The Influence of Nonadsorbed Polymer on the Behavior of Weakly Flocculated Suspensions
Ogden, Andrea Lynn
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https://hdl.handle.net/2142/82869
Description
Title
The Influence of Nonadsorbed Polymer on the Behavior of Weakly Flocculated Suspensions
Author(s)
Ogden, Andrea Lynn
Issue Date
1997
Doctoral Committee Chair(s)
Lewis, Jennifer A.
Department of Study
Materials Science and Engineering
Discipline
Materials Science and Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Materials Science
Language
eng
Abstract
Nonadsorbed polymer effects on the stability of alumina (Al$\sb2$O$\sb3)$-poly(methyl methacrylate) (PMMA)-toluene suspensions were studied by rheological, optical microscopy, and sedimentation measurements. Suspensions contained an appropriate amount of PMMA ($M\sb{avg}$ = 70,000 g/mole) necessary to saturate the Al$\sb2$O$\sb3$ particles (4.8 mg PMMA/m$\sp2$ Al$\sb2$O$\sb3$), as determined by adsorption isotherm studies. The adlayer thickness ($\delta$) was assumed to be equivalent to the hydrodynamic radius ($\approx$11.6 nm) of the PMMA coils. In the absence of excess PMMA, suspensions exhibited strong shear thinning and low shear relative viscosities ($\eta\sb{\rm rel,0}$) in excess of 10$\sp4$, indicative of their weakly flocculated nature. Correspondingly, total interparticle potential calculations obtained by summing van der Waals and steric interactions yielded an attractive minimum of a few $k\sb{b}T$, at interparticle separation distances $h \approx$ 2$\delta$. Upon the addition of nonadsorbed PMMA species, $\eta\sb{\rm rel,0}$ and the degree of shear thinning decreased over a broad range of free PMMA volume fractions (0.0005 $\le\ \phi\sbsp{\rm PMMA}{\rm free} \le$ 0.1). Using the model developed by Mao, Cates, and Lekkerkerker (MCL), a depletion potential was estimated as a function of $\phi\sbsp{\rm PMMA}{\rm free}$. These calculations yielded a repulsive barrier on the order of $k\sb{b}T$ (or higher) at $h >$ 2$\delta$, suggesting that improved stability arises from kinetic (energy barrier) considerations. Aging studies were carried out on suspensions with high $\phi\sbsp{\rm PMMA}{\rm free}$ which appeared to be fully restabilized, as indicated by an initial Newtonian flow response. The minimum relative viscosity and characteristic floc size increased as a function of time, further supporting the kinetic origin of their stability. From the rate of floc growth, a repulsive barrier height was estimated using a model developed for charge-stabilized systems which demonstrated good agreement with the predictions of the MCL model. Substantial shear thickening behavior was also displayed by these weakly flocculated suspensions. Such behavior was considered unusual given that shear thickening has been associated with stable, monodisperse systems. Hard sphere scaling analyses were invoked which revealed that the critical Peclet number exhibits universal behavior as a function of effective solids loading independent of $\phi\sbsp{\rm PMMA}{\rm free}$. The occurrence of such behavior in the Al$\sb2$O$\sb3$-PMMA-toluene system lends support to an emerging view that shear thickening can stem from not only order-disorder phenomena, but from particle cluster formation in disordered systems.
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