## 3 Results and Discussion  
### Formation Mechanism of the Double-Cross-Link Structure  
The simple fabrication process of the coatings is schematically illustrated in Figure 1. For convenience, GPTES, DETA and MXDA are replaced by simple graphics in Figure 1a. In general, Si-O-C\({}_{2}\)H\({}_{5}\) can be catalyzed by acids and alkalis to hydrolyze and produce silanol. At the same time, silanol can also be catalyzed to condense and produce Si-O-Si [4]. Aliphatic polyamine can also catalyze the hydrolysis and condensation of Si-O-C\({}_{2}\)H\({}_{5}\), as discussed in the next section. Low-temperature epoxy groups ring-opening reaction and organic network formation can be achieved by the use of amine curing agents. In principle, each active hydrogen in an amine is capable of opening and linking to one epoxy groups [42]. That is, 1 mol of DETA react with 5 mol of GPTES and 1 mol of MXDA react with 4 mol of GPTES. Therefore, as shown in Figure 1b, DETA and MXDA not only catalyze the hydrolysis of Si-O-C\({}_{2}\)H\({}_{5}\) and the condensation of silanol, but react with GPTES to form the double-cross-link network. As the reaction proceeds, the molecular chains gradually expand to achieve the much larger networks in Figure 1c,d (both amorphous from XRD, not present).  
For the sake of verifying whether the cross-linked network was achieved, the FTIR spectrum was applied to infer the possible groups in coatings. Figure 2 shows FTIR spectrums of NH\({}_{3}\)-OSER, MXDA-OSER, DETA-OSER and GPTES. Absorption peaks at -2920, -2853 and -1457 cm\({}^{-1}\) were observed corresponding to the C-H asymmetric, symmetrical stretching vibration and the in-plane deformation vibration, respectively [43]. This indicated that all the samples contained methylene. Moreover, the characteristic peak shown at -3292 cm\({}^{-1}\) could be attributed to the stretching vibration of hydroxyl [44], including hydrogen bonds, which indicated that the Si-O-C\({}_{2}\)H\({}_{5}\) in three coatings have hydrolyzed, whereas GPTES have not. Since the characteristic peak of O-H is very close to N-H, this might also suggest the existence of -NH\({}_{2}\). The characteristic peak shown at -910 cm\({}^{-1}\) could be attributed to the vibration of epoxy. This indicated incompletely reacted epoxy groups. In addition, in terms of the characteristic peak shown at -1094 cm\({}^{-1}\), which represented the Si-O stretching vibration [45], it could be confirmed that GPTES  
Figure 1: Schematic illustration of the OSER coating. (**a**) Simplification of reactant. (**b**) Reaction process. (**c**) DETA-OSER network. (**d**) MXDA-OSER network.  
contained unhydrolyzed Si-O-C\({}_{2}\)H\({}_{5}\) and this peak corresponds to the cross-link networks of Si-O-Si in the other three samples. Furthermore, the presence of amino was observed at \(\sim\)1649 and \(\sim\)1025 cm\({}^{-1}\), and was assigned to the symmetric N-H bending modes of \(-\)NH\({}_{2}\) groups and C-N stretching modes of R\({}_{3}\)N, respectively [45]. In other words, the open-ring reaction occurred between the GPTES and DETA or MXDA. On the contrary, GPTES was only catalyzed by ammonia to hydrolyze, but not ring-open. In summary, GPTES achieved the single Si-O-Si cross-link with the help of ammonia, but DETA and MXDA both catalyzed hydrolysis and condensation, and reacted with epoxy groups to produce the double cross-links Si-O-Si and R\({}_{3}\)N.