The rheo-optics of shear thickening and structure formation in polymer solutions
Kishbaugh, Alan Jay
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https://hdl.handle.net/2142/19937
Description
Title
The rheo-optics of shear thickening and structure formation in polymer solutions
Author(s)
Kishbaugh, Alan Jay
Issue Date
1992
Doctoral Committee Chair(s)
McHugh, Anthony J.
Department of Study
Chemical and Biomolecular Engineering
Discipline
Chemical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Chemical
Physics, Optics
Plastics Technology
Language
eng
Abstract
Simultaneous measurements of the optical and rheological response of solutions of polystyrene have been made, in-situ, in order to ascertain the connection between structure formation and the phenomenon of shear-thickening. Transient and steady state measurements of the viscosity, dichroism, birefringence and associated orientation angles were carried out in decalin and bromobenzene in the semi-dilute region using a couette device capable of shear rates up to 8,000 s$\sp{-1}.$
Light scattering calculations using the Anomalous Diffraction Approximation (ADA) for large particles, the Rayleigh approximation for isotropic, uniaxial spheroids and the flexible macromolecule model, provide a phenomenologically consistent explanation for the observed behavior. A one-to-one correlation was found between the occurrence of maxima in dichroism and minima in viscosity which demonstrates that shear-thickening is caused by structure formation in solution. Kinetics of the formation of the structures in solution are instantaneous and completely reversible. For solutions that exhibited shear-thinning no maximum in dichroism was seen, the dichroism instead saturated at high shear rates.
Calculations using the ADA suggest that shear-thickening and the associated dichroism behavior result from the flow-induced production and growth of spheroidal, micron-size particles in solution. These particles are isotropic (i.e. liquid-like) and align in the flow direction. In the case of shear-thinning solutions, the Rayleigh approximation for sub-micron, isotropic spheroids was successfully used to account for the magnitude of the saturating dichroism behavior, suggesting that groups of entangled polymer chains may be forming in the solutions which give rise to the optical behavior.
Thus, the size of the particles that form in solution upon imposition of flow determines whether shear-thickening will occur. For particles which remain sub-micron over the entire shear rate range, the solution exhibits shear-thinning. If the particles continue to grow, eventually becoming a few microns in size, the solution exhibits shear-thickening. On the other hand, the birefringence was found to be dominated by the chains or entanglements still present in the dissolved state. This behavior could be modeled on the basis of the elastic dumbell theory, suitably modified to include both macroform and microform optical anisotropies.
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