Understanding the Mechanism of Simultaneously Oelophobic/Hydrophilic Perfluoropolyether (PFPE) Nano-Coatings

This dissertation is focused on understanding the mechanism of simultaneously oleophobic/hydrophilic perfluoropolyether (PFPE) nano-coatings. Four specific topics are presented: 1) investigate why can a nanometer-thick polymer coated surface be more wettable to water than to oil; 2) discuss the effect of end-groups on simultaneous oleophobicity/hydrophilicity and testify the anti-fogging performance of nanometer-thick PFPEs; 3) Fabricate nanometer-thick simultaneously oleophobic/hydrophilic coatings via a cost-effective photochemical approach; 4) study the role of the interfacial interaction in the slow relaxation of nanometer-thick polymer melts on a solid surface. \ud Specifically, chapter 2 and chapter 3 indicate that the simultaneously oleophobic/hydrophilic behavior is kinetic in nature and results from the combination of nanoscale and interfacial phenomena. The end-groups of the nanometer-thick PFPEs are critical to this peculiar wetting behavior: PFPE polymers with different end-groups can interact with the substrate in very different ways, resulting in different packing orders and thus different inter-chain distances within the polymer nanofilms. If the inter-chain distance is appropriately small, smaller water molecules penetrate the nanofilms quickly while larger oil molecules penetrate the nanofilms much more slowly. As a result, the surface shows a higher oil contact angle (OCA) than water contact angle (WCA). Moreover, the effect of simultaneous oleophobicity/hydrophilicity on the long-term anti-fogging capability has been studied by X-ray photoelectron spectroscopy (XPS) and anti-fogging tests. The results indicated that the unique simultaneous oleophobicity/hydrophilicity reduces the airborne hydrocarbon contamination and therefore improves the long-term anti-fogging performance. In chapter 4, an UV-based photochemical approach has been developed to fabricate nanometer-thick simultaneously oleophobic/hydrophilic coatings using cost-effective PFPEs without polar end-groups and the anti-fogging performance and mechanical robustness of the coating were studied. The photochemical approach established here potentially can be applied on many other polymers and greatly accelerates the development and application of simultaneously oleophobic/hydrophilic coatings. In chapter 5, the relaxation of nanometer-thick PFPE melts on a silicon wafer has been investigated by water contact angle measurement. This work reports that the relaxation of nanometer-thick polymer melts on a solid surface is much slower than in the bulk, which has been attributed to the low mobility of the anchored polymer chains and the motional cooperativity between anchored and free polymer chains in the nanometer-thick films