*3.2. Raman Spectroscopy and PCA Model*

The fingerprint region of Raman spectra for different SMI/oil nanoparticle coatings on paper is illustrated in Figure 3 (see also Table 1 for band assignments in Supplementary Materials). For clarity of presentation, only the spectra of the thick coatings are represented, while the thin coatings give similar results within the fingerprint region (of course, with higher intensity of substrate-related cellulose bands).

**Figure 3.** Raman spectra for SMI/oil coatings on paper.

The characteristic Raman bands for coating moieties (styrene, imide, and oil) and substrate (cellulose fibers) are clearly separated and allow for accurate characterization. The spectra have been normalized by integration of the region corresponding to the cellulose bands at 1170 to 1050 cm−1. The C=O ester bands for imide (1765 cm<sup>−</sup>1) and oil (1750 cm−1) are clearly resolved as distinct peaks. The band at 1655 cm−<sup>1</sup> represents cis conformation (C=C) of the fatty acids and naturally occurs in vegetable oils, in contrast with the trans-isomers at around 1680–1670 cm-−<sup>1</sup> in synthetic systems [23]. For fatty acids with the same chain length, the intensity at 1655 cm−<sup>1</sup> increases in parallel with the degree of unsaturation [24], as follows: HCO (saturated fatty acids obtained after hydrogenation); SfO, CaO, RO (monounsaturated fatty acids); CO, SO (polyunsaturated fatty acids).

The imide content and amount of free oil can be quantified from the ratios of integrated Raman band areas of imide I (1765 cm−1), oil (1655 cm−1) and styrene (1602 cm−1), after calibration with a fully imidized pure SMI coating (no oil), as calculated in Table 2. Based on the SMA grade with 26 mol % maleic anhydride, the calculated imide content should be compared to a theoretical maximum imide content (relative to styrene parts) of 35%. Depending on the oil type, the imide content is lower than the theoretical maximum, which can be explained by the interference of oil with the imidization reaction (reactive C=C oil) and consequently remaining ammonolyzed maleic anhydride. A progressively higher amount of unreacted or 'free' oil is present in the coating as the imide content lowers: the amount of free oil is low for polyunsaturated oil (SMI/SO, SMI/CO), intermediate for monounsaturated oils (SMI/RO, SMI/SfO, SMI/CaO), and extremely high for saturated oils (SMI/HCO).


**Table 2.** Quantification of SMI/oil coatings on paper based on Raman spectroscopy.

A model for the classification of the paper coatings based on principal component analysis (PCA) of the Raman spectra is presented in Figure 4. The analysis is used to define parameters that describe the variation in Raman spectra between the different coating types and illustrate major trends. The number of principal components (PC) included in the model is selected from a scree plot (Figure 4a). The variation in data points is most effectively described by a model with nearly zero residual variance and as few PCs as possible: the three principal components account for 97% of total variation (PC-1: 78%, PC-2: 12%, and PC-3: 7%), and the residual variance is very small for the higher-order components. The calibration model was evaluated by a leverage and residual analysis against PC-1, PC-2, and PC-3 to detect eventual outlier samples (Figure 4b). The samples with higher leverage are positioned further away from the mean value and have different characteristics: e.g., the leverage variation between the coating samples is somewhat larger for PC-1, while the leverage for PC-2 and PC-3 is almost the same. PC-1 consequently takes into account differences in oil type, discriminating between polyunsaturated, monounsaturated, and saturated oil. Only samples with high leverage and high residues would damage the model and should be removed from the model: as none of the samples simultaneously indicate high leverage and high residues, all samples fit into the model and each sample adds novel information to the model. The meaning of the different components in relation to the original Raman spectral bands can be recognized form the loading for individual PCs (Figure 4c): (i) the PC-1 (82% of the variation) includes specific characteristics related to the oil type (1655 cm<sup>−</sup>1); (ii) the PC-2 (10% of the variation) includes characteristics of the substrate (cellulose: 1120–1095 cm<sup>−</sup>1); and (iii) the PC-3 (4% of the variation) includes the characteristics of the organic phase (styrene: 1602, 1583, 1452, 1032, 1000, 620 cm<sup>−</sup>1; imide: 1765, 1329 cm−1). As such, single Raman bands make a major contribution to the individual PCs and can be used for statistical discrimination between the coatings. The two-dimensional score plots of individual PCs can be used for classification of the coated papers within the 95% confidence interval (Figure 4d). Test results of the graph include data for some representative samples from the calibration set (red points) and the validation set (blue points) with good agreement. From the score plot of PC-1 versus PC-2, the coated papers are classified according to the coating type in PC-1 and the coating thickness in PC-2. The discrimination of thick and thin coatings is related to the intensity of the Raman bands corresponding to the cellulose substrate, as reflected in PC-2. From the score plot PC-1 versus PC-3, the coated papers are classified according to the oil type, with polyunsaturated oils (SO, CO) having the highest PC values, monounsaturated oils (CaO, RO, SfO) having intermediate PC values, and the hydrogenated oils (HCO) having the lowest PC values. The latter classification is based on the relatively small variations in the organic composition of the coating (similar styrene content and slightly variable imide content), as reflected in PC-3.

**Figure 4.** Statistical classification of SMI/oil paper coatings by statistical principal component analysis (PCA) of Raman spectra: (**a**) residual variance plots; (**b**) leverage plots; (**c**) PC loading plots; and (**d**) score plots with 95% confidence interval.

#### *3.3. Raman Chemical Mapping (Average Intensities)*

The chemical surface maps of coated papers (top view on 5 × 5 mm2 surface area) with different SMI/oil compositions were recorded to study the surface coverage, homogeneity, and distribution of different chemical moieties within the coating layer.

The average intensity maps for thick and thin paper coatings are illustrated in Figure 5, with all maps having the same ordinate scale to make valid comparison between all SMI/oil coating types. The ordinate values are calculated as an average intensity value from the spectrum recorded at each (*x*, *y*) point. The surface maps consequently include data related to the intensities of the dominant Raman bands and illustrate the lateral homogeneity of the coating. As confirmed by the following more detailed analysis of single Raman bands, the ordinate intensities of average intensity maps can be related to the lateral distribution of organic coating moieties, with (i) high intensities (yellow to red) representing thick deposits of coating species and, (ii) low intensities (blue) corresponding to thin coating deposits and poor coverage. The surface areas with low average intensities consequently represent almost uncoated paper. For the thin coatings, inhomogeneities are recognized with striations parallel to the direction of the bar-coating. The average intensities are higher for thick coatings than for thin coatings as an indication for the surface coverage. The contrast in average intensities illustrate local heterogeneities: the coatings with high imide content and low amount of free oil are obviously most homogeneous, while the presence of a high amount of free oil is visualized as an island-like coating phase. The observations for average intensity maps can consequently be related to a combination of coating coverage and homogeneity, which depend on the imide content and amount of free oil. In order to confirm the significance of the average intensity maps and locations of oil, imide and styrene deposits, the features observed in the general surface maps are analyzed below in relation with specific Raman bands for the coating and paper substrate.

**Figure 5.** Raman maps (5 <sup>×</sup> 5 mm2) with average intensities for SMI/oil paper coatings.
