Preparation Of Stimuli-Responsive Nanocontainers And Their Application On Anticorrosion Coatings

Corrosion of metal destroys materials causing a great economic loss. Industrial coatings rely mainly on passive materials for protection of underlying metal, but passive coatings have a limited lifetime. Therefore, introduction of an active process like the self-healing mechanism is urgently needed. In recent years, intelligent anticorrosion coatings have attracted more and more interest. From the current research results, the intelligent anticorrosion coating is one of the most promising alternative for chromate conversion coating. Like the living things, the intelligent anticorrosion coating not only can self-diagnosis but self-repair. In this paper, we have designed and fabricated pH- and light-responsive nanocontainers doped anticorrosion coatings as intelligent anticorrosion coatings, and investigated their ability to respond environmental stimuli arising from corrosion process and working mechanism for self-healing process of corrosion sites. The organic combination of stimuli-responsive nanocontainers and anticorrosion coatings compensated the passive protection of the coatings, and were expected to obtain self-healing functionality and extend the lifetime of underlying alloy. The main contributions of this dissertation are described as follows:1. Base-responsive hollow mesoporous silica nanoparticles for controlled release of corrosion inhibitorIn the present study, a new encapsulation technique for corrosion inhibitor is proposed. The hollow mesoporous silica spheres (HMSs) were synthesized by the co-templates method as nanocontainers for corrosion inhibitor, benzotriazole (BTA) and the supramolecular nanovalves, consisting of curcurbit[6]uril (CB[6]) rings and the functional stalks attached to the surface of HMSs achieved on-demand release. The synthesis process of HMSs and the assembly process of the nanovalves were confirmed by SEM, TEM, N2 adsorption/desorption, FTIR, TGA and solid-state 13C CP/MAS NMR. The results showed that the HMSs assembled with the nanovalves possessed a higher encapsulation capacity for BTA than MCM-41 assembled under the same procedure due to its huge hollow internal structure. The pH-controlled release properties of BTA from the assembled HMSs under different pH environments were monitored by ultraviolet absorption spectra. The release profiles showed that there was only 1.5% of BTA from the assembled HMSs in neutral solution; when pH=8.0,9.0, and 10.0, the cumulative release percentage of BTA from HMSs are 33%,40% and 57%, respectively; when pH raises to 10.5, the cumulative release percentage of BTA is as high as 90%. Such a property makes the HMSs assembled with the pH-responsive nanovalves have great potential applications in smart anticorrosion coatings.2. Preparation of acd/base-responsive supramolecular nanocontainers and their application on intelligent anticorrosion coatingAlkaline-responsive HMSNs based on cucurbit [6]uril (CB[6])/bisammonium supramolecular complex and acid-responsive HMSNs based on αcyclodextrin (α-CD)/aniline supramolecular complex, which operate in water, have been achieved. The two elaborately designed nanocontainers show the pH-controlled encapsulation/release behaviors for benzotriazole molecules. Equal amounts of the alkaline-and acid-responsive nanocontainers were uniformly distributed in the hybrid zirconia-silica sol-gel coating and thus formed the intelligent anticorrosion coating. The self-healing property of AA2024 alloy coated with the intelligent anticorrosion coating is evaluated by electrochemical impedance spectroscopy (EIS). During 96 h immersion in 0.5 M NaCl solution, the sol-gel coating doped with the acid/base-responsive nanocontainers with a thickness of 1.1μm exhibited excellent stability superior to the undoped sol-gel coating, and the coating resistance remained above 2.5×106 Ω·cm2, which is attributed to the release of corrosion inhibitor from the acid/base-responsive nanocontainers and the inhibition of corrosion diffusionafter feeling the changes of environmental pH valves near the corroded areas.3. Controlled release of cargo molecules from hollow mesoporous silica nanoparticles based on acid and base dual-responsive cucurbit[7]uril pseudorotaxanesThe acid and base dual-responsive cucurbit[7]uril pseudorotaxanes have been designed and immobilized on the surface of hollow mesoporous silica nanoparticles as the supramolecular nanovalves to control the release of cargo molecules in response to extensive pH changes. The as-prepared supramolecular nanovalves, which are made up of the CB[7] macrocycles and the stalks containing the hexylammonium units (-NH2+(CH2)6-) and the FcCOOH units, linked by amide bonds were firstly synthesized. It is noteworthy that the release rate is dependent on the pH. When pH=2,3,4,5, the release percentages are 90%,67%,43% and 17% in 160 min, respectively. When the pH was raised to 10, the time required for releasing 90% gemcitabine is only 75 min.4. Acid and alkaline dual stimuli-responsive mechanized hollow mesoporous silica nanoparticles as smart nanocontainers for intelligent anticorrosion coatingsThe present paper introduces an intelligent anticorrosion coating, based on the mechanized hollow mesoporous silica nanoparticles (HMSs) as smart nanocontainers implanted into the self-assembled nanophase particles (SNAP) coating. As the key component, smart nanocontainers assembled by installing supramolecular nanovalves in the form of the bistable pseudorotaxanes on the external surface of HMSs realize pH-responsive controlled release for corrosion inhibitor, caffeine molecules. The smart nanocontainers encapsulate caffeine at neutral pH, and release the molecules either under acidic or alkaline conditions, which make them spontaneously experience the pH excursions arisen from corrosion process and respond quickly. The loading capacity of smart nanocontainers for caffeine is about 20.2%. The intelligent anticorrosion coating was deposited on the surface of aluminum alloy AA2024 and investigated by electrochemical impedance spectroscopy and scanning vibration electrode technique (SVET). Compared with the pure SNAP coating, the surface of smart anticorrosion coating presents no remarkable cathodic and anodic corrosion activity after 16 h immersion in 0.1 M NaCl solution. From the experimental results of SVET, the smart nanocontainers with the acid and alkaline dual stimuli-responsive characteristics can simultaneously suppress corrosion activities on microanodic and microcathodic regions, demonstrating an excellent self-healing functionality.5. Azobenzene-modified hollow mesoporous silica nanocontainers for smart anticorrosion coatingsWe develop a new smart light responsive coating by incorporating azobenzene-modified hollow nanocontainers into a water-based alkyd coating. The functionalized nanocontainers were obtained by immobilizing photoresponsive azobenzene molecular switches in the mesopore inner walls of hollow mesoporous silica nanoparticles. This nanoplatform permits encapsulating of active molecules within the hollow cavity under visible light but releases them exposure to UV irradiation. After immersion in 0.1 M NaCl solution, the current density of the scratch for the coating with light-responsive nanocontainers remains constant at the noise level. In contrast, a significant anodic peak of 13.4 μA cm-2 can be observed on the surface of pure alkyd resin coating. The results of scanning vibrating electrode technique demonstrate that the incorporation of azobenzene-modified hollow mesoporous silica nanocontainers endows the organic hybrid coatings with an excellent continuous self-healing performance.