Investigation of polymer liquid systems through distant dipolar field effects

Date of Completion

January 2007

Keywords

Physics, Condensed Matter

Degree

Ph.D.

Abstract

This thesis is aimed at using distant dipolar field (DDF) effects to obtain correlation information of polymer liquid systems, and to understand the structure of liquid polymer systems through the correlation function. ^ The maximum resolution limit of current NMR imaging techniques are of the order of 20μm. This limit is mainly due to the inherent limitation of the technique itself and not a consequence of hardware limitations. In NMR imaging the location of each volume element within the sample is identified by labeling spins in each volume element with a unique Larmor frequency. Since the NMR signal is proportional to volume of the uniquely labeled volume element, as the resolution is increased the signal to noise ratio decreases, and eventually the NMR signal gets drowned in the thermal background noise. ^ The advantage of using DDF effects to obtain structural information is that the whole sample contributes towards the observed NMR signal, regardless of the resolution of the structural information that is obtained. Thus opening up the possibility of using NMR to obtain structural information of liquid polymer systems at much higher resolution than currently possible. ^ As with regular NMR imaging techniques, the resolution of structural information obtained through distant dipolar field effects is based on the time integral of the applied field gradients. However in the case of polymer systems the spin-spin (T2) relaxation time is of the order of several milliseconds. Currently available commercial gradient probes are not capable of generating the immense gradient fields that are necessary to obtain high resolution structural information, within the time constrains imposed by short T2 relaxation times of polymers. ^ The first chapter of this thesis deals with the problem of design and construction of gradient system that is capable of delivering ultra strong field gradients with time intervals of less than 500μs. ^ The second chapter aims to provide some insight into the COSY revamped by asymmetric z-gradients (CRAZED) response of ordered lattice structures, including single crystal and crystals with multiple domains. ^ The final chapter provides the results of experiments aimed at quantifying the resolution limits that can be achieved in polymers using distant dipolar field effects, and also the CRAZED response of structured polymer samples. ^

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