene to shear against a deuterated polystyrene melt was investigated with neutron reflectometry. The derived grafted polystyrene density profiles showed that the grafted polystyrene was retained on the quartz wafer during the measurements. The profiles suggested that the end-grafted polystyrene response to shear results in a series of metastable states, rather than equilibrium states assumed in the current theory. Except for some possible extension and/or contraction of the grafted polystyrene with shear, there was no obvious correlation between the grafted polymer structure and the shear thinning behavior observed in these samples.
ABSTRACT
Although several other neutron rheometers have been built to study soft matter under nonequilibrium conditions, none of them have the ability to measure the structure and behavior of the polymeric interfacial regions in highly viscous polymer melts which require high torques/high strain rates and high temperatures. A neutron rheometer in the cone and plate geometry has been constructed at the Los Alamos Neutron Science Center to rectify this lack of experimental instrumentation. It is also the first-of-its-kind to perform neutron reflectivity studies concurrently with rheological measurements. The details of both the development and testing of the Los Alamos Neutron Science Center neutron rheometer in the cone and plate configuration are described. Proof of principle neutron reflectivity results of end-grafted polystyrene against an identical melt under shear are presented, showing qualitatively that the structural attributes of the end-grafted polymer change when exposed to shear.
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 degrees 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.
