*3.1. Structure and Morphology of Organo-Clays and Epoxy-Clay Nanocomposites*

The XRD patterns of the parent sodium montmorillonite clay (Na+-PGW) and the two organoclays (Nanomer I.28E and I.30E) are shown in Figure 2.

As can be seen from the patterns and the d-spacing data in Figure 2, the hydrophilic Na+-PGW parent clay exhibits a basal spacing of 12.5 Å (corresponding to the interlayer distance of approximately 2.5 Å), due to the presence of water molecules (12 wt % moisture) in the region between the aluminosilicate clay layers (intragallery). The ion-exchange of Na<sup>+</sup> cations with octadecylammonium ions in both the organo-clay samples was as high as 93%–95% (determined by carbon analysis), resulting in a significant increase of the basal spacing (23.5–24.5 Å) for both Nanomer I.30E (exchanged with primary onium ions) and Nanomer I.28E (exchanged with quaternary onium ions). The relatively broader and less intense (001) peak in the XRD pattern of the organoclay I.30E compared to that of organoclay I.28E indicates that there was greater disorder of the intercalated layered structure and a broader distribution of basal spacings.

**Figure 2.** X-ray diffraction (XRD) patterns of the inorganic Na+-montmorillonite (Na+-PGW) clay and the organo-montmorillonites Nanomer I.30E and Nanomer I.28E modified by primary and quaternary octadecylammonium ions, respectively.

The structure of epoxy-clay nanocomposites, i.e., the degree of clay nanolayer intercalation or exfoliation within the bulk epoxy polymer, was studied by XRD and HRTEM experiments. The XRD results from the epoxy-(organo) clay nanocomposites are shown in Figure 3.

**Figure 3.** XRD patterns of glassy (EPON 828RS + D-230 Jeffamine) epoxy—clay nanocomposites with inorganic clay Na+-PGW and the two organoclays I.28E and I.30E; the weight percent of organoclay addition has been estimated on a silicate basis and was 3 and 6 wt %.

The XRD pattern of the epoxy composite prepared with the parent inorganic clay Na+-PGW (3 wt %) exhibited the characteristic peak (d-spacing of 12.2 Å) of the parent inorganic clay (the epoxy polymer is amorphous), indicating that no intercalation of the epoxy polymer between the clay nanolayers had occurred, as was expected, due to the hydrophilic nature of the inorganic clay surfaces. By comparing the XRD patterns of the nanocomposites prepared by the I.28E (modified with quaternary C18 alkylammonium ions) and I.30E (modified with primary C18 alkylammonium ions) organoclays, we showed that the latter organoclay enabled the formation of an exfoliated clay nanocomposite structure (indicated by the absence of XRD peaks due to ordered nanolayers) in contrast to the former organoclay, which induced the formation of a highly ordered intercalated structure. The actual presence of clay nanolayers in the nanocomposite specimen of I.30E was verified by the XRD peak in the range 60◦–65◦ 2θ, which was attributed to the crystalline structure of the aluminosilicate clay layers (not shown here for brevity). The basal spacing of I.28E increased from 24.5 Å in the powder organoclay to 32.2 Å in the nanocomposite, thus indicating that single or double layers of epoxy polymer chains were intercalated between the organo-functionalized clay nanolayers. The positive

effect of the primary alkylammonium ions versus quaternary ions was attributed to the catalytic effect of the acidic protons from the primary alkylammonium ions, which initiate polymerization within the gallery space, thus facilitating exfoliation of the nanolayers and their dispersion in the bulk polymer [18].

The structure of the nanocomposites and the dispersion of the clay layers in the coatings were further studied with high resolution TEM (HRTEM) images. Representative images of the glassy epoxy-clay nanocomposites prepared by 6 wt % organoclay I.30E and 6 wt % organoclay I.28E are shown in Figure 4A–C, respectively.

**Figure 4.** Representative TEM images of partially exfoliated glassy epoxy—clay nanocomposites prepared by 6 wt % I.30E (**A**,**B**) and of intercalated epoxy—clay nanocomposite prepared by 6 wt % I.28E (**C**). Scale bar = 20 nm.

The main difference between the I.30E and I.28E organoclays is that the first is modified with a primary C18 alkylammonium ion while the second is modified with the same alkylammonium ion but in its quaternary form. Although their powder XRD patterns seem very similar due to the successful incorporation of these two onium ions with similar size, when mixed with the epoxy resin a substantial difference in their dispersion has been identified, resulting in two different nanocomposite structures. The primary ammonium ions (I.30E) provide acidic H<sup>+</sup> ions which can catalyse the fast opening of the epoxy rings, thus enhancing the cross-linking polymerization of the epoxy monomers by the amine curing agents. As a result, a partially exfoliated clay nanolayer structure is formed with enhanced interfacial interactions. On the other hand, quaternary ammonium ions are significantly weaker proton donors. So, when in contact with the epoxide, they react with a slower rate thus resulting in an overall slower cross-linking polymerization and eventually leading to intercalated nanocomposite structures.

The above suggested clay dispersion and nanocomposite structures are supported by the XRD patterns, where in the case of I.30E no diffraction peaks can be observed even in the case of the 6 wt % filler (the actual presence of clay in the XRD specimens was verified by the high angle XRD peaks owing to the crystalline structure of the clay). On the other hand, clear distinct XRD peaks are observed in the pattern of the I.28E nanocomposite. From the increase of the d-spacing when compared to the pattern of the I.28E organoclay, it is clear that polymer chains have been inserted within the clay galleries and have induced a further broadening of the interlayer space.

The exfoliated or at least partially exfoliated clay nanolayer structure of the 6 wt % I.30E nanocomposite has been also verified by the TEM images (Figure 4). The presented images are representative of the bulk nanocomposite and show the presence of either isolated single clay nanolayers or bundles of ca. less than 7–8 layers, with varying interlayer distance. It is also known from the literature that, when the distance among the galleries is higher than 50–70 Angstrom and the number of layers is less than 8–10, no diffraction peak can be observed in the XRD. These observations apply also for the 6 wt % I.30E nanocomposite in our study. On the other hand, as can be seen in Figure 4C, the nanocomposite prepared with the organoclay I.28E consists mostly of bundles with more than ca. 20 highly oriented and stratified nanolayers, capable of inducing the XRD profile observed in the respective patterns.
