## 3 Results
### Benchtop Instrumentation: Confocal Raman Microspectroscopy (CRM)  
Before recording the dataset for the depth profiling analysis, the full-range spectra of each polymer were acquired (Figure 4a). Two characteristic and intense bands were selected for each polymer to evaluate thickness measurement reproducibility. \begin{table}
\begin{tabular}{c c c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multicolumn{10}{c}{**Thickness Measurement (\(\upmu\)m)**} & \multicolumn{10}{c}{**Mean**} & **Std. dev.** \\  & **1** & **2** & **3** & **4** & **5** & **6** & **7** & **8** & **9** & **10** & **11** & **12** & **(\(\upmu\)m)** & **(\(\upmu\)m)** \\ \hline PB-72\({}_{20}\) & 50.8 & 45.1 & 38.0 & 29.2 & 25.5 & 25.7 & 24.0 & 25.1 & 34.0 & 41.4 & 45.5 & 56.3 & 36.7 & 12.7 \\ PB-72\({}_{25}\) & 34.3 & 32.9 & 29.9 & 25.4 & 23.7 & 21.3 & 20.9 & 19.1 & 20.1 & 31.5 & 45.0 & 36.7 & 28.4 & 9.3 \\ PB-72\({}_{20}\) & 21.3 & 20.3 & 13.0 & 11.1 & 11.0 & 12.1 & 12.9 & 12.4 & 11.3 & 17.4 & 18.6 & 20.5 & 15.2 & 5.6 \\ \hline PB-44\({}_{25}\) & 34.3 & 34.8 & 30.7 & 28.4 & 22.0 & 20.5 & 18.5 & 18.9 & 23.7 & 24.8 & 45.8 & 42.4 & 28.7 & 10.0 \\ PB-44\({}_{10}\) & 22.5 & 17.6 & 14.0 & 12.4 & 12.1 & 12.0 & 18.9 & 14.4 & 16.3 & 16.2 & 11.3 & 15.4 & 15.3 & 4.2 \\ PB-44\({}_{6}\) & 7.3 & 7.0 & 7.4 & 8.4 & 8.4 & 8.3 & 7.4 & 9.6 & 8.4 & 8.3 & 8.4 & 6.9 & 8.0 & 1.4 \\ \hline \(\text{Zapon}_{20}\) & 7.9 & 8.0 & 6.9 & 7.1 & 6.8 & 7.0 & 9.3 & 6.9 & 7.0 & 6.7 & 7.1 & 7.0 & 18.8 & 5.0 \\ \(\text{Zapon}_{20}\) & 13.7 & 12.9 & 12.9 & 13.0 & 12.8 & 11.6 & 13.1 & 11.6 & 14.1 & 14.9 & 14.0 & 16.5 & 13.4 & 3.4 \\ \(\text{Zapon}_{20}\) & 7.9 & 8.0 & 6.9 & 7.1 & 6.8 & 7.0 & 9.3 & 6.9 & 7.0 & 6.7 & 7.1 & 7.0 & 7.3 & 1.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Measured thickness results from the OCT profiles. Standard deviation is informative of thickness inhomogeneity. Figure 2: Visible images of (**a**) PB-72\({}_{30}\) and (**c**) Zapon\({}_{20}\) samples, the black arrows indicate the area measured with OCT and the white dashed rectangles represent the region of the profiles (5.6 \(\times\) 1.50 mm\({}^{2}\)) reported in (**b**,**d**). The red arrows highlight the coating defects found in the PB-72\({}_{30}\) sample. For PB-72, the bands at 1725 cm\({}^{-1}\) and 1451 cm\({}^{-1}\), assigned respectively to \(\
u\)(C=O) and \(\delta\)(CH\({}_{2}\)) from methylene groups, were selected. To measure PB-44 films, the bands at 1451 cm\({}^{-1}\) and 812 cm\({}^{-1}\), \(\delta\)(CH\({}_{2}\)) from methylene groups and \(\
u_{\rm sym}\)(C-C) from the main chain, were employed [36]. For Zapon\({}^{\circledR}\) samples, thickness was calculated considering  
Figure 4: (**a**) Raman spectra from the three polymers and (**b**) the spectral depth profiling dataset from sample PB-44\({}_{25}\) (step of stage displacement 1 \(\upmu\)m, 45 spectra recorded in the \(z\) axis), reported here as an example. The 812 cm\({}^{-1}\) and 1451 cm\({}^{-1}\) bands selected for the thickness quantification are marked with arrows. Figure 3: 3D plots of the 12 measured points extracted across three OCT B-scans for (**a**) PB-72\({}_{30}\), (**b**) PB-44\({}_{25}\), and (**c**) Zapon\({}_{20}\) samples showing the shape of the layers measured. The average thickness values are reported in Table 2.  
the relative intensities of the bands at 1284 cm\({}^{-1}\) and 853 cm\({}^{-1}\) from \(\
u_{\rm sym}(\rm NO_{2})\) and \(\
u(\rm NO)\)[37]. After setting up the experimental conditions, a sequence of spectra was recorded by focusing the laser at different depths inside the coating until the disk had been completely crossed. An example of PB-44\({}_{25}\) depth profiling is shown in Figure 4b. The comparison between the apparent thickness data obtained with the two different selected Raman bands shows a negligible difference (\(>\)0.6 \(\upmu\)m) (Appendix A). This confirms the possibility of choosing a distinct band in case of signal overlapping in multilayer systems (e.g., Paraloid(r) B-72 and Paraloid(r) B-44 both having an intense band at 1451 cm\({}^{-1}\)).