Aqueous dispersion of polyurethane nanocomposites based on calix[4]arenes modified graphene oxide nanosheets: Preparation, characterization, and anti-corrosion properties
Aqueous dispersion of polyurethane nanocomposites based on calix[4]arenes modified graphene oxide nanosheets: Preparation, characterization, and anti-corrosion properties
Accepted Manuscript Aqueous dispersion of polyurethane nanocomposites based on calix[4]arenes modified graphene oxide nanosheets: preparation, charact...
Accepted Manuscript Aqueous dispersion of polyurethane nanocomposites based on calix[4]arenes modified graphene oxide nanosheets: preparation, characterization, and anticorrosion properties Abbas Mohammadi, Mehdi Barikani, Amir Hossein Doctorsafaei, Ali Pournaghshband Isfahani, Esmaeil Shams, Behnam Ghalei PII: DOI: Reference:
Please cite this article as: A. Mohammadi, M. Barikani, A.H. Doctorsafaei, A.P. Isfahani, E. Shams, B. Ghalei, Aqueous dispersion of polyurethane nanocomposites based on calix[4]arenes modified graphene oxide nanosheets: preparation, characterization, and anti-corrosion properties, Chemical Engineering Journal (2018), doi: https:// doi.org/10.1016/j.cej.2018.05.111
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Aqueous dispersion of polyurethane nanocomposites based on calix[4]arenes modified graphene oxide nanosheets: preparation, characterization, and anticorrosion properties Abbas Mohammadi a,* Mehdi Barikani b, Amir Hossein Doctorsafaei a, Ali Pournaghshband Isfahani c Esmaeil Shams a, Behnam Ghalei c a
b c
Department of Chemistry, University of Isfahan, Isfahan 81746-73441, I.R. Iran
Department of Polyurethane, Faculty of Science, Iran Polymer and Petrochemical Institute, Iran
Institute for Integrated Cell-Material Sciences (iCeMS), Kyoto University, 606-8501 Kyoto, Japan
Abstract Environmental-friendly waterborne polyurethane/graphene oxides nanocomposites (WPU/GOs) were prepared using p-tert-butyl calix[4]arene (BC4A) and sodium p-sulfonatocalix[4]arene (SC4A) modified GO nanosheets (CGO and SGO) as novel anti-corrosion coatings. Structural, thermal, and morphological investigation of nanosheets by FTIR, XRD, Raman, XPS, TGA, and SEM analysis confirmed their synthesis successfully. Moreover, different properties of WPU/GOs films were also evaluated by ATR-FTIR, XRD, SEM, contact angle, TGA, DSC and tensile analysis. It was found that the modification of GO nanosheets with BC4A and SC4A macrocycles not only overcome the flocculation and coagulation problem of unmodified GO incorporated WPU dispersion (WPU/GO) but also afford better mechanical properties to nanocomposites. The SEM morphological inspection exhibited that the microphase separation degree and dispersion quality of nanosheets within the nanocomposites strongly depends on the type of incorporated nanosheets. Regarding WPU/CGO and WPU/SGO nanocomposites, CGO and SGO nanosheets provide the enhanced storage stability and dispersibility compared to unmodified GO in WPU/GO sample. Anti-corrosion efficiency of the samples was also evaluated by PDS and EIS techniques and the results revealed that the WPU/CGO sample acts as a highly efficient anti-corrosion coating for mild steel and can be introduced as green corrosion protective coating with inhibition efficiency of 99.8 %. KEYWORDS: Waterborne polyurethanes; Calix[4]arenes; Graphene Oxide; Surface modification; Anti-corrosion coatings; Mild steel. *Corresponding authors [email protected] 1
1. Introduction Corrosion is an unavoidable continuing destruction of metallic structures and has main adverse impacts on the chemical, shipping and manufacturing industries [1,2]. One of the most favorable methods for corrosion prevention is barrier protective coatings. In this strategy, polymers act as a physical barrier against transportation of moisture, oxygen, water and corrosive ions [3]. Some polymeric coating such as epoxy resins and polyurethanes are desired candidates that effectively protect the surface of the metal against the corrosive environment [3-5]. Traditional polymeric coating products, due to considerable toxic volatile organic compounds (VOCs) is being gradually replaced by the waterborne polymers with lessened VOCs emissions [6]. Nowadays, environmental-friendly waterborne polyurethanes (WPUs) with fine dispersed polymeric particles in a water medium and high storage stability are extensively developed as green coating for different substrates involving metal, paper, leather, textiles, wood, concrete, and some polymers [7]. The utilization of the WPUs as anti-corrosion coatings is prone to some characteristics such as environmental safety, good adhesion to substrates, suitable chemical resistance, high toughness and flexibility, low dispersion viscosity and easy and good filmforming property [8]. The addition of nanostructures into WPU dispersions provides a beneficial opportunity to improve the anti-corrosion performance of the WPU coatings through improvement in their barrier properties on the metal surfaces [5,11]. In recent years, some efforts have been made on incorporation of nanostructures, including carbon nanotubes [9], nanoclay [10], functionalized rGO [5,11], polyvinyl alcohol (PVA) modified GO/zinc oxide (GO/ZnO), functionalized carbon black/ZnO (CB/ZnO) nanocomposites [12], nano-ZnO [13,14] and nano-silver[15] into WPU dispersion to enhance the anti-corrosion properties of WPUs coatings. 2
Graphene oxide (GO) nanosheets, a honeycomb quasi-two-dimensional carbon atoms with hydroxyl, epoxide, and carboxyl functional groups have attained significant attention; due to their specific properties including sheet-like structure, high specific surface area, electrical insulation, superior mechanical strength, and barrier properties against water, oxygen, and corrosive ions [16, 17,18]. Singh et al. investigated the corrosion resistance of a graphene oxide/polymer composite coatings on the metal substrates. They found that the GO filled coatings act as corrosive protective shields on the metals due to improved barrier effect in exposure to corrosive mediums [19]. Besides, because of high surface area and strong Van der Waals interaction, GO nanosheets tend to agglomerate within the polymer matrix. In this regard, the most convenient strategy to improve the interfacial interactions between nanosheets and polymer matrix and thus enhancement in dispersion quality is physical or chemical modification of GOs [20,21, 22]. There is only one study about preparation of WPU nanocomposites based on surface modified GOs as anti-corrosive coatings. In this connection, Christopher et al. presented fabrication of waterborne polyurethanes reinforced with polyvinyl alcohol (PVA) modified GO/zinc oxide (GO/ZnO) and functionalized carbon black/ZnO (CB/ZnO) nanocomposites. Moreover, they reveal that PVA anchored GO supported nano ZnO provides better anti-corrosion performance than PVA anchored CB supported nano ZnO in WPU dispersion [12]. Macrocyclic and cavity-shaped calix[4]arenes (C4As) are interesting compounds with some superior properties like high thermal stability, host-guest property and modifiable structure which make them attractive candidates in many areas such as molecular recognition, supramolecular chemistry and molecular separation [23,24]. Recently, calix[4]arens are also
3
introduced as new corrosion inhibitors due to their ability to interact with metal surfaces through their aromatic p-electrons [25,26]. Here, as a promising and developed strategy to overcome the flocculation and coagulation problem of the unmodified GO incorporated WPU dispersion, enhance the storage stability of dispersion of nanocomposites, fine dispersion of GO flakes in polyurethane matrices, improve the barrier properties of coatings and eventually strengthen the anti-corrosion ability of coatings, two derivatives of macrocyclic calix[4]arens including p-tert-butylcalix[4]arene and sodium psulfonatocalix[4]arene were used to surface modification of graphene oxide. In the next step, the prepared nanosheets were incorporated into aqueous dispersion of polyurethanes to fabricate green and environmentally friendly waterborne polyurethane (WPU) nanocomposites as anticorrosion coatings. Moreover, the effects of prepared GOs nanosheets on the structural, morphological, thermal and mechanical properties of the obtained films were investigated. The aqueous nanocomposite dispersions were then applied on mild steel substrate to investigate their corrosion protective properties. In addition, to more evaluate the performance of the coatings, the oxygen gas permeability and also pull-off adhesion measurements before and after three weeks immersion in 3.5 wt.% NaCl solution were carried out. To the best of our knowledge, the use of calix[4]arenes modified GOs (C4As-GOs) in preparation of waterborne polyurethanes nanocomposites as anti-corrosion coatings has not been studied up to now.
2. Experimental 2.1 Materials In order to prepare the WPU dispersion, poly (tetramethylene oxide) (PTMO, molecular weight=1000 g/mol), isophorone diisocyanate (IPDI) and dimethylolpropionic acid (DMPA) were purchased from Aldrich. 1,4-butanediol (BDO, Aldrich), Hexamethylenediamine (HMD, 4
Aldrich), Dibutyltin dilaurate (DBTDL, Merck), triethylamine (TEA, Merck), and N-Methyl pyrrolidinone (NMP, Merck) were all provided in analytical reagent grade. The main materials used in the synthesis of unmodified and modified GOs were as follows: graphite powder (200 mesh, DAEJUNG), hydrogen peroxide (30 wt. %, Merck), sulfuric acid (98 wt. %, CARLO ERBA), orthophosphoric acid (85 wt. %, Merck), p-tertbutylphenol (ACROS Organics), formaldehyde solution (37 wt. % in H2O, Aldrich), diphenyl ether (Merck), sodium hydroxide (Merck), ethyl acetate (Merck), and sodium chloride (Merck). All these reagents were used directly without further purification. 2.2. Synthesis of calix[4]arene derivatives (C4As) p-tert-butyl calix[4]arene (BC4A) was prepared based on already reported method [27]. FTIR (KBr, cm−1): 3450 (O–H), 2957 (C–H), 2864 (C–H), 1601 (C=C), 1485 (C–H), 1455 (C–H), 1241 and 1203 (C–O). 1H NMR (400 MHz, CDCl3, δ, ppm): 1.28 (s, 36H, t-Bu), 7.08 and 7.28 (8H, ArH) and 9.67 (s, 4H, Ar-OH).
13
C NMR(400 MHz,CDCl3, δ, ppm): 31.49, 32.33, 34.03,
123.22, 125.53,128.72, 129.75, 144.72,146.62. Sodium p-sulfonatocalix[4]arene (SC4A) was synthesized based on the literature method [28]. FTIR (KBr, cm−1): 3450 (O–H), 2957 (C–H), 2870 (C–H), 1117 and 1053 (S=O). 1H NMR (400 MHz, D2O, δ, ppm): The H of -OH disappeared on exchange with D2O, 3.91 (s, 8H, Ar-CH2-Ar) and 7.38 (s, 8H, ArH). 13C NMR (400 MHz, D2O, δ, ppm): 30.60, 126.21, 128.11, 135.22 and 153.40. 2.3. Synthesis of graphene oxide (GO) The graphene oxide was synthesized by chemical oxidation of graphite flakes according to improved Hummers’ method [29]. Graphite flakes (1.5 g) and KMnO4 (9.0 g) were added into a 5
9:1 mixture of concentrated sulfuric acid and orthophosphoric acid (120:15 mL) under continuous stirring. It was heated to 50 °C and stirred for 24 h. The resulting mixture was cooled to room temperature and poured into mixture of ice (150 mL) and H2O2 (30%, 1 mL) and then centrifuged. The obtained solid was washed sequentially with water, 30% HCl solution, and ethanol. The GO product was finally vacuum dried for 12 h at 80 °C. 2.4. Synthesis of Calix[4]arenes (C4As) modified GOs (C4As-GOs) p-tert-butyl calix[4]arene (BC4A) and Sodium p-sulfonatocalix[4]arene (SC4A) macrocycles can be covalently linked to the GO surface through a ring opening reaction of their phenolic hydroxyl groups with the epoxide groups presented on the basal plane of GO. Therefore, an effective and one-step reaction between C4As and GO produced C4As-GOs. The schematic of the synthesis of C4As-GOs is shown in Figure 1.
2.4.1. Synthesis of BC4A-GO nanosheets (CGO) In the general synthetic procedure of CGO, GO (200 mg) was firstly dispersed in 20 mL of THF and bath-sonicated for 2 h to form a homogeneous dispersion. Afterward, BC4A (400 mg) was gradually added to this dispersion and the mixture was allowed to stir for 72 h at room temperature.In the next step, the resulting mixture was centrifuged, washed with distilled water and ethanol and eventually dried under vacuum. 2.4.2. Synthesis of SC4A-GO nanosheets (SGO) In the general synthetic procedure of SGO, GO (200 mg) was firstly dispersed in 20 mL of deionized water and bath-sonicated for 2 h to form a homogeneous dispersion. Afterward, SC4A (400 mg) was added to this dispersion and the mixture was allowed to stir for 72 h at room 6
temperature. Subsequently, the resulting mixture was centrifuged, washed with toluene, ethanol and distilled water and finally dried under vacuum. 2.5. Synthesis of WPU and WPU/GOs dispersions The WPU and WPU/GOs (WPU/GO, WPU/CGO, and WPU/SGO) dispersions were prepared in accordance with the formulation given in Table 1. The synthesis schematic of the waterborne dispersions is also exhibited in Figure 2. Initially, to prepare the NCO-terminated prepolymer, PTMO, IPDI, DMPA (dissolved in NMP) and DBTDL as catalyst (0.25 wt. % of total solid content) were charged into a 250 mL three-necked round bottom flask equipped with a nitrogen inlet, condenser, and mechanical stirrer. The reaction was carried out with stirring at a speed of 250 rpm at 80 ºC for 2 h. Afterward, a suitable amount of BDO was added to prepolymer mixture at 80 °C for 1 h. Then, the reaction mixture was cooled down to 30 ºC and TEA (DMPA equiv) was added to neutralize the acidic groups. Finally, under vigorous stirring (1000 rpm) certain amount of dissolved HMD in a specific amount of water was slowly dropped for 30 min to accomplish the chain extension and also to get WPU dispersion with 30 wt. % of solid content. Typically, the preparation of WPU/GOs dispersions incorporated with 1 wt. % of nanosheets was as follows: suitable amounts of each GO, CGO and SGO nanosheets were sonically dispersed in deionized water for 2 h. Then, the sonicated nanosheets were added separately to prepared WPU dispersion at room temperature and the resulted mixtures were lastly mechanically stirred for 10 min.
2.6. Preparation of the polyurethane films 7
To evaluate some properties, solid films were obtained by slow water evaporation of suitable amount of WPU and WPU/GOs dispersions in a Teflon mold at room temperature for 48 h. The mold was subsequently put in an oven at 80 ºC for 24 h to complete removal of solvent in the films. The obtained films were stored in a desiccator at room temperature for further analysis. 2.7. Measurements 2.7.1. Characterization of the prepared calix[4]arenes and nanosheets Fourier transform infrared (FTIR) spectra were recorded using a JASCO FTIR-4600 spectrophotometer in KBr pellets. 1H NMR and 13C NMR spectra were collected using a Bruker Avance 400 MHz NMR spectrometer. Deuterated chloroform (CDCl3) and D2O were used as solvents. Thermogravimetric analysis (TGA) was carried out using a Rigaku Thermo plus EVO2 thermal gravimetric analyzer. The temperature range was from 30 °C to 800 °C at a heating rate of 10 °C/min in nitrogen atmosphere. X-ray diffraction (XRD) measurements were performed by a Bruker D8 Advance diffractometer with CoKα radiation (λ = 1.7890 Å). The morphology of prepared nanosheets was also investigated by a Hitachi S-4800 Field Emission Scanning Electron Microscope (FESEM). The Raman spectra of the prepared nanosheets were obtained using a Takram-P50C0R10 Raman microscope spectrometer at the excitation of a 532 nm Nd: YAG laser. X-ray photoelectron spectroscopy (XPS) was carried out with a Perkin–Elmer PHI5702 multi-technique spectrometer (Physical Electronics,USA), using AlKα excitation radiation. The high-resolution transmission electron microscope (HRTEM) images of the nanosheets were obtained with a JEM-2100 transmission electron microscope (JEOL, Japan) at 200 kV. Zeta potentials of the aqueous dispersions of nanosheets were also measured at room temperature and pH 7 by a Malvern Zetasizer (Nano-ZS, Malvern Instrument Ltd.). 2.7.2. Characterization of the WPU/GOs dispersions and their films 8
The pH value of WPU and WPU/GOs dispersions was determined at 25°C using a pH-meter (pH Pen, AZ8665). The viscosity of the dispersions was measured at 25 ºC using a Brookfield DV-II + Pro viscometer. FTIR spectroscopy of WPU and WPU/GOs films were also performed on a JASCO FTIR-4600 spectrophotometer using the attenuated total reflectance (ATR) technique. 1H NMR spectra of WPU sample were recorded on a Bruker Avance 400 MHz NMR spectrometer and deuterated dimethyl sulfoxide (DMSO-d6) was used as solvents. XRD measurement was carried out at room temperature on a Bruker D8 Advance diffractometer using CuKα radiation (λ =1.540598 Å) with a scan rate of 1°/min between 2θ = 5° and 2θ = 70°. The surface morphology of the liquid nitrogen fractured and gold covered polyurethane films was evaluated by a Hitachi S-4800 FESEM. Wettability of the films was measured at room temperature via sessile drop method by a Kruss G10 instrument. TGA and DTG analysis were performed on a Rigaku Thermo plus EVO2 thermal gravimetric analyzer from room temperature to 700 ºC at a ramp rate of 10 °C/min in nitrogen atmosphere. Differential scanning calorimetry (DSC) was measured on a Bruker DSC 3100SA. The measurements were carried out from -100 to 250 ºC under dry nitrogen atmosphere at a scanning rate of 10 ºC/min. At first, to remove the thermal history, all samples were heated to 180 ºC and quenched to -100 ºC and held for 5 min to reach equilibrium. The mechanical properties of WPU and WPU/GOs films were also investigated at room temperature by SANTAM universal testing machine at a crosshead speed of 500 mm/min according to ASTM D1822. The adhesion strength of prepared WPU and WPU/GOs coatings on the surface of mild steel was investigated according to ASTM D4541 by a portable pull-off adhesion testers (DEFELSKO). The adhesion tests were performed before and after two weeks immersion in 3.5 wt.% NaCl solution. Moreover, the oxygen gas permeability of prepared coating was evaluated at room temperature and pressure of 10 bar according to ASTM D3985. 9
2.7.3. Anti-corrosion performance tests The effect of GO and C4As-GOs (CGO and SGO) nanosheets on the anti-corrosion performance of prepared WPU and WPU/GOs coatings on the mild steel substrate were studied by potentiodynamic polarization (PDS) technique and electrochemical impedance spectroscopy (EIS). In this regards, to prepare the WPU and WPU/GOs-coated mild steels as working electrodes, WPU dispersions were first coated onto the mild steels by dip-coating technique and permitted to dry under ambient condition for 48 h. Then, except a 1.0 × 1.0 cm2 of coated area at the center of each specimen, backs and edges of the electrode were sealed by beeswax. The prepared electrodes (with a coating thickness of 50 ± 5 µm) were immersed in aqueous 3.5 wt. % NaCl solution as corrosive environment for one week before corrosion evaluation. PDS and EIS analysis were studied using Potentiostat/Galvanostat (AUTOLAB PGSTAT30, Eco Chemie, Utrecht) in conjunction with frequency response analyzer (FRA), interfaced with a personal computer and controlled by General Purpose Electrochemical System (GPES 4.9) and FRA 4.9 software. The measurements were performed in a three-electrode electrochemical test system including an saturated calomel electrode (SCE) as reference electrode, a platinum counter electrode, and prepared working electrodes which all immersed in a 3.5 wt. % NaCl solution. All EIS measurements were carried out at the frequency range of 0.1 Hz to 10 kHz at the open circuit potential (OCP). Polarization curves were also recorded under potentiodynamic conditions in the potential range of -1.0 to 1.0 V respect to OCP with a scan rate of 0.005 V/s.
3. Results and discussion 3.1. Characterization of the GOs nanosheets FTIR analysis was performed to investigate the modification of GO nanosheets by C4As (BC4A and SC4A). FTIR spectra of GO, CGO and SGO nanosheets are presented in Figure 3a. The 10
FTIR spectrum of GO consist of -OH (3410 and 1420 cm−1), C=O (1740 cm−1), aromatic C=C (1623 cm−1), epoxy C-O (1229 cm−1), and alkoxy C-O (1047 cm−1) bands [22, 30-33]. For CGO, the absorption bands at 2968 cm−1 (C-H stretching), 2868 cm−1 (C-H stretching), 1485 cm−1 (CH bending), 1203 cm−1 (C-O stretching) assigning to the BC4A, all clearly demonstrate that the BC4A macrocycles successfully bonded to the surface of GO nanosheets. Moreover, the appearance of adsorption bands at 2959 cm−1 (C-H asymmetric stretching), 2867 cm−1 (C-H symmetric stretching) and 1115 cm−1 (S=O asymmetric stretching) provides an evidence for successful chemical modification of GO by SC4A macrocycles [34,35]. The symmetric stretching absorption of S=O at near 1053 cm−1 due to overlapping with the strong absorption of GO sheets is not observable. In addition, the decrease in adsorption intensity of the epoxide group at 1226 cm−1 after modification of GO with C4As is justified by the reaction of C4As with epoxide groups through a ring opening reaction [36]. TGA was employed to study the thermal stability and determine the amount of C4As bounded onto the surface of modified GOs. TGA thermograms of GO, CGO and SGO nanosheets under nitrogen atmosphere are shown in Figure 3b. Upon heating, all GOs starts to lose weight below 150 °C due to evaporation of adsorbed water. In the case of CGO, only 9.7% weight loss was observed below 150 °C while GO and SGO showed 24.1 and 14.9% weight losses, respectively. Higher weight losses for GO and SGO below 150 ° is owing to more hydrophilic characteristic of GO and SGO compared to CGO. From TGA curve of GO, The main weight loss of 62.2% occurs between 150 and 300 °C, attributing to the decomposition of the thermally labile oxygencontaining groups (epoxide, hydroxyl, and carboxylic groups) [37]. In relation to CGO and SGO nanosheets which some part of thermally labile groups replaced by more thermally stable calix[4]arenes macrocycles, the weight loss between 150 and 300 °C is lower compared to GO. 11
However, CGO and SGO exhibited better thermal stability below 300 °C with weight losses of 25.9 and 37.3%, respectively. Besides, the sequential weight loss between 300-800 °C is due to decomposition of carbon skeleton and more stable phenolic groups [38,39]. According to remained weights at 800 °C, it can be concluded that the CGO and SGO nanosheets contain 13.1 and 19.2 wt. % of BC4A and SC4A macrocycles, respectively. Furthermore, it should be declared that the amount of SC4A macrocycles attached on the surface of GO is more than BC4A ones. This observation possibly originated from the higher affinity of SC4A to react with epoxy groups on the surface of GOs due to its more nucleophilic phenolic groups compared to BC4A.
The effects of the modification of GO by C4As on the interlayer distance of nanosheets were investigated by XRD analysis. XRD diffractograms of GO, CGO and SGO samples are shown in Figure 4a. In the case of GO sample, the disappearance of (100) peak at 2θ = 31.2° arising from loss of planarity of graphitic planes [29, 30]. In addition, GO has a characteristic diffraction peak at 2θ = 11.7° relating to the (001) plane which according to Bragg's law (nλ = 2d sin θ; λ = 0.1789 nm) exhibited a highly ordered structure with interlayer spacing (d) of about 8.77 Å. This large d-spacing compared with graphite is ascribed to the presence of oxygen functionalities in the gallery spacing of GOs [22, 40]. For CGO and SGO samples, the XRD patterns showed that the (001) characteristic peak shifts to lower values of 2θ at 10.18 and 9.98°, respectively due to intercalation of C4As moieties between GO nanosheets. Moreover, an increase in d-spacing of SGO (10.30 Å) compared to CGO (10.08 Å) indicates that the SC4A bonding to the surface of GO reduces the aggregation of nanosheets more effectively compared to BC4A macrocycles.
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Raman spectroscopy is considered as a powerful analytical technique to identify the defects and disorder degree of graphene-based structures. Figure 4b shows the Raman spectra of GO, CGO and SGO nanosheets and two characteristic D and G peaks were observed for prepared nanosheets. D band arising from the defects and disorder of carbon in graphene platelets and moreover, G band originates from the first-order scattering of the E2g vibration mode and the inplane vibration of ordered sp2-bonded carbon atoms [41]. In order to examine the defects and disorder degree of each sample, the peak intensity ratio of the D band to the G band (ID/IG) was investigated. After modification of GO with BC4A and SC4A macrocycles, the ID/IG values of CGO and SGO nanosheets increased from 0.78 for GO to 0.81 and 0.91, respectively. However, it can be concluded that the defect and disorder degree of CGO and SGO slightly more than GO due to presence of C4As macrocycle on the surface of nanosheets.
X-ray photoelectron spectroscopic (XPS) measurements were performed for GO, CGO, and SGO samples and their high-resolution C1s spectra were presented in Figure 5a. In case of GO, five peaks at 284.7, 285.8, 286.7, 287.3, and 288.1 eV represent the carbon bonds of C-C, C-OH, C-O-C, C=O, and O-C=O, respectively[22,29]. Compared with GO, the C1s XPS spectra of CGO and SGO show a decrease in the peak intensities of C-O-C and O-C=O, together with increase in C-OH peak intensities. Furthermore, a new peak of C-S appeared for SGO nanosheets at 284.4 eV. These observed results all indicate that both of BC4A and SC4A macrocycles were successfully attached on the surfaces of graphene oxide via the reaction of their hydroxyl groups with epoxy groups on the surface of GO during the modification.
13
Hydrophilic/hydrophobic features of the prepared nanosheets were further analyzed with the static contact angle measurement. By using a microsyringe, the water drops were located on three places of the GO, CGO and SGO films. The mean value of the contact angles between water
drops
and
the
surface
of
the
films
was
reported
as
a
criterion
for
hydrophilicity/hydrophobicity evaluation. The results showed that contact angle of GO is 28.2 ° while with BC4A modification, the contact angle of CGO, increased to 95.1°. This result implied that hydrophobicity of CGO sample is significantly higher than unmodified GO which can be arising from the highly hydrophobic structure of BC4A [34]. It should be noted that the hydrophilicity of SGO with a contact angle of 20.1° is slightly higher than GO. Consequently, it can be deduced that the presence of highly sulfonate containing and water-soluble SC4A macrocycles on the surface of nanosheets imparted more hydrophilic characteristics to GO nanosheets [42]. HRTEM images of the GO, CGO and SGO samples exhibit that GOs platelets assembled in multilayer stacks (Figure 5b). The selected area electron diffraction (SAED) indicate differences in crystallinity of the samples. In comparison with GO sample, CGO and SGO show weak diffraction patterns, suggesting that the modification of GO with C4As causes a less regular carbon framework than unmodified GO [29]. The result also shows the increase in interplane spacing for CGO and SGO nanosheets in accordance with XRD results.
Figure 6a-d presents the FESEM images providing the morphologies of Au coated GO, CGO and SGO dried powders. As seen in Figure 6a, unmodified GO sheets are stacked tightly with highly agglomeration tendency and moreover magnified image demonstrates that GO has a slightly 14
smooth surface, owing to its surface tension and the presence of oxygen-containing functional groups [38]. Furthermore, as it can be seen from Figure 6b and c, the CGO and SGO dried powders have a rough surface, attributing to the existence of C4As on the surface of GO sheets which acts as spacer between GO sheets and prevents GO stacking [43,44]. Moreover, regarding to SGO (Figure 6c and d), highly rough surface, more densely thin-layered and wrinkled morphology compared to CGO were clearly observed. This observation may be related to repulsion forces between sulfonate groups of SC4As attached onto the surface of SGO nanosheets that could prevent the stacking and agglomeration of SGO sheets. However, the higher magnified image also represents that SGO comprised of plenty of thin nanosheets with layered structure [45]. Moreover, the higher stacked morphology of CGO than SGO can be ascribed to the self-assembly mechanism of BC4A macrocycles on the surface of GO through to π-π interactions [46]. In order to evaluate the dispersion stability of prepared nanosheets in aqueous medium, GO, CGO and SGO nanosheets at a concentration of 1 mg/ml were sonically dispersed in deionized water for 1 h and their storage stability were assessed after 24 h. As is clear from Figure 6e, GO nanosheets showed substantial dispersion stability in water after 24 h due to its intensely hydrophilic structure arising from polar hydroxyl, epoxy, carbonyl, and carboxyl groups [16,22]. Meanwhile, low storage stability and subsequent sedimentation of CGO nanosheets in water after 24 h is due to the hydrophobic nature of BC4A on the surface of GOs [46]. In contrast with CGO, it was seen that SGO readily disperse and also is dramatically more stable in water after 24 h. This enhanced dispersibility and storage stability is prone to highly hydrophilicity of SGO and improved interfacial interaction between nanosheets and water which prevents nanosheets from aggregating in aqueous solution [34,42]. 15
The zeta potential of aqueous dispersion of prepared nanosheets was also evaluated to confirm the storage stability results. Th zeta potentials of aqueous GO, CGO, and SGO samples were 39.2, -27.4 and -70.1 mV, respectively (Figure 6f). Particles with zeta potentials more negative than -30 mV are usually considered as stable dispersions due to interparticle electrostatic repulsion [41,47]. Therefore, CGO precipitated after 24 h, while GO and SGO form stable dispersions in aqueous medium for a long time. Regarding the results, surface modification of the GO with BC4A caused less negative charge on the CGO surface, while the SGO nanosheets exhibit a more negative zeta potential value. It can be ascribed to the SC4As macrocycles with sulfonate groups which impart high negative charges on the surface of SGO nanosheets dispersed in water [48].
Characterization of the WPU/GOs dispersions and their films WPU/GOs
waterborne
polyurethane
nanocomposites
were
successfully prepared
by
incorporation of unmodified GO, p-tert-butyl calix[4]arene (BC4A) and sodium psulfonatocalix[4]arene (SC4A) modified GO nanosheets (CGO and SGO nanosheets) into neat WPU dispersion according to formulation and schematic given in Table 1 and Figure 2, respectively. Furthermore, the effects of nanosheet types on the structural, morphological, wettability, thermal and mechanical properties of the obtained films were investigated. The WPU and WPU/GOs nanocomposite dispersions were then applied on mild steel substrate to evaluate their anti-corrosion properties. 3.2.1 Characterization of the WPU and WPU/GOs dispersions
16
The effects of nanosheet types on the storage stability, viscosity, and pH of dispersions were investigated and summarized in Table 2. The viscosity of the dispersions was assessed by Brookfield viscosity measurement and the results showed that unlike WPU/GO sample, incorporation of nanosheets has no significant effect on viscosity of WPU/CGO and WPU/SGO dispersions compared to WPU dispersion. This observation originated from coagulation and flocculation formation and thereby deterioration of the stability of WPU/GO dispersion due to increase in particles size through the intense interaction of epoxy groups on the surface of unmodified GO with urethane groups. The storage stability of WPU/GOs dispersions was also evaluated after one week and the results implied that surface modification of GO nanosheets with BC4A and SC4A can overcome the coagulation and flocculation problem of WPU/GO dispersion and furthermore, enhance the storage stability of CGO and SGO in WPU/CGO and WPU/SGO dispersions. In case of WPU/CGO, due to hydrophobic nature of CGO, small precipitation of CGO was observed after two days while WPU/SGO is more stable dispersion without any sedimentation after one week. pH measurements also demonstrated that incorporation of GO, CGO, and SGO generally decrease the pH value of dispersions because of the presence of carboxylic acid groups on the edge of nanosheets. The higher pH values of WPU/SGO and WPU/CGO compared to WPU/GO can be assigned to lower content of carboxylic acid groups in CGO and SGO nanosheets.
3.2.2 Characterization of the WPU and WPU/GOs films Figure 7a shows the ATR-FTIR spectra of prepared films. The WPU sample (without any nanosheets) was characterized with stretching vibrations of urethane N–H bond at 3322 cm-1, stretching vibrations of free and hydrogen bonded urethane carbonyl groups at 1713 and 1701 17
cm-1, respectively. It should be noted that the broad peaks around 1641-1673 cm-1 are related to overlapped vibrations of free and hydrogen bonded urea carbonyl groups and also the DMPA carbonyl groups [49,50]. Absorptions at 1539 cm-1 are due to urethane N–H out-of-plane bending vibration. The C-H asymmetric and symmetric stretching vibrations appeared at 2940 and 2853 cm-1, respectively. The absorption bands at 1463 and 1304 cm-1 were assigned to CH2 bending and CH2 wagging vibrations, respectively and also C-N stretching vibration appeared at 1365 cm-1. The C-O-C asymmetric and symmetric stretching vibrations correspond to the ether oxygen of the PTMO were observed at 1238 and 1103 cm-1, respectively [51,52]. Furthermore, the disappearance of the stretching vibration of the NCO groups at 2270 cm-1 confirms the completeness of the reactions [53]. As seen in Figure 7a, there are almost no notable differences in FTIR spectra of the WPU/GOs composites (WPU/GO, WPU/CGO, and WPU/SGO samples) because of low weight fraction of incorporated nanosheets into WPU dispersions. 1
H NMR spectrum of neat WPU film is shown in Figure 7b. Terminal methyl protons of IPDI
moieties appeared at about 0.85–0.94 ppm. The methyl protons of DMPA and ethyl protons of triethylamine salt were also observed at 0.94–1.39 ppm. Furthermore, the central methylene protons of PTMO, IPDI ring, hexamethylene diamine and butanediol moieties were observed at 1.39–2.37 ppm. Methylene and methine protons of IPDI and also methylene of HMD close to urea group were observed around 2.63-2.75 and ppm. Methylene groups of PTMO moieties attached to ether oxygen atom (-CH2OCH2-), appeared near 3.27-3.30 ppm. Methylene groups of DMPA, PTMO, and BDO attached to urethane oxygen atom appeared at about 3.78-4.07 ppm. In addition, the weak peaks at about 6.80–7.23 ppm were assigned to urethane and urea -NH groups [54,55]. In conclusion, the 1H NMR spectrum of synthesized WPU sample was in accordance with the proposed structure in Figure 7b. 18
XRD measurement was used to investigate the effect of nanosheets incorporation on the crystallinity of WPU and WPU/GOs films. In polyurethanes, the crystallinity is commonly achieved by the ordered structure which can be obtained by the microphase separation of soft segment and hard segment sequences. The soft segments are derived from the polyols, while hard segments derived from diisocyanates and chain extenders [52,56]. As can be seen from Figure 8, the diffractograms of all sample showed a broad peak with different intensities at 19.9º, indicating the crystallinity and microphase separation degree depends on the type of nanosheets incorporated in the polyurethane matrices. Observation of broad peaks for all samples implying that they have a semicrystalline structure because of low microphase separation degree [56]. According to literature, XRD pattern of PTMO shows two sharp crystalline peaks at 2θ of 19.6 and 24º [57]. Since observed peak positions at 19.9 º for all WPUs are well in agreement with crystalline peaks of PTMO, it is possible to say that all samples have some crystalline soft segments domains dispersed in amorphous medium [58]. It should be noted that for WPU/CGO, the peak is more intense than WPU sample while diffraction peaks of WPU/GO and WPU/SGO samples are less intense than WPU. In general, depression of hydrogen bonding interactions and disruption in the regularity of hard segments can obstruct the crystallization of soft segments [59]. Thus, for WPU/GO and WPU/SGO samples, it is proposed that the hydrogen bonding interactions in polar hard segments may be disrupted by hydrophilic GO and SGO nanosheets which inhibit formation of more packed hard segments and eventually the degree of microphase separation is reduced compared to WPU [59,60]. In contrast, CGO nanosheets due to its hydrophobic nature can interact properly with PTMO in WPU/CGO sample and therefore low degree of miscibility between CGO and hard segments is expected. Therefore, it’s concluded that 19
the embedding 1 wt% of CGO into polyurethane matrix favor the formation of more ordered polyurethane structure owing to increase in microphase separation degree compared to WPU sample.
Properties of polyurethanes depend drastically on the morphology inspired by degree of microphase separation [56]. Figure 9 illustrates the SEM morphology of the WPU and WPU/GOs films. The SEM photomicrographs WPU/SGO samples elucidated randomly distributed and smaller hard segments (bright regions) within the continuous soft segments compared to WPU, WPU/GO and WPU/CGO samples due to their low degree of microphase separation, correlating well with morphological XRD results [57]. In the case of WPU/CGO sample, because of its higher microphase-separated morphology, hard domains are more extensive than WPU sample which agrees well with XRD results. Moreover, SEM images were also considered to investigate the quality of nanosheets dispersion in the WPU films. It can be observed that the SGO nanosheets are more homogeneously dispersed in the WPU matrix compared to other samples. This better dispersion arising from the highly layered structure and also easily exfoliation of hydrophilic SGO nanosheets in WPU dispersion during fabrication of WPU/SGO nanocomposite. Contrary to WPU/SGO, the WPU/CGO nanocomposite containing more agglomerated nanosheets due to hydrophobic nature of CGO nanosheets and its low tendency to interact interfacially with water as demonstrated in storage stability investigation (Figure 6e). As SEM images show, WPU/GO sample, exhibited extensive light regions and more agglomerated particles in comparison with WPU/SGO which may be attributed to increase in particles size and subsequently depression of stability of PU particles through extremely
20
hydrogen bonds formation between urethane groups and polar group, such as epoxy and hydroxyl groups, on the surface of unmodified GO [61].
The wettability of WPU and WPU/GOs films was investigated by measuring the contact angles formed between liquid drops and the surface of prepared films. Obtained contact angles for all samples indicating that the polyurethane films due to existence of hydrophilic carboxyl groups in polyurethane chains are hydrophilic [62]. The water contact angle of the WPU/CGO film was 71°, which was higher than that of WPU sample with contact angle of 62°. This originates from the hydrophobic structure of embedded CGO nanosheets. Meanwhile, compared to WPU, WPU/GO and WPU/SGO films are more hydrophilic and their water contact angles are 51 and 45°, respectively. This hydrophilicity enhancement is prone to significant hydrophilic characteristic of GO and SGO nanosheets. As mentioned earlier, it should be declared that the higher hydrophilicity of SGO than GO imparted more hydrophilic characteristics to WPU/SGO compared to WPU/GO [42]. TGA and DTG analysis were used to evaluate the thermal stability and decomposition behavior of the WPU/GOs nanocomposite films under nitrogen atmosphere. The TGA and DTG thermograms and obtained results are presented in Figure 10 and Table 3. The thermal stability of PUs is commonly dependent on the thermal stability of hard segments which are more prone to thermal decomposition than soft segments [63]. The thermal decomposition of all samples was evaluated at 10% weight loss temperatures (T10%) relating to depolymerization of urethane linkages and also known as a criterion for thermal stability of polyurethanes. In comparing with the neat WPU sample with T10% at 293 ºC, for WPU/GOs nanocomposites, T10% shifted toward 21
lower temperature. This lower thermal stability can be justified by reduction in hydrogen bonding interactions in hard segments through incorporation of thermally unstable GOs nanosheets [64]. In addition, the T10% of WPU/CGO is higher than that of WPU/GO and WPU/SGO samples that originated from higher phase-separated morphology of WPU/CGO samples and low degree of miscibility of hydrophobic CGO with polar hard segments which were confirmed by XRD and SEM analysis. It is worth knowing that the thermal degradation process of polyurethanes occurs over two chief steps, the first step takes places from 200 to 350 ºC and is attributed to the decomposition of the hard segments and the second step occurs from 350 to 500 ºC and is attributed to soft segments degradation [65]. As DTG thermograms show in Figure 10b, all samples degraded in two steps and have similar thermal degradation patterns. The obtained results in Table 3 also confirmed that incorporation of prepared nanosheets into WPUs cause an increase in Tmax,1 (temperatures at which rate of hard segments is maximum) and Tmax,2 (temperatures at which rate of soft segments is maximum) values compared to neat WPU sample. The increase in Tmax,1 and Tmax,2 for WPUs/GOs nanocomposites is associated with barrier effect of nanosheets which retard the gas evolution and weight losses.
The DSC thermograms of the WPU and WPU/GOs nanocomposites are shown in Figure 11. All samples showed one glass transition temperature (Tg) corresponds to Tg of soft segments and also one broad endothermic transition, relating melting of the crystalline structures in the hard segments [52]. The DSC thermograms showed that incorporation of GOs in WPUs leads to a 22
slight increase in Tg values of WPU/GOs nanocomposites compared with WPU sample (Table 3). This phenomenon is attributed to a reduction in the chain mobility of soft segments by incorporation of GOs [34]. WPU/SGO sample exhibited the highest Tg value, assigning to the best dispersion quality of SGO nanosheets within PU matrix, whereas WPU/CGO sample exhibited the lowest Tg value because of highly agglomeration of CGO sheets in PU matrix according to SEM analysis. Regarding DSC data, WPU/SGO sample showed lowest Tm value among samples, assigning to high affinity of hydrophilic SGO nanosheets toward polar hard segments which lead to depression of hydrogen bondings in hard segments. Additionally, it has been observed that the WPU/GO has the highest Tm value due to extremely hydrogen bonds formation between urethane groups and polar groups, such as epoxy and hydroxyl groups, on the surface of unmodified GO [61].
The mechanical properties data of the WPU and WPU/GOs nanocomposites including tensile strength, Young’s modulus and elongation at break are summarized in Table 4. It should be noted that all samples demonstrated an elastic stress-strain behavior and moreover, the resulted mechanical properties strongly depend on the nanosheet types. Introduction of unmodified and modified GOs into polyurethane matrices was accompanied by an increase in the elastic modulus and tensile strength due to their reinforcing effect. The WPU/SGO sample showed higher tensile strength and modulus values among samples. This behavior results from strong interphase interactions between homogenously dispersed SGO and polyurethane matrix, causing localization of mechanical stresses at the broad interface [34,52]. On the other hand, WPU/CGO exhibited poor tensile properties compared to WPU/SGO sample which can be assigned to poor interphase interactions of highly agglomerated CGO sheets, and thus localization of stresses 23
applied at less interphase area compared to WPU/SGO sample. Moreover, the WPU/GO sample showed remarkable lower modulus, tensile strength, and elongation at break compared with WPU/CGO and WPU/SGO samples due to presence of some coagulated particles as mentioned earlier.
3.3. Electrochemical studies In this work, the corrosion performance of uncoated and coated mild steels after one week exposure to 3.5 wt.% NaCl solution was studied by potentiodynamic polarization (PDS) technique and electrochemical impedance spectroscopy (EIS). Figure 12a shows the potentiodynamic polarization curves (Tafel plots) of uncoated and coated mild steels. The values of the Ecorr (corrosion potential) and Icorr (corrosion current) derived from Tafel plots are also summarized in Table 4. The %IE (inhibition efficiency) as a useful parameter to evaluate the anti-corrosion efficiency of WPU and WPU/GOs nanocomposite coatings was also calculated using the formula given below [66]: (1)
Where icorr(0) and icorr(i) are the corrosion current values for uncoated and coated mild steels, respectively. The calculated %IE values are given in Tables 5. According to obtained data (Table 5), Icorr values for the coated mild steels are much lower than uncoated steel with Icorr of 3.98E-7 A.cm-2. In addition, the Ecorr of the WPU and WPU/GOs coated mild steels shifted toward more positive potentials in comparison to uncoated steel with Ecorr of -0.39 V. This behavior indicates improvement in corrosion resistance of WPU and WPU/GOs coated mild steels compared to 24
uncoated steel which originated from the barrier effect of coatings on the surface of mild steels [67, 68]. Besides, because of the high barrier effect of GOs nanosheets, the anti-corrosion properties of WPU/GOs nanocomposite coatings were enhanced more effectively compared with the WPU coating [30]. Furthermore, it is evident that anti-corrosion properties of WPU/GOs strongly are influenced by the type of incorporated nanosheets. Among the WPU/GOs samples, WPU/CGO and WPU/SGO samples containing C4As modified GOs (CGO and SGO nanosheets) provide the best anti-corrosion properties on mild steel. The enhanced anti-corrosion properties of WPU/SGO (around -0.19 V in the Ecorr, 5.01E-9 A in the Icorr and IE of 99.2%) and WPU/CGO (around -0.18V in the Ecorr, 3.16E-9 A in the Icorr and IE of 99.8%) in comparison with WPU/GO (around -0.23V in the Ecorr, 3.15E-8 A in the Icorr and IE of 92.8%) can be attributed to lower coagulation and higher dispersion quality of SGO and CGO nanosheets within the coatings on surface of mild steels. However, in comparison with unmodified GO, the SGO and CGO nanosheets can act more effectively as physical barrier to impede the penetration of corrosive electrolytes at the coating/metal interface and subsequently enhance the corrosion protection ability of coatings on mild steels [5,44]. Moreover, the better anti-corrosion performance of WPU/CGO compared with WPU/SGO arising from its high hydrophobicity and microphase separation degree which restrict more strongly the penetration of corrosive electrolytes and hence improve the capability of WPU/CGO to passivate mild steel. < Table 5>
25
EIS measurements were also carried out after one week immersion of uncoated and coated mild steels in 3.5 wt. % NaCl solution to evaluate the anti-corrosion behavior of WPU and WPU/GOs coating on the mild steels. The Nyquist plot, Bode modulus plot and Bode phase plot of uncoated and coated mild steels are presented in Figure 12b-d. In order to analyze the EIS results and explore the mechanisms of the corrosion processes, the proposed equivalent circuits in Figure 13a and b were used to fit Nyquist plots by EC-Lab software. According to literature, the equivalent circuit of R(QR) showing in Figure 13a is appropriate to fit the Nyquist plot of uncoated substrate. This model displays mild steel as working electrode (WE), solution resistance (Rs), charge transfer resistance (Rct) and double layer capacitance (Qdl). The coated mild steel in corrosion solution includes two electrolyte/coating and electrolyte/substrate interfaces. Furthermore, the Nyquist plots of coated mild steels due to the coating layer were fitted using an equivalent circuit of R(QR)(QR) presented in Fig. 13b. In this model, it is postulated that the electrolyte homogeneously diffuses into coatings and causing evenly distributed reaction sites at interface. Considering equivalent circuit model proposed for WPU and WPU/GOs coated mild steel, Rc (coating resistance) and Qc (coating capacitance) are related to electrolyte/coating interface, Rct and Qdl are also related to the charge transfer reactions at the electrolyte/substrate interface [67]. The corresponding parameters derived from two used models are summarized in Tables 6. < Table 6> Rc and Rct are main parameters which generally consider as an indicator of anti-corrosion property and represent the resistance of coatings against electrolyte diffusion [14, 67]. According to EIS data presented in Table 6, the increase in Rct from 19846 Ω cm2 to 46327 Ω cm2 explains 26
that WPU coated mild steel is more resistant against corrosion compared to mild steel. It is worth noting that for WPU/GOs coatings, the values of Rc and Rct values were drastically larger than WPU coating which implies that GO, CGO, and SGO nanosheets greatly enhance the barrier properties of the coatings on the substrates in accordance with schematically shown in Figure 13c. Besides, based on Rc and Rct values, it is apparent that the corrosion protection performance of WPU/SGO and WPU/CGO is much higher compared to WPU/GO sample and furthermore these results are in agreement well with PDS results. It has also resulted that WPU/CGO sample exhibited higher anti-corrosion property than WPU/SGO sample. The anti-corrosion performance of WPU/GOs nanocomposites coating strongly depends on both actual contact area of electrolyte with surface of coatings before water diffusion and also the electrolyte diffusion through the coating to reach the substrate [69]. Regarding WPU/CGO coating, its highly hydrophobic surface is favorable to prevent absorption of water as a medium for the diffusion of electrolytes through the coating. Preventing water from being absorbed by the coating is proportional to impeding in diffusing of electrolyte through the coating, which can decrease significantly the rate of corrosion at the coating/substrate interface [69]. Moreover, for WPU/SGO and WPU/CGO, their superior storage stability and dispersibility of nanosheets in comparison with WPU/GO can deeply extend the diffusion pathway through the composite coating during water diffusion. However, the modification of GO by BC4A not only enhance the hydrophobicity of WPU/CGO coatings to prevent the absorption of water but also induce an improved barrier property against electrolyte diffusion through the coating to reach the substrate. The time constants appeared at high-frequencies range are generally attributed to the coating layers and moreover, the phase angles of near 90º in high frequencies demonstrate the 27
capacitance nature of the coatings [5]. For WPU coating without GOs, the phase angle values decreased dramatically at high frequency, revealing quickly deterioration of the WPU coating and its weak capacitance and barrier properties. Additionally, the tendency of WPU/GOs to reach phase angle of near 90º in highly frequencies demonstrated the capacitance nature of the composite coatings and also a lower delamination of coatings at the coating/substrate interface after one week immersion in the corrosive medium in comparison with WPU coating. The impedance modulus at low frequencies which representing the capability of the coatings to hamper the flow of current between anodic and cathodic areas increased for WPU/GOs nanocomposites which are in agreement with the tremendous increase of their radius in Nyquist plot and higher phase angle in high frequencies [70]. However, the increase in phase angle in high frequencies, the impedance modulus in low frequencies and the radius in Nyquist plot, all confirmed the significant improvement in anti-corrosion properties WPU/GOs coatings. It was also evident that in the high frequencies which attributed to the coating layer, the phase angle and impedance modulus values for WPU/SGO is lower than WPU/CGO. This inferior anti-corrosion property can be implied by the higher hydrophilicity of WPU/SGO coating than WPU/CGO which causes more diffusion of the electrolyte solution through the coatings [5]. Furthermore, Figure 14 describes the effect of immersion time on the impedance (log Z) at frequency of 0.1Hz. The results show that the impedance values decreased for WPU and WPU/GOs coatings with the increase of immersion time. This observation confirms the gradual diffusion of corrosive electrolyte into the coating which results from their deteriorated barrier performance. It is obvious that the WPU/CGO sample acts as the best coating with the lowest depression in anti-corrosion efficiency after three weeks immersion, while the WPU/SGO 28
coating has the lowest performance. The lower reduction in anti-corrosion efficiency of WPU/CGO nanocomposite coating after three weeks immersion, arising from the hydrophobic structure of CGO nanosheets which drastically impede the penetration of corrosive electrolyte and eventually restrict the coating delamination from the mild steel surface. Furthermore, the extent the electrolyte diffusion into the coating/metal interface depends on microscopic delamination of coating [43]. In addition, compared to other studies about WPU/reduced GO nanocomposite [5, 11], the WPU/CGO sample exhibited better performance with the lower reduction in anti-corrosion efficiency after three weeks immersion. However, according to the results, exploiting the modified CGO nanosheets to fabricate aqueous dispersion of WPU/CGO nanocomposite without any coagulation and flocculation, provides an efficient and desired longterm anti-corrosion coating compared to SGO and GO ones. Adhesion strength of WPU and WPU/GOs coatings was measured on the mild steel substrate before and after three weeks immersion in 3.5 wt.% NaCl solution and the results are presented in Figure 15a. The adhesion strength of coatings strongly depends on the nanosheets types in their composition. Before immersion, the pull-off adhesion strength of WPU coating without any nanosheet was 4.52 MPa, while adhesion strengths of 3.93 MPa, 4.62 and 5.32 MPa were obtained for WPU/GO, WPU/CGO, and WPU/SGO, respectively. Among coatings, WPU/GO has the lowest adhesion strength due to the presence of some coagulated particles as mentioned earlier. It can be obviously seen from Figure 15a that after 3 weeks immersion, the adhesion strength of the WPU sample is lower than GO, CGO, and SGO containing samples. It is apparent that WPU/SGO coating with a fine dispersion of hydrophilic SGO nanosheets tend to adhere more strongly to polar mild steel surface in comparison with hydrophobic WPU/CGO coating 29
[71]. Moreover, the WPU/SGO and WPU/CGO showed the lowest and highest adhesion strength after 3 weeks immersion, respectively. The lowest adhesion loss of WPU/CGO nanocomposite coating arising from the hydrophobic structure of CGO nanosheets which drastically impede the penetration of corrosive electrolyte and eventually restrict the coating delamination from the mild steel surface [43]. These results are in agreement with the results of anti-corrosion properties. Figure 15b demonstrates the oxygen gas permeability of WPU and WOU/GOs coating measured at room temperature under pressure of 10 bar. As the results shown in Figure 15b,
the
incorporation of nanosheets in WBPU coating causes a significant decrease in the oxygen permeability of the coatings. It can be noted that the incorporation of nanosheets into WPU coating can increase the path of the O2 gas molecules to permeate through the coatings [72]. The best O2 gas barrier performance was obtained for the WPU/SGO coatings with the best dispersion quality of nanosheets within the coating.
4. Conclusion The objective of this research was to prepare novel environmental-friendly aqueous dispersion of polyurethane nanocomposites based on p-tert-butyl calix[4]arene (BC4A) and sodium psulfonatocalix[4]arene (SC4A) modified GO nanosheets (SGO and CGO) as green corrosion protective coating. The covalently surface modification of nanosheets was confirmed by FTIR, XRD, TGA, SEM and wettability analysis. The modification of GO nanosheets not only overcome the coagulation and flocculation problem of WPU/GO nanocomposite dispersion but 30
also improve the mechanical properties of WPU/CGO and WPU/SGO nanocomposite films, attributing to homogeneous dispersion of CGO and SGO in the polyurethane matrices. Moreover, after one week immersion, the WPU/CGO and WPU/SGO coatings on the surface of mild steels due to a decrease in Icorr, increase in Ecorr, increase in Rc and Rct demonstrated more enhanced anti-corrosion properties compared to WPU/GO sample. Furthermore, anti-corrosion evaluation revealed that WPU/CGO has the best anti-corrosion activity because of its hydrophobicity which prevents the absorption of water and also improved barrier property against electrolyte diffusion. In conclusion, this novel waterborne polyurethane nanocomposite dispersion can be considered as efficient anti-corrosion coatings on mild steel with significant inhibition efficiency of 99.8%.
5. Acknowledgment The authors gratefully acknowledge the support of the University of Isfahan and the financial support from the Iran National Science Foundation (INSF) under grant agreement No 96003365.
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Figure captions Fig. 1 The synthesis schematic of the C4As modified GO nanosheets Fig. 2 The synthesis schematic of the waterborne dispersions Fig. 3 (a) FTIR spectra and (b) TGA thermograms of prepared GO, CGO and SGO nanosheets Fig. 4 (a) XRD diffractograms and (b) Raman spectra of GO, CGO and SGO nanosheets Fig. 5 (a) C1s XPS spectra and (b) HRTEM images of GO, CGO and SGO nanosheets 36
Fig. 6 SEM images of (a) unmodified GO, (b) CGO, (c) SGO, and (d) SGO with higher magnification. (e) visual representation of the storage stability of nanosheets in water after 24 h. (f) Zeta potential of aqueous dispersion of GO, CGO, and SGO at pH 7 Fig. 7 (a) FTIR spectra of WPU and WPU/GOs films, (b) 1H NMR spectrum of neat WPU film Fig. 8 XRD diffraction of WPU and WPU/GOs films Fig. 9 SEM images of WPU and WPU/GOs films Fig. 10 (a) TGA and (b) DTG thermograms of WPU and WPU/GOs films Fig. 11 DSC thermograms of WPU and WPU/GOs films Fig.12 The (a) Tafel plots, (b) Nyquist plots, (c) Bode modulus plots and (d) Bode phase plots of uncoated and coated mild steels in 3.5 wt. % NaCl solution Fig.13 The proposed circuit model for (a) mild steel and (b) coated mild steels Fig.15 Effect of immersion time on impedance value (log Z) at frequency of 0.1 Hz Fig.15 (a) The adhesion strength of prepared coatings on the surface of mild steel, (b) O2 gas permeability of WPU and WPU/GOs films
Table captions Table. 1 Code designation and formulation of prepared dispersions Table. 2 Some properties of the WPU and WPU/GOs nanocomposites Table 3. TGA, DTG and DSC results of WPU and WPU/GOs nanocomposites Table 4. Mechanical properties of WPU and WPU/GOs nanocomposites Table 5. The PDS results of uncoated and coated mild steels 37
Table 6. The EIS results of uncoated and coated mild steels
Tables
Table 1. Code designation and formulation of prepared dispersions Molar ratio Sample
Nanosheets (PTMO:IPDI:DMPA:BDO:HBD)
WPU
-
1:2:0.5:0.1:0.4
38
WPU/GO
GO
1:2:0.5:0.1:0.4
WPU /CGO
CGO
1:2:0.5:0.1:0.4
WPU/SGO
SGO
1:2:0.5:0.1:0.4
Table 2. Some properties of the WPU and WPU/GOs nanocomposites Samples
b
Brookfield viscosity (mPa.s)
pH
Storage Stability a
WPU
15.2
7.30
stable
WPU/GO
11.1
5.50
Not stable
WPU/CGO
15.7
7.00
Slightly stable
WPU/SGO
16.3
6.60
Stable
The storage stability during one week storage at room temperature.
Table 3. TGA, DTG and DSC results of WPU and WPU/GOs nanocomposites Sample
T10%
T50% (ºC)b
Tmax,1 (ºC)c
Tmax,2 (ºC) d
Tg (ºC)
Tm (ºC) 39
(ºC)a WPU
293
361
342
385
-61
191
WPU/ GO
285
362
344
393
-51
205
WPU/ CGO
287
359
343
391
-55
208
WPU/ SGO
283
354
343
389
-45
164
a
Temperature at 10% weight loss Temperature at 50% weight loss c Temperature with the maximum thermal degradation rate of hard segments d Temperature with the maximum thermal degradation rate of soft segments b
Table 4. Mechanical properties of WPU and WPU/GOs nanocomposites Sample code
Tensile strength (MPa)
Young’s modulus (MPa)
Elongation at break (%)
WPU
15.20 0.11
20.75 0.21
556 14
WPU/ GO
11.14 0.08
24.52 0.25
381 5
WPU/ CGO
23.71 0.23
30.18 0.22
426 8
WPU/ SGO
30 0.32
36.55 0.48
430 9
Table 5. The PDS results of uncoated and coated mild steels 40
Samples
Icorr (A cm-2 )
E (V)
Mild steel
3.98E-7
-0.39
-
WPU
1.99E-7
-0.32
50.0
WPU/ GO
3.15E-8
-0.23
92.8
WPU/ CGO
3.16E-9
-0.18
99.8
WPU/ SGO
5.01E-9
-0.19
99.2
IE (%)
Table 6. The EIS results of uncoated and coated mild steels Rs (Ω cm2)
Rc(Ω cm2)
Rct(Ω cm2 )
Qc (F cm-2 )
Qct (F cm-2 )
Mild steel
18.96
-
19846
-
53.15E-6
WPU
2045
234
46327
24.05E-9
22.06E-6
WPU/ GO
2167
1.43E6
4.68E6
0.18E-9
0.39E-6
WPU/ CGO
2781
2.57E6
13.47E6
0.13E-9
2.97E-6
WPU/ SGO
2368
2.19E6
7.21E6
0.14E-9
0.28E-6
Samples
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
Highlights:
WPU/GOs nanocomposites were prepared based on BC4A and SC4A modified GO nanosheets. Modification of GO overcomes the coagulation problem of the WPU/GO dispersion. Modification of GO also affords better mechanical properties to nanocomposites. WPU/CGO coating exhibited the highest adhesion strength after three weeks immersion. WPU/CGO coating showed the best anti-corrosion performance on the mild steel.
