Browsing by Author "Saad A. Khan, Committee Chair"

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Colloidal Gels of Fumed Silica: Microstructure, Surface Interactions and Temperature Effects
(2006-06-29) Sanchez, Angelica Maria; Wendy Krause, Committee Member; Peter S. Fedkiw, Committee Member; Jan Genzer, Committee Member; Saad A. Khan, Committee Chair; Orlin D. Velev, Committee Member
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.
Rheology and Microstructure of Cellulose Acetate in Mixed Solvent Systems
(2005-04-10) Appaw, Collins; Orlin D. Velev, Committee Member; Richard D. Gilbert, Committee Member; Saad A. Khan, Committee Chair; John F. Kadla, Committee Co-Chair
Cellulose is a natural abundant polymer used in a variety of applications. Its use however, is hampered by its poor solubility in various solvents. This is primarily due to the hydrogen bonds between the hydroxyl groups on the anhydoglucose chain. In view of that, various cellulose derivatives have been synthesized to aid dissolution and this impacts a variety of solubility characteristics. Cellulose acetates (CA) are cellulose esters that are partially substituted at the C-2, 3 and 6-positions of the anhydroglucopyranose residue. Their solubility in various solvents depends on the degree of substitution (DS) of the acetyl groups. For instance, CA is soluble in water at low DS of between 0.5-1. But it is insoluble in aqueous solutions at higher degree of substitution (DS > 1). CA is employed in various applications such as textile manufacture, tool handles, specialty papers, cigarette filters and is a polymer of choice in majority of reverse osmosis membrane preparation. These applications often exploit semi- to concentrated cellulose acetate solutions in appropriate solvents. Such systems can be induced to form aggregated structures such as gels which can be initiated by the addition of a non-solvent. Thus, depending on the solvent and non-solvent adopted, the cellulose acetate mixed solvent system can be tailored to exhibit sol-gel characteristics utilizing the inherent intra- and intermolecular interactions present in solution. However, such systems and the interactions influencing their behavior is not very well understood. In this regard, the main objectives of our study are as follows — to develop a ternary mixed solvent system comprising of cellulose acetate, N,N dimethylacetamide and water and manipulate the system to form aggregated structures leading to phase separated gel network. The tools employed in this project to investigate and characterized the macroscopic properties as well as the microstructural changes are rheology, scanning electron microscopy (SEM) and laser scanning confocal microscopy (LSCM). The first part of this study involves addition of water- N,N dimethylacetamide solutions in different ratios to bulk 20% cellulose acetate in N,N dimethylacetamide solutions with emphasis on increasing water content in the system. Using rheology as the main analytical tool, the steady state viscosity was found to increase with water content increase. Above water concentrations of 19%, there is a solution to gel transition, which also showed enhancement in dynamic viscoelastic properties with water content increment. The SEM micrographs showed similar patterns with gels having lower water content exhibiting larger voids in comparison to gels with higher water content at same cellulose acetate concentration. Using LSCM, we obtained microstructural formation with more open networks at lower water content, whiles a more compact homogenous structure was exhibited for higher water content gel samples. In the second part of this study, addition of cellulose acetate to different ratios of N,N dimethylacetamide/water solutions are investigated. At low water content, the system showed steady state viscosity increase with water content as was observed in the first part of this thesis. Typically beyond 19% water concentration, the systems phase separates into two layers consisting of a clear solution on top of a viscous bottom layer. This is in contrast to the first part of the study where we observed a uniform rigid material. Heating the two-phase system to 100°C and cooling back to room temperature led to the formation of a one-phase physical gel matrix. With increasing water content, the elastic and viscous moduli of the gels increased at constant cellulose concentration. Finally, we investigate the gel properties with emphasis on yield stress when mechanical stress is applied to the gels. In addition, the gel-sol transition for the gels are investigated by subjecting them to temperature variations.
Solution Rheology and Microstructure of Associative Polymers
(2003-07-28) Abdala, Ahmed AbdelHay Ahmed; Alan E. Tonelli, Committee Co-Chair; John van Zanten, Committee Member; Richard J. Spontak, Committee Member; Sam S. Hudson, Committee Member; Saad A. Khan, Committee Chair
Water-soluble associative polymers are widely used in a variety of applications because of their ability to modulate rheology and material microstructures. This study focuses on understanding the structure-property relationship for hydrophobically modified alkali soluble emulsion (HASE) polymers with emphasis on their microstructure and rheological properties. These polymers have a complex comb-like structure that is a polyelectrolyte backbone, a copolymer of acrylic or methacrylic acid and alkyl acrylate, with a few hydrophobic macromonomers randomly grafted to this backbone. The hydrophobic macromonomer consists of hydrophobic groups that are separated from the polymer chains by polyethylene oxide (PEO) spacers. Upon neutralization, the polymer backbone adopts a more extended conformation allowing the hydrophobic groups to associate forming a transient network structure that enhances the solution rheological properties. In the first part of this study, we investigate the effect of the polymer composition on their microstructures and rheological properties. In particular, the effects of the concentrations of methacrylic acid (MAA) and macromonomers on the solution rheology are examined. We find that polymers with low MAA content have smaller hydrodynamic size and weaker network structures compared to larger hydrodynamic size and stronger network structure for polymers with high MAA content. However, due to chain increased stiffness at higher MAA and the lower contribution from the aggregation of ethyl-acrylate groups, a broad maximum in the viscoelastic properties of the polymer solution is observed at about 40 mole% MAA. Moreover, the material functions of polymers with different MAA content show different concentration dependences. In the second part of this study, co-solvents of water and propylene glycol (PG) in different proportions are used to investigate the effect of the solvent quality on the solution rheology of these polymers. The steady and dynamic properties show the presence of two regimes with respect to the solvent composition. In "water-rich" solvents, the hydrophobic association dominates the solution rheology. In contrast, in "PG-rich" solvents, the hydrophobic association is suppressed due to the lower tendency of the hydrophobes to aggregate, the smaller coil size of the polymer chains and changes in the PEO spacer conformations. These two different types of behavior are discussed and confirmed by the different concentration dependences in each regime. In the third part of the study, the ability of using diffusing wave spectroscopy (DWS) to probe the dynamics of HASE polymers is examined. We find that DWS accurately probes the structural changes induced by the change in the solvent quality or the polymer concentration. Moreover, comparison with conventional mechanical rheometry data reveals excellent qualitative agreement between the data obtained from the two techniques. Quantitatively, however, there is a discrepancy between the data obtained from each technique. Several reasons for the discrepancy are discussed, including the possibility that the dynamics at the micro-level could be different from the bulk properties. The scaling of the creep compliance, high–frequency elastic modulus and relaxation time with polymer concentration show power-law dependences. The power-law exponents are discussed in light of theoretical predictions and available experimental data. An approach to modulate the hydrophobic association is presented in the last part of the study. The first step in this approach involves the addition of inclusion compound forming hosts (α- or β- cyclodextrin) to the polymer solution. The encapsulation of the hydrophobic groups leads to significant reduction in the solution viscosity and viscoelastic properties The second step requires the addition of surfactants to reactivate the hydrophobic groups and thus recover the solution rheological properties. We are able to recover the solution properties using different nonionic surfactants.