NMR Studies of Complex Fluids Under Deformational Flow

Abstract

In this study, I report on a relatively new method of measuring polymer deformation under shear in which the orientation of bond vectors in determined through nuclear spin interactions. Nuclear magnetic resonance was used to determine the strength of the electric quadrupole interaction of deuterons, a quantity that depends in a very simple manner upon the relative orientation of the electric field gradient axis (the bond axis) with the polarizing magnetic field used to produce the nuclear Zeeman effect. In the experiments reported, the deuteron is present in a small probe molecule (The “SPY”) which rapidly samples the mean alignment of the host polymer giving one the ability to investigate the shear-rate dependence of the interaction, thus investigating essential predictions of a fundamental theory in polymer physics, the Doi-Edwards theory. When a random coil polymer met is subjected to a steady shear, the stress tensor, σαβ exhibits a nonlinear dependence on the shear rate, γ. Under this steady shear the polymer chain suffers a biaxial deformation described by means of the averaged segmental alignment tensor. In the Doi-Edwards formulation of entangled polymer melts, the stress tensor σαβ is shown to be directly proportional to the average alignment tensor Sαβ. When polymer segments are aligned so that Sαβ is non-zero, other physical properties will be anisotropic as well. 2H NMR quadrupole interaction spectroscopy has been used to measure the deformation of polymer melts of varying molecular weight and semi-dilute solutions under shear in a Couette cell. The Rheo-NMR method involves the use of a selective storage precursor pulse, which enables one to observe spectra from pre-selected regions within the flow. I have measured the dependence on the shear rate of the SXX(velocity), SYY(velocity gradient), SZZ(vorticity) and SXY(shear) elements of the segmental alignment tensor as well as the angular dependence of the 2H quadrupole splitting at a fixed shear rate. These results agree well with the Doi-Edwards theory, but significantly better when the convected-constraint release effects are included. Results suggest that the tube disengagement times scale as molecular weight to the power 3.5 ± 0.1, consistent with the usual 3.4 power law. Velocimetry measurements indicate a reproducible and consistent slip occurring at high molecular weights (>1 M Dalton), a phenomenon, which is independently observed in a lower than expected chain deformation, and a review of this is presented. In addition, 1H-NMR spectroscopy as an alternative method for studying complex fluids is presented with spin-spin relaxation (T2) data for a polymer melt under shear. The flow behavior of liquid crystalline polymers (LCP) is more complex than that of simple polymers because LCPs are anisotropic fluids. The same Rheo-NMR methodology used to examine the polymeric liquids of melts and solutions was also used to test the theory of Leslie and Ericksen which, within a rigid rod model, describes the dynamics of a liquid crystal nematic director under deformation. Director dynamics for a side-chain liquid crystalline polymer in the highly ordered nematic phase are studied under extensional flow around a stagnation point. Previous work on a similar LCP reviewed such that a direct comparison can be made with the experimental results presented in this thesis. Values are obtained for the Leslie viscosity coefficients α2 and α3, scaled by the diamagnetic susceptibility. Preliminary methodology for future research in this area.