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FUNCTIONALIZING THREE-DIMENSIONAL SUPERHYDROPHOBIC MATERIALS

Abstract
Traditional bulk superhydrophobic materials are usually characterized by high porosity, low fracture toughness, and extremely low strength due to the low surface energy on the internal surface. Therefore, it is very challenging to achieve a bulk superhydrophobic material with high strength and mechanical durability. This dissertation presents an experimental study of strengthening a superhydrophobic siloxane using carbon-based nanomaterials, emphasizes the effects of microscale dispersion and macroscale distribution of reinforcing graphene on the uniformity and strength of the composite, studies the impact factors on the water adsorption for hydrophilic and hydrophobic materials, and applies chemical modification modify the surface of clay and sands. Carbon-based nanomaterials are ideal materials to reinforce the pure bulk superhydrophobic materials. The water contact angles of all new synthesized carbon-based reinforcements of superhydrophobic polymers are greater than 150°, and the surface energies are less than 1 mJ/m2. The mechanical durability can be indicated by the nanoindentation technique, unconfined compression, and wear testing. Young’s modulus can achieve up to 7 GPa and 5.7 MPa at the micro, and macro-scale, respectively. The mechanisms of reinforcements include carbon-based nanomaterials as nanomaterials fill the pores and bridge the fracture, physically being tangled with aluminosilicones after breaking C-C bonds with the help of the ultrasonic wave and chemical reactions with functional groups. To study and improve uniform dispersion and distribution of graphene nanoplatelets, three different processing methods were adopted: (1) ultrasonication to disperse graphene in the sol; (2) accelerated co-condensation at elevated temperature (i.e., 50℃ and 75℃) to prevent graphene from floating; (3) varying the soil’s viscosity to control graphene’s flotation. Adding only 0.9 wt.% graphene results in highly strengthened superhydrophobic composites with macroscopic compression strengths of up to ~33MPa. However, graphene tends to float upward in the sol, leading to its heterogeneous distribution within the cured samples. On the other hand, despite its functionality of improving microscale dispersion, ultrasonication also detrimentally decreases the composites’ strength due to acoustic cavitation-induced porosity. Similarly, although high-temperature curing accelerates the condensation of the precursor sol and hence results in a more uniform distribution of graphene, it also promotes thermal cavitation and bubble formation. To analyze the difference in isothermal adsorption between hydrophilic and hydrophobic materials, the alkali activation of metakaolin by using materials with different functional groups was synthesized. The biggest difference between the isothermal curves of hydrophilic and hydrophobic is the shape, and also there are several factors to influence the adsorption: surface morphology, surface wettability, the size of pore diameter, and specific surface area. In addition, inspired by the sol-gel method with different functional groups, modification clay and sands with other surfactants can also be hydrophobic materials, the largest water contact angle is 146.9 ± 0.4º. The surface roughness and size of pore diameter are vital to achieving a high-water contact angle. Due to their hydrophobic properties, the modified clay and sands are promising to be used in geotechnical and transportation engineering.
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