## 3 Results and Discussion
### Morphology and Optical Property of Coatings  
In order to confirm the hardness quantitatively and figure out the surface morphologies of the coatings, the coatings' Young's modulus and surface roughness were determined by atomic force microscope. Young's modulus describes the ability of a solid material to resist deformation. That is, the higher Young's modulus of the coating, the stronger its ability to resist bending and the greater its hardness. The black lines marked in Figure 4 are the center line average, which represents the average of Young's modulus of the coatings. The results shown in Figure 4 revealed that MXDA-OSER obtained the highest value of \(\sim\)10.8 GPa, while TM obtained a minimum value of \(\sim\)1.5 GPa. This result was approximately consistent with the scratch-resistance observation shown in Figure 3. The stability of the double cross-links structure was verified. Figure 5 showed the surface morphologies of the coatings. The corresponding root-mean-square deviation roughness (Rq) is given in  
\begin{table}
\begin{tabular}{c c c c c} \hline  & **0.5** & **1.0** & **1.5** & **2.0** \\ \hline DETA-OSER & 2H & 5H & 7H & 8H \\ MXDA-OSER & 3B & H & 6H & 8H \\ \hline \end{tabular}
\end{table}
Table 1: Pencil hardness of DETA-OSER and MXDA-OSER in different molar ratios: H:epoxy.  
Figure 3: Optical microscope images of the scratches on various coatings. (**a**) DETA-OSER, scratched with an 8H pencil. (**b**) DETA-OSER, scratched with a 9H pencil. (**c**) MXDA-OSER, scratched with an 8H pencil. (**d**) MXDA-OSER, scratched with a 9H pencil. (**e**) NH\({}_{3}\)-OSER, scratched with a 3B pencil. (**f**) TM, scratched with a 6B pencil.  
Figure 5. Spherical clusters on the surface could be observed as shown in Figure 5c,d. The roughness of NH\({}_{3}\)-OSER and TM (Rq = 2.024, 7.651 nm) was much higher than that of the DETA-OSER and MXDA-OSER (Rq = 0.316 nm, 0.274 nm). The result reflects that the DETA-OSER and MXDA-OSER had extremely smooth surfaces, which was attributed to their tightly linked molecular chains with double-cross-link structures. The smooth surface also greatly reduced the light scattering, resulting in a high transmittance of visible light. It is noticed that the scratch resistance of TM and NH\({}_{3}\)-OSER was much weaker. Therefore, the further investigation on these two coatings will not be carried out.  
According to Figure 6, the transmittance of the DETA-OSER was a little higher than that of bare glass. This slightly difference is caused by the refractive index. The refractive index and thickness of the coatings and glass, measured by spectroscopic ellipsometry, are given in Table 2. The result indicates that DETA-OSER can also be used as an antireflective coating. However, when using MXDA as a curing agent, phenyl was introduced into the molecular chains, which increased the density of particles in the molecular chains to a certain extent. Nevertheless, the large phenyl enhances the rigidity of the molecular  
Figure 4. Histograms of the [PERSON]’s modulus distribution of the coatings. (**a**) DETA-OSER. (**b**) MXDA-OSER. (**c**) NH\({}_{3}\)-OSER. (**d**) TM.  
Figure 5. The 3D surface morphology of the coatings. (**a**) DETA-OSER. (**b**) MXDA-OSER. (**c**) NH\({}_{3}\)-OSER. (**d**) TM.  
chains. Consequently, chains are more difficult to deform, curl and shift, which makes the coating extremely hard and ultra-scratch-resistant. Back to Figure 3c, there were no scratches, scratched with an 8H pencil, whereas in DETA-OSER, without phenyl, there were minor scratches on it. In a word, MXDA-OSER was more scratch resistant than DETA-OSER, but at the cost of a slight decrease in transmittance.