3.1.2. PIL Gel

As described above, PIL gels are one form of PILs which recently have been used in a number of applications such as electrolytes for batteries and supercapacitors [70], drug delivery [71], agriculture, and biomedical fields [72]. Indeed, they are showing a multitude of characteristics that make them very versatile materials with tuneable properties. Gordon and co-workers pioneered the synthesis of large PIL beads in the micron meter size scale through direct polymerizations of 1-butyl-3-vinylimidazoium TFSI in presence of a 1,8-di(vinylimidazolium)-octane TFSI as a crosslinker. Furthermore, the resulting gel-type beads were swelled in acetone, and loaded with palladium nanoparticles to catalyze C–C coupling reactions [58]. The authors suggested the application of such gel beads in catalysis, separation technology, and ion-exchange resins. Xiong and co-workers [73] reported a facile one-step synthetic strategy for the preparation of cross-linked polymeric nanogels by the conventional radical copolymerization of a phosphonium-based IL for use as catalysts. Recently, Rahman et al. used a microfluidic method to fabricate monodisperse spherical PIL microgel beads [74]. The authors showed the anion exchange can enable fine-tuning of size and swellability of these beads. By incorporating diverse anions, they were able to impart a multitude of functionalities to these beads, ranging from redox capabilities, controlled release of payload, magnetization, toxic metal removal and robust, reversible pH sensing. These chemically switchable stimulus-responsive PIL beads have potential applications in portable and preparative chemical analysis, separations and spatially addressed sensing (Figure 7) and also have potential for use as cargo for corrosion inhibitors or slow release inhibitor.

**Figure 7.** Schematics illustrating microfluidic method to generate switchable stimulus-responsive PIL microgels. (Reprinted with permission from [74]. Copyright 2013 American Chemical Society). (**a**) Stereomicroscope image of a prepolymer droplet flowing in the transparent capillary tube; (**b**) Chemical structures of IL monomer and cross-linker; (**c**–**e**) Stereomicroscope images of PIL microgels showing their monodispersity and transparency and (**f**) FESEM image of synthesized PIL[Br].

With the aim of using such PIL as corrosion inhibitors, we reported the novel and facile fabrication of multifunctional PIL gel beads using vinyl imidazolium based ionic liquid through click-type reactions (Figure 8) [75]. A detailed study into the effect of reactant ratios is examined. The gel formation is confirmed through fourier transform infrared spectroscopy (FTIR), thermal analysis, and kinetic studies. These PIL gels exhibited multiple characteristics including (1) self-healing characteristics due to their rubbery nature, (2) the ability to uptake active molecules which acts as corrosion inhibitors, and (3) pH sensing through the incorporation of indicator molecules. These functionalities demonstrate the potential of PIL gel family as multifunctional autonomous platform material for the control, detection and inhibition of corrosion.

**Figure 8.** The facile fabrication of multifunctional PIL gel beads using vinyl imidazolium based ionic liquid through click-type reactions. (Reprinted with permission from [75]. Copyright 2015 American Chemical Society).

## **4. Graphene as Green Corrosion Inhibitor in Anticorrosion Coating**

Graphene is a nanofiller with one-atom-thick planar sheet of two-dimensional carbon with *sp*<sup>2</sup> bonded carbon atoms that are densely packed in a honeycomb crystal lattice or an unrolled single-walled carbon nanotube [76]. Different approaches for preparing graphene sheets have been investigated like graphite exfoliation, including mechanical cleavage of graphite, chemical exfoliation of graphite, thermal-induced exfoliation, and direct synthesis, such as epitaxial growth, and bottom-up organic synthesis. Prasai et al. [77] studied the corrosion inhibition effect of copper and nickel by either growing graphene on these metals by chemical vapor deposition (CVD) method, or by mechanically transferring multilayer graphene onto them (Figure 9). Graphene grown by chemical vapour deposition (CVD) technique has shown superior anticorrosion coating but it is also demonstrated that these coating cannot be used over a long-term duration. It has been reported that transferring multiple graphene layers onto the metal surfaces will increase the degree of protection with building thicker and more robust films.

**Figure 9.** Schematic demonstration of thin layers of graphene as a protective coating that inhibits corrosion of underlying metals. (Reprinted with permission from [77]. Copyright 2012 American Chemical Society).

*Coatings* **2017**, *7*, 217

High thermal conductivity, better gas barrier, extraordinary electronic transport properties, superior mechanical stiffness combined with a wide set of other unusual properties of graphene-based composites made them promising and cheaper alternative to carbon nanotubes-based composites [78–81]. Graphene and graphene derivatives (e.g., graphene/graphite oxide, functionalized graphene, etc.) could be used in various applications such as hydrogen storage [82], sensors [83], transparent conductive films [84], batteries [85,86], super capacitors [87], solar cells [88] and nanocomposites coatings [89–91]. Due to the high surface area of graphene sheet (2630 m2/g), improvement of mechanical, thermal, and electrical properties of composite graphene based coating could be achieved with very low loading [92]. Chang et al. [93] applied polyaniline/graphene composites (PAGCs) for corrosion inhibition of steel. The composites display outstanding barrier properties against O2 and H2O. Figure 10 depicts the corrosion inhibition behaviour of bare steel and PAGCs coated steel with different amount of graphene loading in a corrosive medium (3.5 wt % aqueous NaCl electrolyte) under potentiodynamic polarization conditions. As it can be observed, as the PAGCs loading was increased further, the corrosion inhibition ability was enhanced evidenced by the highest *E*corr and lowest *I*corr values (which corresponds to a lower corrosion rate). In fact, using graphene in coating matrix could increase the length of the diffusion pathways for reactive gases such as oxygen and water vapour in polymer coatings and lead to a remarkable improvement of the corrosion inhibition of metallic substrate compared to normal polymer coating.

**Figure 10.** Tafel plots for (**a**) bare steel; (**b**,**c**,**d**,**e**,**f**) PANI-coated with different amount of graphene loaded. Electrodes measured in 3.5 wt % NaCl aqueous solution (Reprinted with permission from [93]. Copyright 2012 Elsevier).

Wang et al. [79] suggested that incorporation of graphene sheets into the epoxy polymer composite improved thermal conductivity and reduced coefficients of thermal expansion (CTEs). Their results also indicate that due to the high thermal-stability of graphene, they can be used in microelectronics coating applications. Since it is easy to obtain the graphene precursor, graphite, as it is naturally abundant, and the functionalized graphene can serve as a conductive nanofiller for other polymers (such as epoxy, polyimide, polyurethane, etc.), polymer/graphene based composite coatings will emerge as a new area of corrosion inhibition technology.

Stronger interface have been achieved using graphene platelets (GP) comprising one or more layers of a graphene plane. Yasmin et al. [94] have developed epoxy/graphite nanocomposites by mixing epoxy with graphite in solvent; 4 wt % graphite increases Young's modulus by 10% and glass transition temperature (*T*g) marginally from 143 to 145 ◦C. Better results have been obtained using sonication and shear mixing, 1 wt % GP increasing modulus 15%, but leads to a reduction of

tensile strength. The mechanical properties of epoxy/GP nanocomposites have been investigated by Koratkar et al. [95] showing improvement in epoxy fracture toughness from 0.97 to 1.48 MPa m1/2 at 0.1 wt % filler fraction. Therefore, it could be used as toughening agent for coating.

Despite the tendency of graphene nano-sheets to re-aggregate and stack due to their high surface area and strong van der Waals force has limited their applications in polymer nanocomposites. However, several studies have focused on improving the dispersion and interface interaction of graphene in a polymeric matrix using functionalised graphene. Novel method for functionalization of GP has been presented by Miller [96] using a coupling agent to form covalent bonding between fillers and soft matrix (0.78 GPa Young's modulus), resulting in 50% modulus improvement at 1 wt % filler fraction. Chiang and Hsu [97] have improved the fire resistance of epoxy/GP nanocomposite following a similar method. Martin-Gallego et al. [98] studied the effect of functionalized graphene sheets (FGS)/epoxy coatings which are prepared using cationic photopolymerization on mechanical properties of coating. Their results indicate increased stiffness and *T*<sup>g</sup> values of the cured epoxy network with better storage modulus properties in higher temperature.

Jeong and co-workers [99] investigated the effect of graphene content on structures and electrical properties of graphene/epoxy composite films which are prepared by solution casting and following thermal curing of diglycidyl ether of bisphenol-A with an amine-functionalized agent mixed on a polyimide film. The graphene/epoxy composite films can be utilized as high performance electric heating elements in various applications. They found that the graphene content as well as the applied voltage are two key elements in controlling the maximum temperatures of the composite films. Bao et al. [100] enhanced the mechanical, electrical and thermal properties of the epoxy nanocomposites utilizing functionalized graphene oxide. In situ thermal polymerization has been used to functionalize graphene oxide (FGO) via surface modification by hexachlorocyclotriphosphazene and glycidol. Strong interfacial interaction between FGO and epoxy matrix improved the thermal stability, storage modulus and hardness in a polymeric matrix.

Thus, graphene as an anti-corrosive agent is very attractive as it may protect metals by keeping their intrinsic properties, which cannot be achieved using three dimensional protective paints, oxides or polymers. In the field of using graphene as corrosion protective material the biggest hurdle is that the graphene sheets synthesised using current methods still contain too many defects. So the main challenge in near future is to improve the quality of sheet produced, the poor quality of sheet drastically reduces the performance as an anti-corrosive material. The keys factors affecting the quality of sheets are defects or abnormalities in graphene sheets like:


All these factors can represent the centre of damage accumulation also, other than altering the properties of graphene. Local defects can lead to accumulation of oxygen which ruins the chemical properties of sheet. Presently the functionalization of GO via non-covalent and covalent route with organic compounds has become a matter of rigorous research for production of innovative hybrid nano composites with new advanced functions and applications.
