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Browsing by Author "Chen, Chiahong"

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    Studies of adsorption and stabilization of silica suspensions using well-defined polymeric dispersants
    Chen, Chiahong (Virginia Tech, 1994-06-06)
    Solutions of poly(2-ethyl-2-oxazoline) and poly(2-methyl-2-oxazoline) in water and several alcohols were characterized by light scattering and cloud point measurements. The second virial coefficients in water were found to decrease with increasing temperature, reflecting lower critical solution behavior, which is consistent with the cloud point measurements. The temperature dependence of the second virial coefficients revealed that specific interactions between polymer and water dominated the free energy of mixing. The Flory-Huggins x parameter determined from light scattering was in the range 0.48 - 0.49 in water and 0.32 - 0.41 in ethanol. The Kuhn length for PEOX was determined to be 0.77 nm which corresponds to less than two monomer units, indicating relatively flexible chains of PEOX. The segmental adsorption energy, xspo, of PEOX was measured using a desorption/displacement technique. PEOX was desorbed from silica with five low molecular weight organic displacers in two solvents - water and ethanol - to obtain values of the critical volume fraction of the displacer at which desorption was complete, Φcr. The high adsorption energy parameters are consistent with the polymer adsorbing principally by hydrogen bonding between the carbonyl groups on the polymer and surface silanol groups. The difference in adsorption energies in water and ethanol reflect specific solvent effects that may be related to the formation of hydrogen bond bridges between PEOX and silanol groups in water. Adsorption of PEOX from water, alcohols and chlorobenzene onto silica was investigated by measuring PEOX adsorption isotherms using a depletion method. A linear relationship of the plateau adsorption amount, Γp vs. log (molecular weight) was obtained, which agreed qualitatively with the Scheutjens-Fleer (S-F) mean field adsorption theory. The values of Γp, varied significantly with solvent type as well as with pH and electrolyte concentration in water. These variations in Γp, were due to changes of the polymer solvency and the silanol density on the silica particles. Competitive adsorption experiments of PEOX with various polymers were performed, including poly(ethylene oxide) (PEO), poly(propylene oxide) (PPO) poly(vinyl methyl ether) (PVME), and poly(dimethyl siloxane) (PDMS). PEOX showed a higher affinity to the silica surface than other polymers. This suggested that PEOX had good potential for serving as an anchor block for diblock copolymer stabilizers for metal oxides in water. The solubility of homopolymers PEOX, PEO, PPO, PVME, and PDMS and copolymers PEOX-PDMS and PEOX-PVME was investigated in water, alcohols, and chlorobenzene using static light scattering (SLS). The steric stabilization effect of silica dispersions in chlorobenzene by PEOX-PDMS was measured by dynamic light scattering (DLS). The stability was qualitatively related to the average particle hydrodynamic diameter against time. The adsorbed amount and layer thickness of diblock copolymer poly(dimethyl amino ethyl methacrylate-b-n-butyl methacrylate) (DMAEM-BMA) on silica surfaces from isopropanol was measured. The linear dependence of the adsorbed amount and thickness with respect to the tail block length was obtained. This is consistent with the Marques-Joanny model.