## 3 Results and Discussion  
### Characterization by Equilibrium Swelling, MDR, and DSC  
In Figure 1, the values of the maximum torque (\(M_{\rm h}\)) obtained by MDR measurements and of the crosslink density determined by equilibrium swelling experiments are reported as a function of the initial sulfur content for the IR and NR samples vulcanized at 140, 150, and 170 \({}^{\circ}\)C. At each vulcanization temperature (\(T_{\rm vulc}\)), \(M_{\rm h}\) progressively increases by increasing the sulfur content (Figure 1a), while it decreases with the increase in curing temperature, especially on passing from 150 to 170 \({}^{\circ}\)C. The same behavior can be observed for the crosslink density determined by equilibrium swelling (\(v_{\rm c,sw}\)), as shown in Figure 1b. Moreover, a linear dependence of \(M_{\rm h}\) on \(v_{\rm c,sw}\) can be found (Figure S1), ascribable to the increase in viscosity of the polymer network. In fact, a direct proportionality between \(M_{\rm h}\) and crosslink density is documented in the literature [8; 36]. It is worth noting that since the \(v_{\rm c,sw}\) values are in the range 2-8 \(\times\) 10\({}^{-5}\) mol/g, our samples can be considered as moderately crosslinked. The decrease in \(v_{\rm c,sw}\) by increasing \(T_{\rm vulc}\) can be explained with a reduction of the vulcanization efficiency, as clearly shown in Figure 1c. Indeed, at elevated curing temperatures, it is more likely that sulfur and sulfurating complexes undergo various uncontrolled side reactions at the expenses of the formation of elastically effective crosslinks. Possible side reactions lead to modifications of the polymer chains, including the formation of cyclic sulfidic structures and unreactive sulfidic accelerator-terminated pendant groups, as well as to the production of zinc sulfide [15; 16; 66]. Moreover, the curing temperature can also influence the amount and distribution of different crosslink structures in the polymer networks. As shown in Figure 2, at each \(T_{\rm vulc}\), the percentage of mono- and disulfide bridges progressively decreases by increasing the sulfur content, probably due to the concomitant decrease in A/S ratio from 3 (EV system) to 1 (SEV system), which favors the formation of polysulfide bridges [5; 10; 30]. Different trends can be obtained as a function of the curing temperature for samples with different sulfur contents. In particular, a decrease in the percentage of mono- and disulfide bridges occurs for samples containing 1 phr sulfur by increasing the vulcanization temperature, which is ascribable to the faster vulcanization kinetics and shorter curing times, which make the conversion of polysulfide bridges (formed at the initial stages of vulcanization) into more stable mono- and disulfide bridges less efficient [13; 14]. This effect is more pronounced when passing from 140 to 150 \({}^{\circ}\)C. For IR samples, at 2 phr sulfur the percentage of polysulfide bridges increases from \(T_{\rm vulc}\) = 140 \({}^{\circ}\)to \(T_{\text{vulc}}=150\)\({}^{\circ}\)C, and then it remains constant until 170 \({}^{\circ}\)C. At 3 phr sulfur, the trend of the percentage of polysulfide bridges with \(T_{\text{vulc}}\) shows a slight maximum at 150 \({}^{\circ}\)C. On the other hand, for NR samples, the percentage of polysulfide bridges shows a maximum at 150 \({}^{\circ}\)C at both 2 and 3 phr sulfur. The observed trends could be related to the decrease in vulcanization efficiency observed at high \(T_{\text{vulc}}\) and low A/S (Figure 1c), which causes a lower amount of sulfur to be available for the formation of crosslinks. An analogous behavior of the relative amount of polysulfide bridges by increasing the vulcanization temperature was also reported for NR by other authors [14; 15; 16] and ascribed to a combined effect of vulcanization kinetics and efficiency. In Figure 3, the values of \(T_{\text{g}}\) measured by DSC are reported vs. \(v_{\text{c,sw}}\) for all the IR (Figure 3a) and NR (Figure 3b) samples.