Rheology of complex fluids: mechanical hole burning spectroscopy and organic complex fluids

The nature of frozen disordered structure in glass-forming materials has long been of great interest in fundamental research as well as in practical applications. Considerable effort has been carried out in the past half century to understand the underlying origin(s) of the commonly observed broadened relaxation response. The present dissertation is composed of three major works.

The first part focuses on the study of the thermal and rheological behavior of polystyrene solutions to facilitate understanding of the nature of dispersive relaxation processes by targeting the issue of how a small molecule affects the macromolecular responses. Solutions of polystyrene in ortho-terphenyl have been investigated using both thermal and rheological methods.

The second part focuses on the rheological studies of complex small organic molecule glass-forming fluids in and out of their equilibrium states to provide direct mechanical shear relaxation information, a response that has been little investigated for such materials in the past. Direct physical aging experiments reveal the structural recovery impact on physical properties of simple organic glass formers.

The third part focuses on the development of a method to study mechanical heterogeneity in polymeric systems. This dissertation is the first in-depth study of dynamic heterogeneity of low dielectric constant polymer melts and their solutions in the terminal relaxation regime using a Mechanical Spectral Hole Burning (MSHB) technique. The MSHB method is conceptually an extension of the nonresonant hole burning technique in dielectric measurements. The fact that the systematic and distinguishable mechanical holes are successfully burned in the systems is consistent with an inhomogeneous relaxation dynamics in these systems studied. MSHB shows great potential as a tool to study dynamic heterogeneity as well as nonlinear viscoelastic behavior in polymer systems. It is suggested that future research applying this MSHB technique to study polymer blends, which have known heterogeneity be undertaken.