Title

Fabrication and the rheology of crystalline polyethylene microgels for thermo-responsive suspensions

Date of Completion

January 2010

Keywords

Chemistry, Polymer|Engineering, General|Engineering, Materials Science

Degree

Ph.D.

Abstract

Microgel suspensions have been studied and characterized extensively by many research groups in recent years. However, most of these suspensions are aqueous in nature with those exhibiting thermo-responsive characteristics being almost exclusively aqueous. The original impetus for this research dissertation was to develop a thermo-responsive microgel suspension that was completely organic in nature for use in high-performance lubricants. These suspensions would maintain or even increase in viscosity with increasing temperature. The focus of the dissertation eventually broadened to a more general approach encompassing soft, swellable particles in organic media. ^ PE microgels were developed using mechanical fragmentation techniques as well as from immiscible blends of PS and PE. The microgels ranged in size from 127 ± 2 µm to 0.944 ± 0.003 µm and the size distributions were well-described using Log-Normal and Weibull distributions. The mechanical fragmentation method produced PE microgels with long surface chains that were capable of interparticle interactions when suspended in squalane. To study the effect of these interactions, the PE microgels produced from immiscible blends were modified to have minimal terminal PE chains or PS chains grafted on the surface. ^ A variety of spectroscopy and electron microscopy techniques were employed to characterize the PE microgels especially the topology of PE microgels with grafted PS chains. Steady flow and small-strain oscillatory viscoelastic experiments were performed on the PE microgel suspensions to characterize the thermal response and interparticle interactions. The PE microgel suspensions exhibited an increase in steady-shear viscosity when the PE microgels melted and the magnitude of the increase was dependent on the PE microgel concentration. Dynamic temperature sweeps of the PE microgel suspensions showed evidence of interparticle interactions as evident from the formation of a gel. ^

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