Study and Application of Underwater Self-cleaning Behavior of Zwitterionic Polymer-based Superhydrophilic Surface

Author:He Ke

Supervisor:yang wen sheng


Degree Year:2014





Design and synthesis of self-cleaning materials have attracted world wide attention recently,which arises from not only the fundamental research interest in material science but also the practical applications of self-cleaning materials and technology in industry,agriculture,military,energy,architecture and etc.Inspired by the nature,self-cleaning materials with superhydrophobicity were first developed.However,it was found later that superhydrophobic surface could still easily be wetted by organic liquids such as alcohol and oil,which possess low surface energy.Thus the fabrication of superoleophobic or superamphiphobic materials is of great importance for both scientific research and the commercialization.Because no substance in nature is oleophobic,the construction of a superoleophobic surface requires well designed micro-nano structures with specific topography.Although some methods have been reported to realize superoleophobicity,however,most of the reported protocols are complicated and not facile to reproduce.Further more,though these superoleophobic/superamphiphobic materials resist bulk oil contamination,they are vulnerable to organic contaminants with small scale such as oil vapor due to the constructing theory.Recently,hydrophilic or superhydrophophilic surfaces based underwater superoleophobic surfaces have attracted extensive attention.The underwater superoleophobicity can be achieved by either constructing surface structures on hydrophilic surface or surface modification of hydrophilic or superhydrophilic polymers.Zwitterionic polymers have been well studied for their excellent performance in antifouling,including resistance to both protein adsorption and bacteria adhesion.The zwitterionic polymer grafted surface presents superhydrophilicity and underwater superoleophobicity due to the strong interactions between the zwitterionic groups and water molecules,which is a promising underwater self-cleaning material and may solve the problems that conventional superoleophobic materials cannot overcome.In the thesis,we focused on the study of the underlying physical chemistry process and the mechanism for the underwater self-cleaning behavior of the zwitterionic polymer grafted hydrophilic surface and explored the applications of such self-cleaning properties in resistance of oil vapor condensation and in oil-water separation.The thesis consists of four parts as following:1.Zwitterionic Poly(2-methacryloyloxyethyl phosphorylcholine)(PMPC)brush was grafting on to gold substrates using surface-initiated atom transfer radical polymerization(SI-ATRP)method.The prepared surface was characterized by X-ray photoelectron spectroscopy(XPS)and ellipsometry.The polymerization was well controlled in the presence of both free initiator and Cu2+ ions suggested by the linear growth of polymer brush with polymerization time.The wettability was examined by conventional static contact angle measurement for water and oil in air and underwater static contact angle for oil.The PMPC-grafted gold substrates presented superhydrophilicity and underwater superoleophobicity.Though the surface was superoleophilic and could be wetted by oil in air,dewetting of the oil occurred on the surface underwater and the oil droplet detached from the surface spontaneously.The in air and underwater contact angles of other hydrophilic/superhydrophilic surfaces were measured to analyze their wettability.Fouling by oil in air and dipping underwater experiments were also conducted to these surfaces.We proposed for the first time that presenting underwater superoleophobic(CAoil>150°)is only the basic stage of self-cleaning,i.e."no adhesion of oil underwater".The advanced stage requires the oil wetted surface to enable a complete oil dewetting underwater to realize the underwater self-cleaning of the surface polluted by oil in air,i.e "wash the oil away underwater".To evaluate the self-cleaning effectiveness,dyed oil was used to pollute the PMPC-grafted surface and no fluorescence could be detected by naked eyes after water rinsing.Furthermore,the surface polluted by fluorinated oil and rinsed by water was examined by XPS spectroscopy and found no residue of the oil left on the surface.2.Superoleophobic surface with high static contact angle and low rolling off angle for oil was fabricated according to the literature.By applying this surface into oil vapor,we found that such surface-structure based superoleophobic surface could not resist oil vapor and could be contaminated by oil condensation.The self-cleaning behaviors of PMPC-grafted surfaces prepared by "grafting from" and "grafting to"methods were compared.Both of these two sample presented in air superhydrophilicity(CAwater<5°)and underwater superoleophobicity(CAoil>150°)with no oil adhesion.By examining the wettability and underwater oil dewetting,it was shown that there was no obvious difference between "grafting to" and "grafting from" samples in the self-cleaning for bulk oil contamination,whereas the "grafting to" sample could no longer be self-cleaned after oil condensation.Surfaces functionalized by various hydrophilic/hydrophobic groups were also studied for oil condensation.Only PMPC-grafted surface using "grafting from" method showed unchanged wettability after water rinsing,demonstrating its excellent underwater self-cleaning effect against oil vapor condensation.3.Zwitterionic PMPC was grafted on to silica nanoparticles via SI-ATRP method.The silica nanoparticles with a diameter of 240 nm were synthesized using typical Stober method.The immobilization of initiator was conducted via aminopropylsilanation followed by reaction with 2-bromo-propionyl bromide.XPS spectra confirmed the immobilization of initiator and grafting of PMPC.Dynamic light scatter(DLS)and zeta potential measurements were performed to monitor the colloid properties of the particles dispersed in water.The PMPC-grafted silica nanoparticles showed great colloid stability even in the presence of high concentration of salt.Whereas,aggregation was found for bare particles treated with the same concentration of salt.In addition,the PMPC-grafted particles also showed antifouling property against high concentration of BS A.Different halide anions were introduced to affect the particles and we found that the zeta potential was increased by halide ions following the sequence Cl-<Br<I-,suggesting the difference in adsorption of different halide anions to the zwitterionic groups of PMPC chains,which may help to understand the interactions between different halide anions and the lipid molecules.4.Zwitterionic PMPC was grafted on to stainless steel mesh by SI-ATRP method.The initiator was grafted onto the stainless steel surface via aminopropylsilanation followed by reaction with 2-bromo-2-methylpropionic acid.XPS spectroscopy was used to monitor the immobilization of the initiator and to confirm the success of grafting PMPC.The grafting was conducted to meshes with two different sizes(25μm and 100 μm)and underwater self-cleaning behaviors were found for both two different meshes.As comparison,PDDA/PSS multilayer was deposited on to the stainless steel mesh by layer-by-layer method.And ZnO nanorod was deposited on to the stainless steel mesh by hydro-themral method.Both PDDS/PSS-mesh and ZnO-mesh presented underwater superoleophobicity and were able to accomplish oil-water separation if they were wetted by water prior to using.But they lost their oil-water separation ability permanently when they contacted oil in dry state.However,even PMPC-grafted mesh was polluted by oil first,simply rinsing by water could help it restore the separation ability.