3.2. Characterizations
UV-vis spectroscopy was applied to gain insights into the formation of the supramolecular network and its complexation with Fe
3+. UV-vis absorbance displays a strong peak at 395 nm, corroborating the existence of quinone motifs converted from catechols (
Figure 2a). There are two observable peaks contributing to the complexation of catecholato-Fe
3+. Theoretically, the maximal absorption values for individual mono-, bis-, and tris-catecholato-Fe
3+ complexes are located at ~759 nm, ∼575 nm, and ∼492 nm, respectively [
14,
15,
16,
17,
18]. In this case, the observable catecholato-Fe
3+ peaks are located at ~501 nm and ~622 nm, which are likely caused by the overlapping of the three individual peak signals. The peak with stronger intensity at ~501 nm signifies that the tris-complex is predominant, whereas the bis-complex is in secondary dominance and the mono-complex is quite minor in complexation.
The supramolecular network was further analyzed via Raman spectroscopy (
Figure 2b). The chelation of Fe
3+ ions by the C3 and C4 oxygen atoms of catechol can be represented by a band consisting of peaks in the range of 490~694 cm
−1, whereas the corresponding ring vibrations of catecholato-Fe
3+complexes output peaks near 1323 cm
−1 and 1484 cm
−1. Specifically, the peaks at 587 cm
−1 and 624 cm
−1 are assigned to catecholato-Fe
3+ complexation, and the other two peaks within this band are likely caused by the quinone-Fe
3+ complexes. As expected, the peaks related to Fe
3+-O chelation and ring vibration cannot be observed for PEA-dhba. However, apparent peaks are observed for PEA-dhba-Fe
3+ with 1:3 and 1:5 feed ratios, indicating the formation of catechol-Fe
3+ and quinone-Fe
3+ coordination. In particular, the intensity of the peak at 587 cm
−1 is much higher than that of the peak at 624 cm
−1, indicating the dominance of tris-catechol-Fe
3+ complexes in the polymer networks, which is in agreement with the UV-vis absorbance analysis. It can be also deduced from
Figure 2a that the complexation modes remain nearly identical at pH = ~5 and pH = ~8, despite the fact that researcher-reported pH can have an intense effect on complexation modes [
5,
19,
20]. This is presumably due to the inadequate amount of catechol motifs. The increase in the Raman intensity of the peaks at 587 cm
−1, 1323 cm
−1, and 1484 cm
−1 for PEA-dhba-Fe
3+ with 1:3 compared to 1:5 verifies this assumption.
It is universally acknowledged that the quinone form is likely to lead to coupling between catechol and quinone [
1,
4,
5,
21]; as a result, the oxidized PEA-dhba may experience chain extension. However, the GPC results show a nearly unchanged molecular weight and polydispersion index (
Figure 2c), which is evidence against the chain extension thought to be caused by dopa-quinone coupling. To this end, the predominant chemistry of PEA-dhba and PEA-dhba-Fe
3+ can be displayed in
Figure 2d,e. The catechol motifs dangling in the chain ends are sensitive to oxidation, even with trace amount of oxygen in the ethanol. Nevertheless, further transition to coupled catechols nearly does not take place. Fe
3+ ions are far more ready to perform complexation with catechol motifs, and the tris-complex is the dominant form, meaning that the individual PEA-dhba chain ends are assembled into supramolecular networks through coordination cross-linking. The SAXS results obtained from dried PEA-dhba-Fe
3+ bulk show a conspicuous scattering peak and ring (
Figure 2f). The inter-cluster spacing is calculated to be 3.70 nm, which can be considered to be phase separation between the soft PEA chains and coordination-induced clusters. It strongly confirms the formation of clusters of coordination that serve to construct physical cross-linking networks.
The chemical change of PEA-dhba and PEA-dhba-Fe
3+ can be reflected by its color. As shown in
Figure S2, the dark-brown solution of PEA-dhba is probably due to the formation of imine bonds. However, further addition of Fe
3+ ions turns the solution even thicker and darker, which indicates the derivation of catechol into quinone and the formation of a catecholato-Fe
3+ complex. This phenomenon was previously revealed by Marina Faiella et al., who showed that Fe
3+ promotes the oxidization of catechols to quinones and mutually forms a dark-brown complex [
22]. The change of rheological state is another sign of coordination. Unlike conventional waterborne adhesives, such as latex-containing high molecular weight epoxy, polyurethane, polyolefins, and so on [
3,
20,
22], PEA-dhba-Fe
3+ solutions in alcohols contain low molecular weight polymer chains. Whether or not the well-designed PEA-dhba-Fe
3+, created according to the above protocols, could evolve into elastic supramolecular polymeric bulk has to be inspected in terms of rheological parameters. As shown in
Figure 2g,h, the elastic and loss modulus of dried PEA-dhba-Fe
3+ at 1.0 Hz are measured to be 28,587 Pa and 43,274 Pa, much higher than those of PEA-dhba (0.82864 Pa and 0.52505 Pa) and PEA (0.93545 Pa and 1.8709 Pa). The results indicate that the rapid cross-linking between catechol groups of PEA-dhba and Fe
3+ ions significantly improves the mechanical properties. This is because the elastic modulus reflects the rigidity of the material, and the loss modulus represents the viscosity of the material. It is worth mentioning that the loss modulus of PEA-dhba-Fe
3+ is obviously greater than the elastic modulus, which means that the viscosity of PEA-dhba-Fe
3+ gives it its fundamental properties. This is because the catechol structure has strong adhesion. This phenomenon is also in line with our design.
Figure S3 shows the EDS mapping results of the cross-section of the PEA-dhba-Fe
3+ bulk sample. It is clear that the C, N, O, and Fe elements are evenly distributed in the testing area, indicating the uniform structure of the polymer sample.
3.3. Adhesion Properties
This work develops a strategy for designing an environmentally friendly adhesive liquid polymer adhesive, which is composed of non-toxic polymeric components and diluent, non-covalent cross-linking networks that give it elasticity, non-reactive molecular motifs, and strong adhesion. Catechol motifs have the remarkable ability to adhere to a broad variety of metal and metal oxide surfaces, which makes it an ideal anchoring group for surface modification [
2,
3,
4]. However, the binding strength to metal substrates is highly dependent on the number of its phenolic groups, which means its binding strength will drastically reduce when the catechol is oxidized to quinone and coordinated with the incorporated Fe
3+ ions (
Figure 2). Assuming that predominant catechols exist in their reduced form, the amount of residual phenolic groups in the four samples at different states is shown in
Table S2.
Lap-shear tensile strength tests were conducted by using dried PEA-dhba-Fe
3+ films (
Figure 3a) to adhere the aluminum substrates. By comparing the shear strength under different ratios and different pH values, it was found that the shear strength firstly increases and then decreases with the increase in the Fe
3+:dhba ratio and pH. With a pH fixed at 5, the highest shear strength was 1.13 ± 0.15 MPa, as the ratio of Fe
3+:dhba ratio was equal to 1:5 (
Figure 3b,c). This ratio was then selected for the subsequent investigation of the adhesive properties. By adjusting the pH value of the precursor PEA-dhba-Fe
3+ solutions, the highest shear strength reached 1.36 ± 0.14 MPa at pH = 8, which is approximate to that of seawater (
Figure 3d and
Figure S4). It explains that the cross-linking density of catechol-Fe
3+ and the amount of residual -OH are both optimal for binding under mild conditions. The tested steel plates indicate the mechanical strength and shear strength of the bulk (
Figure 3e). A low feed ratio and a high pH value lead to complete cohesive failure and low shear strength. The feed ratio at 1:5 and pH = 8 offer appropriate bulk mechanical strength that leads to the highest shear strength in spite of partial cohesive failure.
Then, we tested the shear strength of the lap-shear samples at 25 °C, 40 °C, 60 °C, and 80 °C for 15 min and 8 h, and the results are shown in
Figures S5 and S6. When treated at 80 °C for 15 min and 8 h, the highest shear strength can reach 1.01 MPa and 1.38 MPa, respectively, indicating that catechol-terminated PEA with Fe
3+ can be bonded very firmly, even in a short time. Bonding tests on different substrates show that the PEA-dhba-Fe
3+ has strong bonding ability to aluminum and steel (
Figure 3f,g), and minor adhesion to epoxy substrates, PET, and PVC. The emission of volatile organic compounds (VOCs) by adhesives is a major concern for consumers. Commercial adhesives developed with organic solvents or synthetic precursors release VOCs, which may cause undesirable skin and respiratory diseases. VOCs are released from commercial adhesives (i.e., the ERGO 0113 acrylic adhesive and the two-component epoxy adhesive) and PEA-dhba-Fe
3+ in vessels. A gas chromatograph equipped with a mass detector (GC-MS) was used to monitor the number of instantly released VOCs over time. As shown in
Figure 3h, areas of peak integration directly reflect the number of VOCs. It can be found that PEA-dhba-Fe
3+ released very few VOCs within the first 25 min, whereas the epoxy adhesive had an abrupt emission of certain chemicals at ~10 min, and the acrylic adhesive constantly intensively emitted VOCs within the 25 min. On the basis of these comparative results, PEA-dhba-Fe
3+ has significantly better environmentally friendly features. Hence, this liquid glue can be widely used in many industrial applications with consideration to its strong adhesion, low VOCs emission, and feasible operation.
The impressive adhesion to metal substrates in comparison to polymeric substrates indicates that PEA-dhba-Fe
3+ might have unique interfacial interactions derived from its molecular structure. The mechanisms of formation of the metal/polymer interfaces depend on three main features: the availability of reactive functional groups at the polymer surface, the nano- and micro-structure of the surface, and the valence of the metal adsorbate(s). Metals that easily form oxides, such as Al, Cr, Fe, or Ni, will form stable metal (M-)O-C covalent bonds; for instance, Al atoms attack carbonyls and the in-chain C=O of polycarbonate to from C=O-Al [
23,
24,
25]. It has been shown that the polymer/metal interface derived from coordination bonding, i.e., the electron donor (groups within organic components) and the electron acceptor (metal atoms or ions) produce a charge transfer to form an electric double layer, would produce desirable adhesion [
26].
3.4. XPS Analysis and Theoretical Calculations
We performed XPS characterizations and theoretical calculations in order to figure out the science behind this phenomenon. The interfacial interaction between PEA-dhba-Fe
3+ and metal substrates (i.e., iron and aluminum) can be qualitatively interpreted by the shift of binding energy in XPS spectra. This helps us to obtain a signal revealing the interface, which is revealed by evaporation and drying the adhesive in an oven. The signal was collected by electron-penetrating the coating layer for roughly 10 nm.
Figure 4a compares the spectra of Al 2p for the substrate matrix and the interfacial layer. The peaks at 74.4 eV and 71.7 eV correspond to the 2p electrons of Al
3+ in Al
2O
3 and Al atoms, respectively. The relatively stronger peak for Al
3+ than that of Al is caused by the feasible oxygenation of the surface of the aluminum plate under an ambient atmosphere. It can be notably observed that a downshift of binding energies for both Al
3+ and Al takes place when the excitation electron beam bombards the adhering surface. This is telling evidence of the formation of coordination bonds. Here, the oxygen-donating electron fills the empty orbital of Al
3+ and Al. One more interesting phenomenon is that the peak intensity for Al←O surpassed that of Al
3+←O, which indicates that the supramolecule of PEA-dhba-Fe
3+ thoroughly wetted the substrate and can permeate into the bulk Al. Likewise, the spectra of Fe 2p for the substrate matrix and the interfacial layer are shown in
Figure 4b. The Fe 2p core peaks for the bulk steel sample are split into two components because of spin-orbit coupling (Fe 2p
3/2 and Fe 2p
1/2) with an area ratio of 2 and present multiplet structures and satellites [
27]. It can also be observed that the peaks for 2p
1/2 and 2p
3/2 undergo obvious downshifts, giving them lower binding energies when they adhere to the interface sample, which also corroborates the formation of coordination bonds (Fe←O) between the steel substrate and PEA-dhba-Fe
3+. The O 1s spectrum is further deconvoluted in order to reveal the exact oxygen-containing groups involving the coordination. High-resolution scans of the O 1s for the drop-coated aluminum substrate (
Figure 4c) show the presence of C
Ph-OH/C-O-C and (-C=O)
quinone functional groups with deconvoluted binding energies at 533.0 eV and 531.4 eV, respectively. There are two peaks occurring at 533.8 eV and 532.2 eV when adjusting the cumulative fit to overlap the raw curve. They are attributed to the complexed form of C
Ph-OH/C-O-C and (-C=O)
quinone, namely, C
Ph-O-Al/C-O-C and (-C=O-Al)
quinone. A similar process happens to the drop-coated steel substrate (
Figure 4d), i.e., the peaks for C
Ph-OH/C-O-C and (-C=O)
quinone at 532.5 eV and 530.7 eV undergo an upshift of binding energies to 533.5 eV and 531.5 eV. To this end, the XPS results firmly demonstrate the existence of coordinative interactions at the adhering interfacial layer. The interfacial complexation serves to generate overwhelming adhesive force compared to other polymeric substrates.
It is revealed by the quantum simulation results that the quinone and catechol motifs spontaneously perform strong adsorption toward Al or Fe atoms. In this study, density functional theory (DFT) was used to calculate the electronic property and adsorption energy of Fe and Al (100) surface-adsorbed organic molecules (see supplementary chemical formula). According to the calculation of the adsorption energy, the adsorption energy of organic molecules to Al and Fe is −2.227 eV (
Figure 5a) and −1.439 eV (
Figure 5g), respectively. According to the bond energy manual, when the absolute value of the energy is higher than 1eV, the interaction is a strong interaction, namely, chemical adsorption occurs instead of a van der Waals interaction. Therefore, formation bonds take place between organic molecules and metals, which greatly contributes to the notable improvement of the adhesion to metal substrates. The charge density of the organic molecules adsorbed on the metal surface is shown in
Figure 5b,h. The yellow cloud symbolizes the area of withdrawing electrons, and the blue cloud symbolizes the area of donating electrons. It can be seen from the figures that both the Al and Fe atoms lose electrons after adsorbing organic molecules, while the counterpart organic molecules gain electrons. The transfer of electrons from metal to organic molecules is further demonstrated by the cross-sectional analysis of the charge density (
Figure 5c,i) in the adsorption region, where the red region is electron-enriched, while the blue region is the electron-poor region, further indicating that the electrons are transferred from metal to organic molecules. In addition, we calculated the electron localization function (ELF) and electrostatic potential distribution for the adsorption of organic molecules on the Al and Fe metal surfaces. As the redder color indicates a higher degree of localization, it is found that the degree of delocalization of electrons on the metal surfaces is significantly higher than that of the organic molecules (
Figure 5e,j), which further indicates the aggregation of electrons on the surface of organic molecules. The electrostatic potential distribution proves that the electrostatic potential of the metal surfaces is higher than that of the organic molecules (
Figure 5f,k). Regarding to the above analysis, the adsorption of organic molecules on the Al or Fe surfaces leads to the transfer of electrons from the metal to the organic molecules, and there is an obvious gain and loss of electrons at the adsorption site, which indicates that the metal and organic molecules form a strong ionic bond interaction.
According to the above results, a model revealing this environmentally friendly supramolecular glue can be explicitly demonstrated by the schematic illustration in
Figure 6. As for the PEA-dhba-Fe
3+ solution in ethanol, the amount of Fe
3+ ions is notably insufficient to perform complexation with all catechol and quinone groups, meaning that the system remains as a fluidic liquid, except that its viscosity apparently increases. After the ethanol is removed, this can result in a dark-brown elastomer, which is the consequence of the clusters formed by the complexation of Fe
3+ with catechol and quinone groups (
Figure 2f). The clusters act as a cross-linking network within the bulk and, as a result, the material exhibits the features of an elastomer. One more peculiar merit of this elastomer is that it can be feasibly processed in molten state or re-dissolved in alcohol solvents. As for the origin of adhesion to metal substrates, the abundance of remaining catechol and quinone groups plays a pivotal role in the construction of a polymer/metal interface with coordination bonds. Even though the transition of catechol moieties into quinone is unavoidable due to oxygenation, the resultant quinone groups do not have negative impacts on adhesion, but rather they readily perform interfacial coordination bonds, a kind of strong interaction that contributes to substantially improving the shear adhesive strength with reference to the conventional van der Waals interactions.