Preparations, characterization, and flow behavior of dispersions of monodisperse microspheroids
Abstract
An original scheme was developed for producing polymer microspheroids of precise size, shape, and monodispersity by the following method: (i) polymer microspheres were embedded in an elastic matrix; (ii) the composite material was deformed under uniaxial tension to produce microspheroids; and (iii) the microspheroids were recovered by chemically degrading the matrix. Graft-copolymer-stabilized poly(methyl methacrylate) (PMMA) prolate spheroids with sphere-equivalent diameters of 0.50 to 1.40 $\mu$m and axial ratios of up to 8 were prepared. Deviations in the dimensions and axial ratios were typically less than 5%. These novel dispersions of model particles are ideal systems for elucidating the effects of shape and orientation on the behavior of colloidal dispersions. PMMA microspheres with average diameters of 0.1 to 1.4 $\mu$m were prepared via dispersion polymerization. The particles were incorporated into silicone elastomers of poly(dimethylsiloxane) (PDMS). These composite elastic materials contained 1 to 10wt% PMMA particles and were capable of elongations of up to 500%. Deformation produced monodisperse spheroids with a good prolate shape even when the particles were uncrosslinked. The stability of particles in mixtures of alkanes and PDMS was examined. Stable dispersions of spheres and spheroids were prepared in various suspension media. Shape effects on the rheological behavior of dispersions of sterically stabilized microparticles were examined. The shear viscosity of the dispersions decreased with increasing shear rate. Intrinsic viscosities ranged from 3 to 5 and increased with decreasing particle size. The particle shape had only a minimal effect on the dispersion rheology for volume fractions of up to 0.1 and then only for the largest particle axial ratio. These unexpected results were attributed to the softness of the steric layer and interparticle colloidal interactions. Because measured viscosities differed from theoretical predictions by Scheraga (1955), it was inferred that even for dilute dispersions hydrodynamic interactions were important.
Degree
Ph.D.
Advisors
Franses, Purdue University.
Subject Area
Chemical engineering
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