*3.2. Properties of Epoxy-Clay Nanocomposites*

The mechanical and thermomechanical properties of the pristine epoxy polymer as well as the glassy epoxy nanocomposites are summarized in Table 3.


**Table 3.** Tensile and thermo-mechanical properties of glassy epoxy–clay nanocomposites.

As can be seen from the data in Table 3, the glassy epoxy nanocomposites prepared by 3 wt % inorganic sodium MMT (Na+-PGW) exhibited slightly improved tensile strength and elastic modulus with a concomitant small decrease of the strain at breaking point. Although the XRD data (Figure 3) showed that the clay nanolayers were not intercalated by the epoxy resin in the (nano)composite sample, it can still be suggested that the observed changes in mechanical properties, with even 3 wt % filler, were derived from the interaction of the epoxy matrix with the external surfaces of the micro-particles of the inorganic clay. The absence of organic modifiers in the inorganic clays allows direct adhesion of the polymer chains to the stiff inorganic external surfaces of the clay particles. In the case of the epoxy nanocomposite prepared using 3 wt % of the I.28E organoclay (modified with quaternary C18 alkylammonium ions), the stress and elongation at breaking load were slightly reduced (the elastic modulus remained unchanged), while with the I.30E organoclay (modified with primary acidic C18 alkylammonium ion), the tensile strength was increased by 12% (with a slight reduction of elongation at the breaking load) and the elastic modulus increased by 10%. In the exfoliated structure of this nanocomposite, the nanolayers were highly dispersed, thus increasing the polymer volume fraction that was being affected by the presence of the stiff clay layers. In addition, the catalytic function of the acidic/primary alkylammonium ions in initiating the epoxy polymerization via the epoxy ring opening led to a stronger interaction of the polymer chains with the surface of the clay layers, compared to the case of the non-reactive quaternary ion modifiers such as those in the I.28E organoclay. An increase in the organoclay concentration from 3 to 6 wt % led to an increase in the elastic modulus with both organoclays (up to a 16% increase compared to the pristine glassy epoxy), with a concomitant decrease in the stress and strain at breaking load, most pronounced in the case of the I.28E organoclay. This behavior was typical for glassy polymer matrices such as the glassy epoxy resins, as increasing the stiffness of the nanocomposite (increase of the modulus) due to the effect of the clay nanolayers leads to more brittle materials that break at lower strain [27,28].

The storage modulus (DMA measurements, Table 3) of the glassy epoxy matrix at 40 ◦C (glassy region) changed slightly (±5%) with the addition of 3 wt % (silicate basis) of the inorganic sodium MMT, but increased remarkably with the addition of 3 wt % (silicate basis) of both the C18 alkylammonium modified organoclays (70% increase with the I.30E organoclay modified with primary C18 alkylammonium ions and 50% increase with the I.28E organoclay modified with quaternary C18 alkylammonium ions). Both the organoclays also exhibited substantial improvement of the storage modulus at 100 ◦C (elastic region) (≥60% increase compared to the pristine polymer). The *T*<sup>g</sup> generally showed a slight fluctuation (up to ±2 ◦C) compared to that of the pristine glassy epoxy polymer with the addition of the inorganic and organo-modified clays. The small decrease of *T*<sup>g</sup> in the case of the nanocomposite prepared by the I.30E organoclay can be attributed to the plasticizing effect induced by the chains of C18 alkylammonium ion modifier, which becomes more pronounced in this exfoliated clay nanocomposite compared to the intercalated structures formed with the I.28E organoclay. As mentioned above, in the exfoliated nanocomposite structure a larger volume of the polymer matrix is affected by the organo-modified clay surfaces due to the high dispersion of isolated clay layers. The direct interaction with the stiff inorganic layers increases the modulus of the nanocomposite while, on the other hand, penetration of the modifier's dangling chains within the polymer network reduces the *T*g. A greater decrease of the *T*<sup>g</sup> is observed as the organoclay loading increases, due to the increasing concentration of the C18 alkylammonium modifier within the polymer matrix. The storage modulus at 40 ◦C decreased with the addition of 6 wt % organoclay compared to the nanocomposites with 3 wt % clay, but it was still higher compared to that of the pristine epoxy polymer. The addition of a small percentage of organoclay (≤3 wt %) is improving the thermo-mechanical properties without causing a distortion of the epoxy crosslinked network in the glassy region. A further increase of the organoclay content (≥6 wt %) may cause defects in the polymer network, increase the stiffness substantially, and restrict the flexibility of the polymer chains (glassy region). While the temperature increases, a small degree of elasticity (polymeric chains movement) is induced in the system thus allowing the organoclay sheets to penetrate among the epoxy chains without interrupting the epoxy network (rubbery state). The result of this thermo-mechanical behavior is a small deterioration of the storage modulus values in the glassy region when increasing the organoclay content and in contrast, an increase of these values in the rubbery region.

The TGA curves of the pristine glassy epoxy polymer and the respective epoxy nanocomposites are shown in Figure 5.

**Figure 5.** Thermogravimetric analysis (TGA) curves of (A) glassy epoxy (EPON 828RS + D-230 Jeffamine) and epoxy—clay nanocomposites with (B) Na+-PGW, (C) I.30E, (D) I.28E (3 wt %, silicate basis), and (E) I.30E (6 wt %, silicate basis).

From the curves in Figure 5, it can be seen that the presence of the clay filler had a minor effect on the thermal stability of the epoxy polymer, which can be evaluated by comparing the percent weight loss at a certain temperature (for instance at 350 ◦C, as can be seen in the inset of Figure 5). This behavior can be attributed to the relatively low clay loading, at least for the nanocomposites with 3 wt % clay, which is not sufficient to significantly affect the thermal stability and decomposition rate of

the polymeric matrix. The addition of 6 wt % I.30 organoclay induced a slightly faster decomposition, as can be seen in the inset of Figure 5, possibly due to the acidic function of the primary alkylammonium ions of this organoclay, which can catalyze the pyrolysis/decomposition of the polymeric chains.

With regard to the barrier properties of the epoxy polymer and the epoxy-clay nanocomposites, a 30% and 40% reduction of oxygen permeability was observed for the nanocomposites prepared by the addition of I.28E and I.30E organoclays (6 wt %), respectively, compared to that of the pristine polymer (oxygen permeability: 105 cc mm/m2 day). The reduction of permeability with the addition of the inorganic clay was not significant.
