*2.1. Materials*

The formulated samples are composed by a hosting toughened epoxy matrix consisting of the precursor "3,4-Epoxycyclohexylmethyl-3 ,4 -epoxycyclohexane carboxylate" (ECC—Empirical Formula C14H20O4) (see the chemical structure in Figure 1a) (Gurit Holding, Wattwil, Switzerland) and the hardener agent "Methylhexahydrophthalicanhydride" (MHHPA—Empirical Formula C9H12O3) (see the chemical structure in Figure 1b) (Gurit Holding, Wattwil, Switzerland), both used in a ratio 1:1. As toughening agent (5 wt% with respect to the total mixture) the liquid rubber Carboxyl-Terminated Butadiene Acrylonitrile Copolymer was employed (R, see the chemical structure in Figure 1c) supplied by Hycar-Reactive Liquid Polymers, with Mn = 3600, containing terminal carboxy groups (COOH content of 0.67 × <sup>10</sup>−<sup>3</sup> equiv/g of CTBN and 18 *<sup>w</sup>*/*w*% of CN). The compound triphenyl phosphine (PPh3, Merck KGaA, Darmstadt, Germany), added in an amount of 10 wt%, was employed as a catalyst to promote the functionalization reaction of the epoxy precursor (ECC).

**Figure 1.** Chemical structures of the: (**a**) epoxy precursor (ECC); (**b**) hardener (MHHPA); (**c**) liquid rubber (R).

Table 1 shows the amount in grams of each component to prepare 23.58 g of a complete mixture Ep-R-120 or Ep-R-160.

Considering the chosen composition, the first Nucleophilic attack by triphenylphosphine (see reaction scheme in Section 3.1.1 FT-IR Analysis ) opens 9.15 × <sup>10</sup>−<sup>3</sup> mol of oxirane rings, leaving 7.0 × <sup>10</sup>−<sup>2</sup> mol of oxirane rings still unreacted (not opened). After the first Nucleophilic attack of PPh3, the unreacted oxirane rings represent 88% of the initially available rings. The moles of the terminal carboxylic groups are 0.804 × <sup>10</sup><sup>−</sup>3. This amount (in mol) is slightly defective compared with the opened oxirane rings (9.15 × <sup>10</sup>−<sup>3</sup> mol). This choice has been made to avoid big domains of elastomeric phase in the resin (as highlighted later through SEM investigation). The question of the dimensions of the rubber domains has been a nontrivial issue that has been addressed before choosing the chemical composition of the epoxy mixture. Based on data already reported in literature [52] and authors' experience, a lower amount of triphenylphosphine (or a higher amount of rubber phase with respect to the same amount of PPh3) determines a higher dimension of the elastomer domains, causing local inhomogeneity in the order of hundreds or units of microns with consistent local inhomogeneity.

**Table 1.** Amount in grams of each component to prepare 23.58 g of a complete mixture Ep-R-120 or Ep-R-160.


The self-healing fillers 1.3-Dimethylbarbituric acid (DBA), 2-Thiohydantoin (T), and Murexide (M) (all purchased from Merck KGaA, Darmstadt, Germany) have been added in a percentage of 0.42 wt% (see Figure 2). The ability of these fillers to activate selfhealing mechanisms, based on hydrogen bonding interactions, was already demonstrated in previous work with a nano-charged resin based on a tetrafunctional epoxy precursor hardened with 4, 4-diamino diphenyl sulfone [47]. The matrix was loaded with conductive nanofillers, and the healing efficiency was evaluated for the formulation with 0.5% by weight of carbon nanotubes to confer functional properties to the resin. Generally, the presence of conductive fillers confers many functional properties to the hosting polymeric matrix [53–55]. In particular, electrically conductive fillers such as CNTs, graphene-based nanoparticles, or expanded graphite have been dispersed in optimized epoxy resins to impart them self-sensing for the damage monitoring [56,57] or for activating the anti/deicing function through the joule effect [58–60], or to make possible energy saving curing processes (electro-curing processes) of resins/composites [61], and for enhancing adhesive properties [62].

**Figure 2.** Chemical structure of molecules acting as self-healing filler.

*2.2. Formulation of Epoxy Samples*

2.2.1. Functionalized Epoxy Precursor

The functionalized precursor liquid blends ECC-R-120 and ECC-R-160 were composed of the epoxy precursor covalently modified with the rubber phase.

The two blends were obtained by mixing, under mechanical stirring, the epoxy precursor ECC, the elastomer R, and the catalyst PPh3 for a time of 15 h, at temperatures of 120 and 160 ◦C, respectively.

#### 2.2.2. Functionalized Epoxy Samples

The cured epoxy samples Ep-R-120 and Ep-R-160 were composed of the precursor functionalized at the temperature of 120 and 160 ◦C, respectively. They were obtained by mixing by magnetic stirring for 20 min at room temperature, the hardener MHHPA and the functionalized precursor (ECC-R-120 and ECC-R-160, respectively). After a degassing process for 2 h at room temperature, the mixture was polymerized in an oven by a curing cycle of 1 h at 80 ◦C, followed by 20 min at 120 ◦C and 1 h at 180 ◦C. To perform this curing cycle, the samples were placed in the oven for the first cure step, with the oven temperature at 80 ◦C. At the end of the cure treatment at 80 ◦C (1 h), a heating speed ramp was set to go from 80 ◦C to 120 ◦C at 10 ◦C/min. The sample's actual heating rate (verified with a probe) is 4 ◦C/min. The same thing happened for the transition from 120 ◦C to 180 ◦C.

In this paper, for comparison, the Ep sample, corresponding to the cured epoxy matrix, without the presence of the rubber phase, was prepared by mixing the ECC precursor and the hardener MHHPA and following the procedure previously described. Finally, the functionalized epoxy samples with the selected molecules, reported in Figure 2, were obtained by dispersing the self-healing filler in the functionalized precursor ECC-R-160 by ultrasonication process for 30 min at room temperature. The addition of the hardener and the curing cycle followed the same procedure described above. The obtained samples containing DBA, T, and M were labeled Ep-R-160-DBA, Ep-R-160-T, and Ep-R-160-M, respectively.
