**3. Results and Discussion**

*3.1. Functionalization of the Epoxy Prcursor*

3.1.1. FT-IR Analysis

FT-IR investigation was performed to evaluate the best conditions to choose in the functionalization procedure. A key role is played by the catalyst triphenylphosphine (PPh3), which is necessary to promote the reaction between epoxy groups of the matrix (ECC) and the carboxylic groups of the rubber phase (R), as depicted in the reaction scheme of Figure 3.

**Figure 3.** Reaction mechanism supposed for the functionalization of the epoxy precursor: (**a**) nucleophilic attack of PPh3; (**b**) reaction of the intermediate with the carboxylic group of the rubber phase.

The presence of PPh3 is a necessary step to avoid phase separation phenomena between the precursor and the elastomer.

The spectroscopic analyses were carried out on the ECC-R-120 and ECC-R-160 liquid mixtures to investigate the functionalization reaction and the effect of temperature (120 ◦C and 160 ◦C, respectively). FTIR spectra of the epoxy precursor ECC, liquid rubber R, and ECC-R blend were compared with the spectrum of the ECC-R-160 blend (see Figures 4 and 5) and the spectrum of the ECC-R-120 blend (see Figures 6 and 7). The liquid blend ECC-R corresponds to the epoxy mixture precursor/elastomer raw before the heat treatments for 15 h. Focusing the attention on Figure 4, in the range between 1650 cm−<sup>1</sup> and 1850 cm−<sup>1</sup> (see inset on the left), the spectrum of the rubber phase shows two absorption bands for the ester carbonyl group, as a consequence of the hydrogen bond interactions established among the molecules of the liquid rubber [64]. In particular, the band at 1738 cm−<sup>1</sup> is ascribed to the free ester carbonyl group, while the band at 1710 cm−<sup>1</sup> belongs to the H-bonded carbonyl group, involved in the hydrogen bond interactions with the hydroxyl groups of the same rubber molecules. In the same range of wavenumber, the spectrum of the precursor shows the ester C=O stretching band, around 1730 cm<sup>−</sup>1, while the spectrum of the ECC-R blend displays broadband always at 1730 cm<sup>−</sup>1, which belongs to the carbonyl groups of both the components. In the same region of wavenumber, it is possible to observe the presence of a shoulder peak at 1780 cm−<sup>1</sup> that could be considered the experimental evidence of the functionalization reaction between the oxirane ring of the epoxy precursor and the carboxylic groups of the rubber phase. This effect is due to the presence of an electron-withdrawing group (hydroxyl group in β position) that can determine the shift of the carbonyl signal to higher values of wavenumber for inductive effect [65]. The peak at 1120 cm−<sup>1</sup> (see inset on the right of Figure 4), assigned to the C-O stretching of the secondary alcohol generated by the opening of the epoxy group during the functionalization reaction, supports the hypothesized mechanism.

Further confirmation of the occurred functionalization is deduced by the results depicted in Figure 5, in the range of wavenumber between 3700 cm−<sup>1</sup> and 3100 cm−1. ECC-R-160 sample shows an absorption band at 3350 cm−<sup>1</sup> ascribed to the –OH groups generated during the reaction. In addition, the absorption band at 3230 cm−1, ascribed to the hydroxyls of the –COOH group of the R elastomer, disappears after the functionalization reaction.

**Figure 4.** FTIR spectra of the precursor ECC (black curve), the liquid rubber R (green curve), the blend of ECC-R (red curve), and the blend of ECC-R-160 (blue curve), in the range 2000–400 cm<sup>−</sup>1.

**Figure 5.** FTIR spectra of the precursor ECC (black curve), the liquid rubber R (green curve), the blend of ECC-R (red curve), and the blend of ECC-R-160 (blue curve), in the range 4000–800 cm<sup>−</sup>1.

Similar considerations can be made for the ECC-R-120 system, as shown in Figures 6 and 7. Compared to the previous case in the spectrum of the precursor functionalized at 120 ◦C, the peak around 1780 cm−<sup>1</sup> is not detectable (see the inset on the left of Figure 6). The reason for this is probably attributable to not very effective functionalization obtained at 120 ◦C. To evaluate the effectiveness of the functionalization reaction at the two different temperatures (120 ◦C and 160 ◦C), a further investigation was carried out in the range between 850 and 950 cm−1, i.e., the area spectrum attributable to the oxirane ring of the precursor. As a consequence of the functionalization reactions, the peak at 913 cm−1, ascribed to the oxirane group of epoxy precursor, decreases in intensity (see the right inset of Figure 5).

**Figure 6.** FTIR spectra of the precursor ECC (black curve), the liquid rubber R (green curve), the blend of ECC-R (red curve), and the blend of ECC-R-120 (blue curve), in the range 2000–400 cm<sup>−</sup>1.

**Figure 7.** FTIR spectra of the precursor ECC (black curve), the liquid rubber R (green curve), the blend of ECC-R (red curve), and the blend of ECC-R-120 (blue curve), in the range 4000–800 cm<sup>−</sup>1.

More in particular, the reduction of the peak relative to the oxirane group was evaluated, normalizing the peak a 913 cm−<sup>1</sup> to the peak at 1435 cm−<sup>1</sup> associated with the CH2 stretching of six terms ring, which is assumed chemically unmodified during the reaction [66]. The ratio (R = Apeak 913/Apeak 1435) of the subtended areas was evaluated for precursor-liquid rubber system before and after the functionalization process, respectively. An algorithm based on the Levenberg–Marquardt method [67] to separate the individual peaks in the case of unresolved, multicomponent bands, was applied. To reduce the number of adjustable parameters and to ensure the uniqueness of the result, the baseline, the band shape, and the number of components were fixed. The minimum number of components was evaluated by visual in the section based on abrupt changes in the slope of the experimental line shape. The program calculated, by a non-linear curve fitting of data, the

height, the full-width half height (FWHH), and the position of the individual components. The peak function was a mixed Gauss–Lorentz line shape of the form, reported in Equation (3) [68]:

$$f(\mathbf{x}) = (1 - L)H \exp\left[ -4 \ln(2) \left( \frac{\mathbf{x} - \mathbf{x}\_0}{w} \right)^2 \right] + LH \left[ 4 \left( \frac{\mathbf{x} - \mathbf{x}\_0}{w} \right)^2 + 1 \right]^{-1} \tag{3}$$

where *x*<sup>0</sup> = the peak position; *H* = peak height; *w* = FWHH; *L* = fraction of Lorentz character.

The results of this deconvolution procedure for the functionalized precursor at 160 ◦C, in the above-mentioned ranges of wavenumbers, are shown, respectively, in Figure 8a,b. This procedure was repeated for the systems rubber-precursor before and after the functionalization process at 120 and 160 ◦C.

**Figure 8.** FT-IR spectrum of the ECC-R-160 sample; deconvolution relating to the region of the: (**a**) epoxy ring; (**b**) peak at 1435 cm−<sup>1</sup> associated with the CH2 stretching.

A reduction of 5.3% and 13.5% was found for ECC-R-120 and ECC-R-160 systems, respectively. This is further proof that the higher temperature value allows for obtaining a greater amount of bond between the carboxyl group of the rubber and the epoxy ring of the precursor, making the precursor functionalization more efficient. The most effective functionalization process affects the resin structure and consequently the thermal and mechanical properties, as described below.
