Colloidal Gels of Fumed Silica: Microstructure, Surface Interactions and Temperature Effects

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Title: Colloidal Gels of Fumed Silica: Microstructure, Surface Interactions and Temperature Effects
Author: Sanchez, Angelica Maria
Advisors: Wendy Krause, Committee Member
Peter S. Fedkiw, Committee Member
Jan Genzer, Committee Member
Saad A. Khan, Committee Chair
Orlin D. Velev, Committee Member
Abstract: The interactions of fumed oxides with organic solvents, polymers and biological systems are of great interest as they can be utilized as viscosity modifiers, fillers or adsorbents when mixed in or in contact with such materials. Fumed silica is of particular interest due to the large surface area that its branched structure provides for establishing interactions with specific matrices or chemical reactive groups. If these small particles are dispersed in an adequate medium, they can form suspensions, flocculated systems or three-dimensional networks; thus an understanding of how to control this microstructure is of paramount importance. Extensive research involving fumed silica dispersions has been conducted in areas such as inks, cosmetics, and paints. More recently, alternative novel applications such as fiber optic cables and composite polymer electrolytes, our major research effort in the past years, have gained especial importance. In this work, various types of fumed silica particles have been dispersed in different oligoethers and poly(ethylene oxide) PEO and their properties evaluated with the aim to finely tune them for improved performance during end use. Rheology, a reliable, easy, and readily available technique has been employed not only to characterize the systems, but also to study their microstructure and establish correlations that can be subsequently employed to tailor the material for the particular application. In particular, we examined dispersions of hydrophobic and hydrophilic fumed silica in oligoethers of different molecular weights and end group composition at different temperatures by using dynamic rheology. We observed that hydrophilic fumed silica particles form gels in the less polar oligoethers, whereas the hydrophobic ones form a network in all the oligoethers employed. Increasing the temperature increases irreversibly the gel modulus of the system containing hydrophilic fumed silica and the oligoether with the largest end group content, poly(ethylene glycol)dimethyl ether PEGdm(250). We also studied the effect of fumed silica particle concentration in PEGdm(250); a larger relative change in the gel modulus was observed for the materials containing lower concentration of fumed silica. A "concentration" effect due to polymer adsorption and chemical reaction on the particles' surface seems to explain this anomalous observation. We also study how the hydrophobic group length attached on the fumed silica particles affects the rheological properties, in particular the yield stress of the dispersions. Additionally, we take advantage of a material instability known as wall slip to explore how chemical composition of the shearing surface modifies the flow behavior of gels containing particles with different surface functionalities. Dynamic stress sweep experiments with hydrophobic and hydrophilic surfaces suggest that specific interactions between the nanoparticles contained in the gel and the plates' surface control the extent of wall slip. By combining dynamic mechanical rheology and flow visualization, it was possible to accurately determine the yield stress of the gels and differentiate the observed rheological behavior from slippage. Mixtures of fumed silica particles, hydrophobic and hydrophilic, were dispersed in PEGdm(250) and the effects of temperature in the rheological properties of the systems evaluated. The mixtures showed a negative deviation from the log-additive mixing rule within the temperature range studied. This indicates that the two types of particles form independent networks that provide less mechanical stability than each individual component in the system. In order to explore the effects of adding a low-molecular weight oligoether to PEO containing hydrophobic and hydrophilic fumed silica, blends of high- and lo- molecular weight (MW) PEOs were prepared by melt and solution mixing. In the composition range studied, the blends containing hydrophilic fumed silica showed to be more susceptible to the presence of the low-MW component. Blends containing larger amounts of the high-MW component behave liquid-like as the low-MW concentration in the blend increases. This behavior reverses at the 50⁄50 high- to low-MW composition, where the gel formation mechanism of the fumed silica in the low-MW component dominates and a gel-like behavior is observed. Both hydrophobic and hydrophilic fumed silica showed the same trend. Our results are encouraging and establish a new approach for designing methods that facilitate processing of these particulate materials and the control of their flow and ?at rest? properties while establishing the underlying mechanisms dictating such behavior.
Date: 2006-06-29
Degree: PhD
Discipline: Chemical Engineering

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