Cross-linking of polystyrene by Friedel-Crafts chemistry and the flame retardancy of polymer-clay nanocomposites
The thesis consists of two parts. In Part I, Cross-Linking of Polystyrene by Friedel-Crafts Chemistry, Polystyrene has been copolymerized with 4-vinylbenzyl alcohol, 4-(2-hydroxyethyl)styrene, and 4-(3-hydroxypropyl)styrene and it has been shown that thermal cross-linking of these copolymers is dependent upon the alcohol content. When the alcohol content is low, no thermal cross-linking is observed. When various phosphate esters are present as catalysts with these low alcohol content copolymers, cross-linking is observed at temperatures of about 250°C but not at lower temperatures. Cross-linking enhances the thermal stability of the copolymers. There is little difference in the thermal stability of all three copolymers and their blends with the phosphate additives. Finally, various difunctional compounds have been prepared to investigate the possibility of functionalized polystyrene formed in an extruder. In part II, The Flame Retardancy of Polymer-Clay Nanocomposites, various organic modifiers have been synthesized and their modified clays were obtained. The polymer-clay nanocomposites (PCN) were prepared from in-situ polymerization of monomer with organic modified clay. The structures of nanocomposites were measured by x-ray diffraction, XRD, and transmission electron microscopy, TEM, or atomic force microscopy, AFM. Both intercalated and exfoliated polymer nanocomposites have been obtained. The nanocomposites greatly improve thermal stability of polymer and show higher flame retardancy through Cone Calorimetry. Structural affinity between monomer and the organic group of modified clay is an important factor in preparing well-dispersed nanocomposites. For example, the benzyl-containing clay is better for polystyrene, but not good for poly (methyl methacrylate). In addition to the char barrier mechanism proposed by Gilman, paramagnetic effect also contributes to the thermal stability and flame retardancy of nanocomposites. The mechanical properties of nanocomposites are much better than those of pure polymer, such as strength, modulus, etc. The mechanism of clay exfoliation in nanocomposites has been proposed, and possible techniques to form exfoliated nanocomposites have been discussed.
"Cross-linking of polystyrene by Friedel-Crafts chemistry and the flame retardancy of polymer-clay nanocomposites"
(January 1, 2001).
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