Deuterium nuclear magnetic resonance (NMR) and rheology of microgel colloids at ambient and high pressure

Sbeih, Suhad Adnan (2018) Deuterium nuclear magnetic resonance (NMR) and rheology of microgel colloids at ambient and high pressure. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Microgel colloids exhibit a polymer collapse transition resulting in a large reduction in colloid particle volume at high temperatures or pressures. They are of interest because of their potential for applications in areas, like drug delivery and chemical separation, that involve the uptake, encapsulation and release of small or biological molecules. The goal of this work is to obtain a microscopic understanding of the internal structure and microscopic dynamics of microgels by examining the temperature and pressure dependence of the collapse transition. Deuterium NMR (²H NMR) has been used to probe the microscopic dynamics of crosslinked poly N-isopropylacrylamide (polynipam) chains, in microgel colloids, as a function of temperature and pressure for a series of four crosslink densities (Cd). Each crosslinked microgel colloids was synthesized with deuteron labels on the nipam backbone (d3 nipam). Macroscopic properties of unlabeled colloids having the same crosslink densities were characterized by dynamic light scattering (DLS) and rheology. Rheological characterization as a function of temperature (T) and particle concentration (C), and for 4 crosslink densities, showed that the microgel viscosity decreases as temperature is increased. The Krieger- Dougherty model of the relative viscosity as a function of volume fraction was employed for viscosity analysis. The measured viscosity in the colloidal regime (at high T, low C) collapsed onto one line when plotted against volume fraction. This yields a measure of the water content in the particles as function of T. The water volume fraction in the microgel particle at 45ᴼC was always found to be 0.6 for any Cd. ²H NMR spectra of the d3 nipam suspensions for all Cd indicated freely moving chains at low temperature and a nearly immobilized fraction above 35ᴼC. This is consistent with DLS observations of a transition from swollen to collapsed colloids. Nipam segments in the collapsed phase of the d3 nipam suspension were more mobile than those in the dry powder. This suggests significant amounts of water in the collapsed phase, a finding consistent with the rheology observations. Variable pressure NMR (up to 90 MPa) showed a slight increase in transition temperature with pressure for all Cd values studied.

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