3.1 Characterization of chemical structure
The FTIR spectra of V-CNT containing different CNTs contents are displayed in Figure 1. The mixture of epoxy/CNSL/CA before curing is depicted in Figure 1(a). The main characteristic peak of epoxy was observed at 915 cm-1, which was attributed to epoxide rings. The C−H and C−CH2 stretching vibrations of epoxide rings were also found at 2864 and 1453 cm-1, respectively (26). The absorption bands at 1007 and 912 cm-1 were assigned to the phenolic group of CNSL. The C=O stretching of citric acid and anacardic acid appeared at 1650 cm-1 (16,27). Figure 1(b) shows FTIR spectra of samples after the curing process. All samples display similar FTIR spectra. The peak intensity at 915 cm-1 remarkably decreased, which indicates the opening of the epoxide rings followed by crosslinking (16). The peak at 1734 cm-1 was assigned to C═O stretching of ester, which was formed by esterification between epoxy and the carboxyl groups of CA and anacardic acid (28). The 1112 cm-1 peak was attributed to C─O─C stretching of ether, which confirms the etherification between epoxy and hydroxyl groups of cardanol and cardol in CNSL (29). Figure 1(c) shows the peak shift from 3444 cm-1 to 3436 cm-1 after adding CNTs at various contents. This shift was due to H-bonding interaction between C═O group of CNTs and the OH groups of epoxy matrix during the ring opening polymerization reaction (30). The possible reactions of epoxy, CNSL, CA and CNTs are depicted in Scheme S1.
3.2 Thermomechanical properties of vitrimer nanocomposites
The relationships between storage modulus and temperature of V-CNT are revealed in Figure 2. The storage moduli at glassy state (0 °C) of V-CNT0, V-CNT0.1, V-CNT0.3, and V-CNT0.5 vitrimers were 5773, 6658, 7366 and 8602 MPa, respectively. This increase in storage modulus with increasing CNTs content was attributed to the reinforcing effect of the homogeneous dispersion of CNTs nanofiller (12). The cross-sectional micrographs of all samples were used to confirm the dispersion of CNTs without aggregation as shown in Figure 3. Saha and Bal (31) suggested that the carboxylic groups on CNTs could form covalent bonds with epoxy which enhanced the interfacial stress transfer and positively improved dispersability of CNTs. Figure 4 depicts the Tg which was determined from the peak loss modulus. The Tg of the V-CNT increased from 27.3 to 30.6 °C when 0-0.5 wt% CNTs were incorporated in the epoxy matrix. The increase in CNTs content improved the glassy storage modulus and Tg of the nanocomposites, which could be explained by the effect of CNTs reinforcement. Uniformly dispersed CNTs in the network obstructed the mobility of the polymer chains, leading to a higher resistance for deformation (32). Furthermore, the strong covalent bonding between epoxy and COOH groups on CNTs promoted dissipation of energy between CNTs and matrix, enhancing the thermal properties of the epoxy composite (30).
3.3 Stress relaxation of vitrimer nanocomposites
Figure 5 depicts the relationship between the normalized relaxation modulus (G/G0) and time at 70, 80, 90, and 100 °C for the V-CNT specimens with different CNTs contents. A relaxation time for viscoelastic fluid can be defined at the G/G0 of 0.37, according to Maxwell model (33). It can be observed that all specimens can reduce the G/G0 value to 0.37, indicating the completion of BER via esterification to transesterification in the specimens. Figure 6(a) shows the linear relationship between relaxation time and temperature. The transesterification activation energy can be obtained from the slope of the plot according to Eq. 3 (34). Moreover, the topology freezing transition temperature (Tv) of the vitrimer can be evaluated at ln(t*) value of 13.8 via extrapolation (35,36).
![](data:image/png;base64,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)
where t* is relaxation time (s), T is temperature (K), Ea is the activation energy (kJ/mol), and R is the universal gas constant.
Legrand and Soulie ́-Ziakovic (37) suggested that the stress relaxation of vitrimers containing functionalized fillers depended on two main phenomena, i.e. (i) the bond exchange of linkages between filler and polymer matrix and rearrangement and (ii) rearrangement of the filler in the polymer matrix. Figure 6(b) shows the activation energy and Tv of V-CNT with different CNTs contents. The Ea values ranged from 55.7 to 62.4 kJ/mol and increased with CNTs content. The increase of CNTs in the network hindered the polymer chain mobility, hence a higher energy was required for transesterification. This observation was in good agreement with a previous report on polymethacrylate vitrimer nanocomposites (38). The Tv values of V-CNT0-0.5 were in the range of -26.1 to -12.0 °C. This Tv was also observed to increase with CNTs in the specimens, which was related to the hindrance effect of CNTs on the chain mobility. The delay in transesterification rate due to the increasing CNTs therefore led to the increase in Tv (39).
3.4 NIR Self-healing ability of vitrimer nanocomposites
Dynamic thermosetting is a new class of polymers that has been developed to resolve the limitations (unrepairable and unrecyclable) of conventional thermosetting polymers. Figure 7 demonstrates the self-healing ability of V-CNT specimens before and after NIR exposure. After NIR exposure for 1 h, the scratch sizes were obviously reduced, particularly for V-CNT0.5. Since the photo-activation resulted in a higher thermal energy, transesterifications in the vitrimer network were initiated to dissociate the crosslinked network. This phenomenon allows the polymer chain diffusion to fill the scratch, resulting in self-healing (40). Without CNTs, the V-CNT0 showed self-healing because of the ᴨ-ᴨ interactions between polymer chains (41), but the self-healing ability was only 31%. The addition of CNTs improved self-healing ability of V-CNT up to 93%, three times as high as that of V-CNT0, as shown in Figure 8. The addition of CNTs in the vitrimer network directly promoted the photothermal conversion effect, leading to the higher thermal energy activating transesterifications (42). In addition, CNTs promoted heat dissipation in the polymer matrix to improve heat transfer throughout the network, according to their high thermal conductivity (43). Previously it was reported that an increase in CNTs content in the composites could physically retard the chain mobility, resulting in poor self-healing ability (44). The self-healing results in this work demonstrate the opposite, which is likely due to the functionalization of the CNTs. The CNTs in this work contain COOH groups, which increase the compatibility to bond with epoxide groups to create reversible linkages that participate in the self-healing process (45,46).
3.4 Self-healing efficiency of vitrimer nanocomposite coating
The Shore D hardness was measured to investigate the ability of the coatings to resist indentation using a durometer scale in range of 0-100, as shown in Table 1. A higher value represents a harder coating material. The hardness values of virgin samples increased with CNTs content and ranged from 27.53 to 30.13, which were comparable to those of bio-based epoxy cured with anhydride and organic acids (47) and epoxy/polycaprolactone copolymer (48). After healing, the hardness values slightly decreased for all samples. The self-healing efficiency of the coating increased from 94.34% for coated steel with V-CNT0 to 97.37% for coated steel with V-CNT-0.5. It is postulated that adding CNTs into the epoxy matrix was beneficial to the self-healing efficiency. Since V-CNT0.5 exhibited the highest performance of self-healing in coating, the vitrimer with this composition was used to coat steel plates for anti-corrosion tests.
3.5 Corrosion properties of vitrimer nanocomposite coating
Figure 9 illustrates the Tafel plots of bare steel and steel samples coated with V-CNT-0 and V-CNT-0.5 after immersion in 3.5 wt% NaCl for 7 days. The current (Icorr), corrosion potential (Ecorr), protection efficiency (P.E.) and corrosion rate (Rcorr) from the Tafel plots are summarized in Table 2. The P.E. and Rcorr values can be evaluated following Eq. 4 and Eq. 5 (49).
![](data:image/png;base64,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)
where I0corr and , Iccor (ampere cm-2) are the corrosion current of steel plate and coated steel plate with vitrimer nanocomposite, respectively.
![](data:image/png;base64,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)
where Rcorr is corrosion rate in milli-inches per year (MPY), Ew is equivalent weight of sample (g equivalent-1), K is a constant value (1.288 × 105 milli-inches per year/ ampere -cm-year), A is the surface area of sample (cm2), d is density of the sample (g cm-3) and is the corrosion current (ampere ).
The two main parameters, Icorr and Ecorr, were used to analyze the anticorrosion performance. The Icorr represents a corrosion dynamic rate involving the cathodic reduction of oxygen and anodic dissolution of metal ions whereas Ecorr indicates the corrosion resistance performance (50). The negative shift of Icorr and positive shift of Ecorr implies the enhancement in corrosion resistance with low corrosion dynamic rate (51). The anticorrosion performance was in the following order: coated steel with V-CNT0.5 > coated steel with V-CNT0 > uncoated steel. The higher P.E. and lower Rcorr of coated steel with V-CNT0.5 were also observed. The hydrophobic character of V-CNT0.5 (see supplementary information in Figure S2) repelled the NaCl solution, resulting in diffusion away from the steel substrate. In addition, the uniform dispersion of CNTs can change and hinder the diffusion pathway of oxygen molecules, chlorine ions and H2O molecules in the epoxy matrix (50). Harb et al (52) suggested that the negatively charged CNTs in a polymer matrix acted as a repulsive agent for chloride anions resulting in an increase in ionic resistance. Furthermore, the presence of CNTs with high aspect ratio increased the surface area for oxygen cathodic half reaction resulting in a decrease in over-potential for the reduction of oxygen molecule. At a low over-potential, the passive film formation might occur due to an increased metal dissolution at the anode in low corrosion current density (53).
The corrosion products of steel coated with V-CNT0 and V-CNT0.5 after immersion in 3.5 wt% NaCl solution for 7 days were characterized using XRD, as shown in Figure 10. The main corrosion products of coated steel V-CNT0 and V-CNT0.5 were found at 28.8o (Fe3O4), 30.1 o (Fe2(OH3)Cl), 35.4 o (α-FeOOH), 43.2 o (Fe), 57.1 o (Fe3O4) and 63.5 o (Fe). Differently, the peak at 31.9o (Fe2O3) was only observed in the steel coated with V-CNT0.5. The Fe3O4 peaks were derived from the steel substrate and corrosion products (6,54). This oxide film acted as a barrier to prevent oxygen from invading into the mediate layer (54). The proposed corrosion protection mechanism of the steel coated with V-CNT0 and V-CNT0.5 is illustrated in Scheme 1.