Corrosion-Responsive Self-Healing Coatings

Organic coatings are one of the most popular and powerful strategies for protecting metals against corrosion. They can be applied in different ways, such as by dipping, spraying, electrophoresis, casting, painting, or flow coating. They offer great flexibility of material designs and cost effectiveness. Moreover, self-healing has evolved as a new research topic for protective organic coatings in the last two decades. Responsive materials play a crucial role in this new research field. However, for targeting the development of high-performance self-healing coatings for corrosion protection, it is not sufficient just to focus on smart responsive materials and suitable active agents for self-healing. A better understanding of how coatings can react on different stimuli induced by corrosion, how these stimuli can spread in the coating, and how the released agents can reach the corroding defect is also of high importance. Such knowledge would allow the design of coatings that are optimized for specific applications. Herein, the requirements and possibilities from the corrosion and synthesis perspectives for designing materials for preparing self-healing coatings for corrosion protection are discussed.

Degradable polyprodrugs: design and therapeutic efficiency

Prodrugs are developed to increase the therapeutic properties of drugs and reduce their side effects. Polyprodrugs emerged as highly efficient prodrugs produced by the polymerization of one or several drug monomers. Polyprodrugs can be gradually degraded to release therapeutic agents. The complete degradation of polyprodrugs is an important factor to guarantee the successful disposal of the drug delivery system from the body. The degradation of polyprodrugs and release rate of the drugs can be controlled by the type of covalent bonds linking the monomer drug units in the polymer structure. Therefore, various types of polyprodrugs have been developed based on polyesters, polyanhydrides, polycarbonates, polyurethanes, polyamides, polyketals, polymetallodrugs, polyphosphazenes, and polyimines. Furthermore, the presence of stimuli-responsive groups, such as redox-responsive linkages (disulfide, boronate ester, metal-complex, and oxalate), pH-responsive linkages (ester, imine, hydrazone, acetal, orthoester, P-O and P-N), light-responsive (metal-complex, o-nitrophenyl groups) and enzyme-responsive linkages (ester, peptides) allow for a selective degradation of the polymer backbone in targeted tumors. We envision that new strategies providing a more efficient synergistic therapy will be developed by combining polyprodrugs with gene delivery segments and targeting moieties.

Fighting corrosion with stimuli-responsive polymer conjugates

Corrosion is a financial and enviromental plague which leads to the deterioration of our infrastructures. Using corrosion inhibitors at low concentrations in coatings is one effective method for preventing corrosion. Inspired by the development of polymer-drug conjugates, corrosion inhibitors are incorporated in various polymer structures to create novel materials for hindering corrosion. We discuss the strategies to covalently integrate corrosion inhibitors in polymer structures to form polymer-inhibitor conjugates. Inhibitors are conjugated to polymers via non-labile or stimuli-labile linkages to allow the release of the inhibitors upon onset of corrosion. The application and anticorrosion performance of representative polymers are also discussed.

Nanocontainer-Based Active Systems: From Self-Healing Coatings to Thermal Energy Storage

We highlight the development of nanocontainer-based active materials started in 2006 at the Max Planck Institute of Colloids and Interfaces under the supervision of Prof. Helmuth Möhwald. The active materials encapsulated in the nanocontainers with controlled shell permeability have been first applied for self-healing coatings with controlled release of the corrosion inhibitor. The nanocontainers have been added to the paint formulation matrix at 5-10 wt % concentration, which resulted in attaining a coating-autonomous self-healing ability. This research idea has attracted the attention of many scientists around the world (>1500 publications during the last 10 years) and has already been transferred to the commercialization level. The current trend in nanocontainer-based active systems is devoted to the multifunctionality of the capsules which can combine self-healing, antibacterial, thermal, and other functionalities into one host matrix. This article summarizes the previous research done in the area of nanocontainer-based active materials together with future perspectives of capsule-based materials with antifouling or thermoregulating activity.

Nanocontainer-based self-healing coatings: current progress and future perspectives

Nanocontainers add more functionalities to the standard coating formulations.

pH-Sensitive Polymer Conjugates for Anticorrosion and Corrosion Sensing

In 2015, the global cost of corrosion in the world was estimated to be around 2.5 trillion dollars and has been continuously increasing. The active protection by corrosion inhibitors is a well-known technique for protecting metals against corrosion. However, one major disadvantage is that corrosion inhibitors can be leached in the environment, even when corrosion does not occur. We design and synthesize smart polymer/corrosion inhibitor conjugates as a new generation of materials for corrosion protection. These materials release inhibitors upon acidification, which may occur either by acidic rain or as a consequence of the metal corrosion process itself. A polymerizable derivative of 8-hydroxyquinoline (8HQ), an effective corrosion inhibitor, is prepared so that it contains acid-labile β-thiopropionate linkages. The monomer is copolymerized with ethyl acrylate, and the obtained functional polymer is processed to form nanoparticles. Under acidic conditions, >95% 8HQ is released from the nanoparticles of the polymer conjugates after 14 days. However, the release was significantly slower under neutral conditions, reaching only 15% during the same period. Additionally, nonconjugated 8HQ can be physically entrapped in the nanoparticles of the polymer conjugates by encapsulation. The nonconjugated 8HQ is then released in less than 30 min so that the coexistence of both conjugated and nonconjugated 8HQ in the nanoparticles allows a release profile, which is a hybrid of sustained and burst releases. Furthermore, the nanoparticles are advantageously used as nanosensors. The 8HQ released from the nanoparticles displays enhanced fluorescence upon chelation with aluminum ions. Therefore, the nanoparticles can be used simultaneously for corrosion sensing and protection.

Redox-Responsive Polymer with Self-Immolative Linkers for the Release of Payloads

Previous couplings of corrosion inhibitors to redox-responsive polymers via covalent bonding suffer from several drawbacks. It is presented here novel redox-responsive polymer-corrosion inhibitor conjugates that contain self-immolative linkers in their side chains. Very fast redox-induced release of tryptamine, a drug and a corrosion inhibitor, is observed after applying a reductive trigger.

Designing Smart Polymer Conjugates for Controlled Release of Payloads

Incorporating labile bonds inside polymer backbone and side chains yields interesting polymer materials that are responsive to change of environmental stimuli. Drugs can be conjugated to various polymers through different conjugation linkages and spacers. One of the key factors influencing the release profile of conjugated drugs is the hydrolytic stability of the conjugated linkage. Generally, the hydrolysis of acid-labile linkages, including acetal, imine, hydrazone, and to some extent β-thiopropionate, are relatively fast and the conjugated drug can be completely released in the range of several hours to a few days. The cleavage of ester linkages are usually slow, which is beneficial for continuous and prolonged release. Another key structural factor is the water solubility of polymer-drug conjugates. Generally, the release rate from highly water-soluble prodrugs is fast. In prodrugs with large hydrophobic segments, the hydrophobic drugs are usually located in the hydrophobic core of micelles and nanoparticles, which limits the access to the water, hence lowering significantly the hydrolysis rate. Finally, self-immolative polymers are also an intriguing new class of materials. New synthetic pathways are needed to overcome the fact that much of the small molecules produced upon degradation are not active molecules useful for biomedical applications.

Hemiaminal ether linkages provide a selective release of payloads from polymer conjugates

Hemiaminal linkages allow for a selective and pH-responsive release of triazoles from polymer conjugates.