Hydrophilic-oleophobic stimuli -responsive materials and surfaces

John A Howarter, Purdue University

Abstract

Due to their high surface energy, hydrophilic surfaces are susceptible to contamination which is difficult to remove and often ruins the surface. Hydrophilic-oleophobic coatings have a diverse engineering potential including applications as self-cleaning surfaces, extended life anti-fog coatings, and environmental remediation in the selective filtration of oil-in-water mixtures. A successful design model for hydrophilic-oleophobic behavior has been developed using perfluorinated surfactants covalently bound to a surface. Within this design model, a variety of materials have been explored which the surfactants are covalently bound to a substrate; similarly, the surfactants may also be incorporated as a monomer into bulk copolymers. Surfactant based surfaces exhibited simultaneous hydrophilicity, necessary for anti-fogging, and oleophobicity, necessary for contamination resistance. The combination of these features rendered the surface as self-cleaning. Surfactant based brushes, composed of polyethylene glycol and perfluorinated constituents were grafted on to silica surfaces. The relationship between brush density and stimuli-responsiveness was determined by varying grafting conditions. The resultant surfaces were characterized with respect to chemical composition, brush thickness, and wetting behavior of water and hexadecane. Optimized surfaces exhibited stimuli-responsive behavior such that the surfaces will be wetted by water but not by oil. Surfactants were incorporated into random copolymers to create self-cleaning polymers which could be easily coated on to surfaces post-synthesis. Acrylic acid, methyl methacrylate, and hydroxyethyl methacrylate were used as comonomers; feed ratio was varied to establish compositional limits of stimuli-responsive behavior. Polymer composition dictated coating durability and self-cleaning performance as determined by water and hexadecane contact angle. The ability of select coatings to mitigate fogging was assessed in two extreme environments: transition from -20°C to humid laboratory environment; exposure to steaming water vapor. Silica membranes of varying pore size were modified with stimuli-responsive surfactants. Membranes showed the ability to selectively pass water and restrict passage of oil, demonstrating a reusable method of separating oil-in-water emulsions. Selectivity of oil-in-water emulsions and permeate flow rate of each individual fluid were quantified for the surfactant modified membranes. Permeate flow rate was characterized with respect to individual droplets and bulk fluid.

Degree

Ph.D.

Advisors

Youngblood, Purdue University.

Subject Area

Polymer chemistry|Materials science

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