USANS as a Probe of Large-Scale Structure in Attractive Colloidal Glasses of Block Copolymer Micelles
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USANS as a Probe of Large-Scale Structure in Attractive Colloidal Glasses of Block Copolymer Micelles Mark A. Crichton, Neil S. Forbes, Surita R. Bhatia Department of Chemical Engineering University of Massachusetts Amherst 686 North Pleasant Street, Amherst, MA 01003 U.S.A. ABSTRACT We have used ultra-small-angle scattering (USANS) and fluorescence microscopy to show the existence of large-scale structure in attractive colloidal glasses composed of block polyelectrolyte micelles. Our systems display evidence of surface scattering, with the scattered intensity I at low scattering vector q scaling as I ~ qx with x in the range –3 to –4. We believe this is due to surface scattering from large, highly polydisperse aggregates with rough interfaces. USANS may provide an ideal way to distinguish fractal colloidal gels and colloidal glasses.
INTRODUCTION Arrested or jammed states of soft matter, such as colloidal glasses and fractal colloidal gels, are interesting from both a fundamental point of view and in applications such as emulsions, foams, cosmetics, and foods. Colloidal gels and glasses can exhibit similar rheological properties and dynamics, making them difficult to distinguish experimentally [1]. However, there are key structural differences. In colloidal gels, attractive forces dominate, leading to an "open," percolated structure characterized by a fractal dimension. Colloidal glasses can form from either repulsive or attractive particles; however, repulsive forces are dominant and lead to slowing of the dynamics. The microstructure is much denser and more "crowded" than in a fractal colloidal gel, and the expectation is that no large-scale structure should be present in such systems. This behavior has been verified experimentally for colloidal glasses of repulsive particles, also referred to as "repulsive glasses." However, there is little data on the large-scale structure of glasses formed from attractive particles, referred to as "attractive glasses." Recently, we performed ultra-small-angle neutron scattering (USANS) on aqueous solutions of poly(styrene)-poly(acrylic acid/ethyl acrylate) (PS-PAA/EA), which forms spherical micelles in water (Figure 1). Because of the hydrophobic EA groups, the micelles have an effective attraction that leads to formation of a viscoelastic solid or "gel"-like state at polymer concentrations above ~2 wt% [2,3]. We have shown previously that this viscoelastic solid state can be considered an attractive glass [2] where the strength of attraction is inversely related to the degree of hydrolysis of the PAA/EA block, f. In this study, we combine results from smallangle neutron scattering (SANS), USANS, and fluorescence microscopy to determine how the morphology of the jammed state changes with the strength of attraction.
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hydrophilic polyacrylic acid block glassy polystyrene core ethyl acrylate “sticker”
Figure 1. PS-PAA/EA micelles associated via ethyl acrylate (EA) stickers.
EXPERIMENTAL DETAILS The diblock polymers was supplied by Rhodia Inc. as polystyrene-
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