Rheo-NMR Investigations of Flow and Alignment in Complex Fluids
Authors:   , Douglass Bradley 
Published: November 2021

<p>This thesis reports the use of Rheo-NMR, that is, a class of techniques within the realm of magnetic resonance which are both confirmatory and complementary to rheometric experiments on materials which can best be classified as complex fluids. The physical properties of such fluids are both hybrid and, in general, vaguely defined. In displaying characteristics attributable to both ideal fluids and elastic solids, the term `complex fluid', in a very real sense, epitomises all the fluids with which every human deals with (and is comprised of) daily. With a multitude of potential candidates for further research then, here we confine ourselves to fluids of molecules and aggregates which are either linear polymers or at least maintain the curvilinear one-dimensional topology of linear polymers. Magnetic resonance is an ideal research tool in this regard, as it is in many respects a rather statistical and insensitive tool from a signal-to-samplevolume perspective, precisely the regime in which the dynamics of a macroscopic collection of macromolecules is relevant. Material deformation is the mechanism upon which rheological measurement depends, and the first research presented here reports on a numerical simulation of the NMR signal of sheared polymer melts. Proton NMR relaxation times of such melts have previously been measured experimentally and found to depend on the shear rate applied by a horizontal Couette geometry, presumably due to the alignment of the mean-field boundaries of the space in which the polymer may reside, known as the polymer tube. The restrictions forming the tube are the other polymers in the bulk, around which an exemplar polymer molecule must meander. In diffusing through this tube, whose direction between entanglements is random in equilibrium, at any time, the return-to-origin correlation for a single spin returning to its locally anisotropic environment generates the least NMR transverse relaxation, as the sum contribution from all tube segments is random. When a deformation-related transformation matrix is applied to the coordinates of entanglements in the polymer, tube segments are no longer isotropically distributed, and an enhanced relaxation process results. Here we present the results of a numerical simulation of this procedure, based on the earlier model of Ball, Callaghan and Samulski, in addition to measurements of the transverse NMR relaxation by Cormier. Not only does it demonstrate qualitative agreement, the NMR signal can be simulated quantitavely or conversely, the size of several key polymer physics parameters can be found through fitting to the NMR signal. Proton NMR spectroscopy is inherently simpler than deuteron NMR spectroscopy, in which the nucleus of interest is quadrupolar. However, a large section of this thesis deals with the structures and response of worm-like micellar structures in solution, for which alignment data cannot reasonably be measured with the proton alone. The most used sample in this thesis is that of the BASF nonionic block copolymer Pluronic P105 in aqueous solution (5% w/w), and a small amount of 1-phenylethanol is required to stabilise cylindrical micellar structures. 1-phenylethanol is a small molecule perfectly suited to act also as a deuterated probe molecule to observe alignment, as it resides in the core of the micelle. By using a variety of Rheo-NMR techniques, such as velocimetry, spatially resolved spectroscopy, and diffusometry, many different flow and alignment behaviours were observed for this solution in Couette flow. Following the measured temperature-dependent viscosity of the P105 solution, which shows an elevated viscosity in a temperature region 15K wide centred at 297 K, we use temperature and applied shear rate as independent variables in our experiments, first identifying spectral features through diffusometry, and then observing a range of behaviours including shear-banding and quadrupolar splitting indicating alignment. Finally we present some experimental work performed in the extensional flow geometry known as the semi-hyperbolic converging die. Extensional flow, inherently, is a transient and nite procedure, and such a geometry is designed to produce a constant extension rate along the axis of its constricting pipe, which, compared to the mill geometries, improves the volume and time over which extension occurs. We investigate the flow and alignment measuring capabilities of Rheo-NMR in this geometry.</p>