*3.4. Raman Chemical Mapping (Band Ratios)*

The intensity ratios of specific Raman bands related to the oil, styrene, imide, and cellulose components are further detailed to provide information on the local chemical composition and distribution of these elements over the paper coating.

The distribution of oil is evaluated from the intensity ratios of Raman bands for oil (1655 cm−1), relative to styrene (1602 cm<sup>−</sup>1) and cellulose (1095 cm−1), as illustrated in Figure 6: the styrene band is chosen as a reference band for the distribution of oil within the coating, while the cellulose band is chosen as a reference for the distribution of coating over the paper substrate. The progressive increase in oil at the surface for different SMI/oil coatings agrees with previous calculations (see Table 2). The band intensity ratios and surface coverage of oil is comparable for thin and thick coatings when expressing the relative intensity of oil versus styrene within the coating itself: indeed, the same coating dispersions for deposition of thin and thick coatings were used. Contrarily, the differences in oil exposure between thin and thick coatings become clear when using the relative band intensity of oil versus cellulose. The surface patterns and locations covered with oil in the coating and those covered with oil are relative to the paper substrate overlap. Interestingly, the inhomogeneities for thin coatings observed in the average intensity maps are not reflected in the oil distribution over the surface. It is obvious that the inhomogeneities of the coating relative to the paper substrate are not expressed when taking the coating (styrene) as a reference. However, homogeneous distribution of the oil is also observed when taking the substrate (cellulose) as a reference. Therefore, the oil seems to be homogeneously spread over the surface and provides a continuous phase. The coating defects in thin coatings with striations can likely be attributed to the viscosity of the coating dispersions, where the presence of a free oil phase locally enhances the formation of a continuous coating due to better flow properties.

The distribution of organic coating moeities is evaluated from the intensity ratios of Raman bands for imide (1765 cm<sup>−</sup>1), relative to styrene (1602 cm−1) and cellulose (1095 cm−1), as represented in Figure 7. The relative intensity of imide progressively decreases for different SMI/oil coatings, in agreement with previous calculations (see Table 2). The relative intensity of imide is comparable for thick and thin coatings when expressed against styrene, while the differences in imide between thick and thin coatings become clear relative to the paper substrate: the thick coatings are relatively homogeneous with even distribution of the imide, while thin coatings show striations at the same locations as observed in the average intensity maps. Therefore, the distribution of the organic imide phase plays a major role in the coating homogeneity. The imide phase is likely more rigid and viscous compared to the oil phase, with more risk of coating defects. There is good agreement in the surface maps of oil/cellulose versus imide/cellulose for the thin coatings, confirming that an amount of free oil is present between the imide phase and contributes to the formation of a homogeneous coating layer by 'filling' the non-imide zones as a binder. The complementarity of the oil and imide phases is visible for thin coatings with poly- and monounsaturated oils (e.g., SMI/CO, SMI/RO), where the calculated amounts of free oil are relatively small. The latter is less visible for thin coatings with polyunsaturated oils (e.g., SMI/SO) as there is only a limited amount of free oil, while it is partly visible for the coatings with unsaturated oil (e.g., SMI/HCO) as the oil coverage is much larger than the zones in between the imide phase.

**Figure 6.** Raman maps (5 <sup>×</sup> 5 mm2) with band intensity ratios illustrating exposure of oil at the surface for SMI/oil paper coatings (left two columns: oil/styrene; right two columns: oil/cellulose).

**Figure 7.** Raman maps (5 <sup>×</sup> 5 mm2) with band intensity ratios illustrating imide moieties at the surface for SMI/oil paper coatings (left two columns: imide/styrene, right two columns: imide/cellulose).

The distribution of organic coating moieties from the intensity ratios of Raman bands for styrene (1602 cm−<sup>1</sup> and 1000 cm−1), relative to cellulose (1095 cm−1), are shown in Figure 8. The maximum intensity of styrene bands is comparable for all SMI/oil coating types and does not strongly differ with the oil type, because all copolymers contain a fixed amount of 74 mol % styrene that remains inert during the synthesis of the SMI/oil nanoparticles. The differences in intensities of styrene therefore mainly correspond to variations in coating coverage. In particular, the effects of thin and thick coatings are visualized. The locations for coverage of styrene components represented by Raman bands at 1602 cm−<sup>1</sup> and 1000 cm−<sup>1</sup> are in agreement, but slight differences in intensity may be attributed to the specific orientation of the styrene parts. The styrene-related absorption bands are characteristic for either the aromatic C=C stretch (1602 cm<sup>−</sup>1) or the aromatic C–H bending (1000 cm−1). Their intensities depend on the bonding length of the π-conjugation between the vinyl group and benzene ring and change with local conformation of the aromatic styrene groups [25]. The styrene orientation in the SMI copolymer is induced by a specific molecular structure of the high-molecular-weight SMA precursor and induces nanoparticle formation through self-assembly [26]. Therefore, the variations in ratio of both styrene bands at 1602 and 1000 cm−<sup>1</sup> can eventually be attributed to configuration or orientation effects: the influences of segment orientation in styrene bloc copolymers with polarized Raman scattering were detected [27]. In particular, the locations corresponding to the aromatic parts (1602 cm−1) are more confined that the aliphatic parts of the styrene, as the aromatic styrene groups are likely more influenced by self-organization within the nanoparticles.

**Figure 8.** Raman maps (5 <sup>×</sup> 5 mm2) with band intensity ratios illustrating styrene moieties at the surface for SMI/oil paper coatings (left two columns: 1602 cm−1/cellulose, right two columns: 1000 cm<sup>−</sup>1/cellulose).

In conclusion, the locations for styrene coverage strongly overlap with the previous locations for imide coverage (see Figure 6). The chemical mapping for the band ratios of organic coating moieties (imide and styrene) generally corresponds with the chemical mapping based on average intensities: the areas with high intensity in styrene (see Figure 7) overlap with the areas of high average intensity (see Figure 4). In case of coatings with polyunsaturated oils (SMI/SO, SMI/CO, SMI/SfO), the surface coverage with high intensities of band ratios for oil moieties (see Figure 5) overlap with the areas of styrene coverage (see Figure 7), as the oil is largely encapsulated in the organic parts. For coatings with monounsaturated oils (e.g., SMI/CaO), the surface areas with high intensities of oil moieties (see Figure 5) are complementary to the surface areas with high intensities of styrene moieties (see Figure 7), as there is a large amount of free oil in between the organic coating phase.

### *3.5. Raman Chemical Mapping (Single Bands)*

The surface maps with intensities of single Raman bands provide direct chemical information on the surface composition of coated papers. As only a single band is considered, the spectra are first normalized (cellulose region 1170–1050 cm<sup>−</sup>1) and baseline-corrected before selecting a specific band. An example of chemical surface maps for different single Raman bands of thick and thin coatings of SMI/SO and SMI/HCO is shown in Figure 9 (the maps for other coating types are given in Figures S2–S5 in Supplementary Materials). The selected Raman bands provide complementary information on the surface coverage of different chemical components.

The surface areas covered with high intensities of the styrene band (1000 cm−1) are similar to those covered with high intensities of the imide band (1765 cm−1). The same surface areas have also been covered by high intensities in previous surface maps with band intensity ratios of organic coating moieties. The surface maps of the cellulose bands (1378 cm−<sup>1</sup> and 1095 cm−1) are complementary to the holes observed in the surface maps with organic coating deposits, indicating that the paper is locally penetrating through the coating at those locations. Both cellulose bands cover the same surface areas and exposure of cellulose bands at the surface is more intense for thin coatings than for thick coatings. The intensity of the single band related to hydroxyl groups (3100 cm<sup>−</sup>1) is highest at locations where cellulose fibers are exposed and lowest at locations covered by the coating. The exposure of cellulose hydroxyl groups at the surface is evidently more intense for thin coatings than for thick coatings. Therefore, interactions between the coating and the paper substrate are expected to happen through hydrogen bonding between hydroxyl groups at the cellulose fibers and the imidized SMI/oil nanoparticles. In a search for other complementary bands, the locations with low intensities in the 1765 cm−<sup>1</sup> band (imide) correspond to locations with high intensities of the 1565 cm−<sup>1</sup> band. The latter band can be assigned to the amide II band (NH bend + CN stretch) and represents amic acid groups as a remaining intermediate product after imidization (i.e., ammonolyzed maleic anhydride). The non-imidized parts obviously induce minor local defect spots and influence the formation of a fully homogeneous coating layer. These parts do not interact well with the paper substrate as they appear at similar surface areas with high intensities of hydroxyl bands. The surface areas with high intensities of the single oil band (1655 cm−1) fully overlap with the imide zones for coatings with polyunsaturated oil (e.g., SMI/SO), corresponding to previous conclusions that oil is embedded in the organic phase and there is very little free oil. The overlap of oil coverage and imide locations is much less for coatings with monounsaturated oil (see Figures S3–5 in Supplementary Materials). In particular, the oil coverage largely exceeds the locations with imide deposits in the case of unsaturated oil types (e.g., SMI/HCO) due to the large amount of free oil.

**Figure 9.** Raman maps (5 <sup>×</sup> 5 mm2) with single wavenumbers representing different coating and substrate components at the surface for SMI/oil paper coatings.

The surface maps with simultaneous coverage of the different coating components have been constructed after image processing (Figure 10). The summation images with imide and oil coverage ("imide ADD oil") illustrate the degree of overlap and inclusion of oil within the organic coating phase, with following color code (see online for color figures: red = oil, black = imide). The quantification of thick and thin coatings is visible from the surface areas covered with the red phase. Almost full inclusion of the red surface areas within the black surface areas is recognized for SMI/SO, SMI/MO, and SMI/RO coatings, while the amount of red surface areas outside the black surface areas increases for SMI/SfO, SMI/CaO, and SMI/HCO coatings, which is in parallel with the localization of free oil. The difference in images between oil and imide ("oil SUBTRACT imide") directly illustrate the surface locations with free oil, following the color code (see online for color figures: blue = free oil). From the blue surface areas it can be observed that the amount of free oil increases in the formerly reported order of SMI/oil coating types, and is higher for thick coatings than for thin coatings. In conclusion, the processed images and original Raman surface maps allow us to accurately visualize the different components and local chemical composition at the coated paper surface, and systematically illustrate the effect of oil encapsulation depending on the degree of saturation of the vegetable oil.

**Figure 10.** Processed images of single Raman wavenumbers for imide (1765 cm<sup>−</sup>1) and oil (1655 cm−1), illustrating lateral distribution of oil within the organic phase (+) and free oil (−).

#### **4. Conclusions**

Dispersive Raman spectroscopy and microscopy provide an efficient analytical tool for the characterization, discrimination, and quantification of thick and thin barrier coatings on paper with nanoparticle deposits of poly(styrene-co-maleimide) that are filled with six different types of vegetable oils, soy oil, corn oil, rapeseed oil, sunflower oil, castor oil, and hydrogenated castor oil. Depending on the degree of oil saturation (iodine value), the imide content gradually decreases and the amount of free oil gradually increases.

A statistical classification of the coatings can be made by the partial least squares model with three principal components (PC), as each of them include information about the different coating components, i.e., oil phase (PC-1), cellulose substrate (PC-2), and the organic coating components (PC-3). The overall classification of the paper coatings in score plots is based on the degree of saturation for the different oils and coating thicknesses.

The chemical surface maps with average intensities illustrate the coating coverage and homogeneity, where high amounts of free oil result in thick deposits with island-like locations. The thin coatings often result in the formation of striations parallel to the direction of coating, and can mainly be related to local deposits of the organic (imide) coating components. The presentation of free oil in

different amounts depending on the coating type is complementary to the average intensity maps. The chemical surface maps with relative band ratios have been analyzed relative to the organic coating phase and the substrate. The distribution of styrene and imide over the surface agrees fairly well and largely corresponds with the chemical mapping based on average intensities. For coatings with polyunsaturated oils, the surface coverage with high intensities of oil overlaps with the areas of organic coating components, as the oil is largely encapsulated in the organic parts. For coatings with monounsaturated oils, the surface coverage with high intensities of band ratios for oil is complementary to the surface areas with organic coating components, as there is a large amount of free oil as a binder in between the organic coating phase.

In conclusion, the selection of specific oils incorporated in poly(styrene-co-maleic) anhydride allows us to create various distributions of chemical moieties at the surface. Depending on the type of oil chosen, the amount of imide, styrene, and free oil at the coating surface varies. As the resulting hydrophobic properties and barrier performance depend on both the chemical surface composition and topography, the coatings are expected to serve as a candidate for barrier coatings in future packaging applications.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2079-6412/8/5/154/s1, Figur S1: Reference SEM image of uncoated paper and the same magnifications of the coated paper samples; Figures S2–S5: Raman maps (5 <sup>×</sup> 5 mm2) with single wavenumbers representing different coating and substrate components at the surface for SMI/oil paper coatings, Table S1: Raman shift assignment for SMI/oil coatings on paper.

**Acknowledgments:** The author acknowledges the support of Mr. Peter Mast (Ghent University—Department of Materials Science and Engineering, Belgium) with FEG-SEM. Samples were kindly supplied by Topchim N.V. (Wommelgem, Belgium).

**Conflicts of Interest:** The author declares no conflict of interest.
