## 3 Results and Discussion
### Preparation and Characterization of Rubber Composites
#### 3.2.3 Dynamic-Mechanical Tests  
The mechanical properties of rapidly varying stress conditions over a broad range of temperatures of the material grades were studied using the DMA method. The storage modulus (\(E^{\prime}\)) and loss factor along the temperature range are shown in Figure 5a,b, respectively. The storage modulus reveals the elastic component of material behavior, which is related to the stiffness of the material. The peak of the loss factor corresponds to the glass transition temperature (\(T_{\text{g}}\)), and the peak intensity, as well as the width, revealing the damping properties of the material [80; 81]. Figure 5: Dynamic mechanical analysis of vulcanized NR-based composites containing either HSAG or HSAG-SP (**a**) Storage modulus deviation versus temperature; (**b**) Loss factor deviation versus temperature. \(E^{\prime}\) is affected by the presence of nanographite, independent of the functionalization: the storage modulus increases with the nanofiller amount, to a larger extent above the glass transition temperature. In the case of 5 phr- and 15 phr-filled nanocomposites, the functionalization with serinol pyrrole only slightly influenced the E\({}^{\prime}\) values, whereas a lower E\({}^{\prime}\) was obtained at 24 phr with HSAG-SP, even though the difference is still pretty low. The \(T_{\mathrm{g}}\) appears to be substantially unaffected by the nanofiller type and content, except for the nanocomposite containing 24 phr of filler: in the presence of HSAG-SP, the \(T_{\mathrm{g}}\) value was about 1.5 \({}^{\circ}\)C lower than that obtained with HSAG. The intensity of the loss factor peak clearly depends on nanographite type and amount. Such intensity decreased by increasing the HSAG amount. As to the filler type, larger damping was obtained with HSAG-SP, at all the filler contents. These findings are in line with what is reported in the literature for composites based on graphene related materials (GRM) and NR. Enhancement of \(E^{\prime}\) with GRM content, to a different extent, was found by using NR latexs; and thermally reduced GO, up to 4 phr (in prevulcanized natural rubber) [45], chemically reduced GO, up to 2 phr [43] or 5 phr [46], GO, up to 0.5 phr [54]. Particular enhancement of \(E^{\prime}\), of two orders of magnitude, was found with GO up to 5% in the composite. In this case, it was commented that GO acted as a physical crosslinker [51]. Indeed, the interpretation for the \(E^{\prime}\) enhancement is based on the hydrodynamic effect promoted by rigid inextensible filler particles and on their interaction with the polymer chains. The larger \(E^{\prime}\) increase, with respect to NR, observed in this work, in comparison with those commented above for composites based on reduced graphene oxides, has to be attributed to the larger content of HSAG (up to 24 phr). To explain the larger \(E^{\prime}\) observed in Figure 6 for the composite with 24 phr HSAG, a lower mobility could be hypothesized for the polymer chains trapped in the network of HSAG aggregates. In fact, the HSAG percolation threshold in a poly(isoprene) matrix was reported to occur at about 21 phr [39]. Less aggregates and more exfoliated layers are present in HSAG-SP based composites, as is shown by TEM pictures (discussed in the text below). It is also worth adding that HSAG-SP was reported to exfoliate in a water suspension [68]. Further explanation for the larger \(E^{\prime}\) of HSAG-based composites could be based on the consideration that graphene layers stacked in a crystalline aggregate are estimated to have larger volume than exfoliated layers. Hence, HSAG aggregates could be expected to give more mechanical reinforcement than exfoliated HSAG. Figure 6: Stress-strain curves of vulcanized NR-based composites containing either HSAG or HSAG-SP. The number of each composite is close to the corresponding curve. Moreover, the chemical structure of serinol pyrrole is shown close to the curves of composites based on HSAG-SP.