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Synthesis, Characterization and Surface Modification of Polydimethylsiloxane and Its Composites

Silicone elastomers are widely employed in composite, coatings, biomedical, sealant and electronics applications. This dissertation highlights new methodologies and formulation approaches to improve the performance of siloxane polymers, including the mechanical properties, surface properties, optical properties and transport properties. It also provides an in-depth understanding about dip-coating deposition on chemically patterned surfaces. The first chapter presents the process of transforming sand into siloxane polymers of excellent mechanical strength. History, properties and applications of siloxane polymers are systematically discussed. Another focus of this chapter is wetting, including the concepts of contact angles and capillary bridge ruptures. Five interconnected projects are discussed following the introduction. Base-catalyzed siloxane equilibration is fully utilized in the first and second projects. First, extremely stretchable silicone elastomers are prepared by copolymerizing silica particles and octamethylcyclotetrasiloxane with siloxane equilibration. It is demonstrated that Stöber silica particles can actively participate in the chemical cross-linking of linear PDMS, serving as SiO4 linking group reservoirs. Second, permanently hydrophilic silicones are obtained by grafting PEO-PDMS copolymers to cross-linked silicones via siloxane equilibration. High power light-emitting diodes require a transparent encapsulant of high refractive index and low water permeability. The third project concerns the uniform incorporation of high refractive index inorganic fillers into siloxane polymers. Silicone composites with tailored refractive indices, different molecular structures, tunable mechanical properties and excellent transparency are achieved via sol-gel chemistry. In the fourth project, the high water transport rate in pure silicone is clarified, for both linear and cross-linked silicones. This high water transport rate results from the special structural features of silicone. Incorporation of water-resistant coating layers, silanized fillers and polyimide blocks are shown to be several effective approaches to decrease the water permeability of silicones. The fifth project deals with dip-coating deposition on chemically patterned surfaces. The mechanisms of capillary bridge deformation and rupture for dip-coating deposition on chemically and topographically patterned surfaces are compared. Ordered inorganic and organic individual crystal arrays are successfully patterned on smooth and chemically patterned surfaces prepared by photolithography.
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