## 3 Results and Discussion  
### Results of Chemical Characterization Tests  
Results obtained from SARA analyses performed on the neat bitumen are synthesized in Table 3, which contains the average values of the fraction percentages calculated from the five most representative rods out of the 10 rods subjected to testing. Table 3 also lists the values of the colloidal instability index (I\({}_{\text{c}}\)), calculated as the ratio between the sum of asphaltenes and saturates and the sum of resins and aromatics. Such a parameter quantifies the expected degree of instability of the colloidal system constituted by asphaltenes micelles peptid by resins in the saturates and aromatics medium.  
It can be observed that the considered bitumen was characterized by a high percentage of aromatics, which changed only slightly as a result of ageing. Such an outcome suggests that the binder may be adequate for modification, with the presence of a relevant quantity of macromolecules available for interaction with added polymers [20]. The very low values of the I\({}_{\text{C}}\) parameter also indicate that the available bitumen possessed a stable colloidal structure, with a well-balanced combination of the four component fractions.  
FTIR spectra recorded for the neat bitumen, added polymers, and PMBs are provided in Figures 1 and 2. Highlighted peaks are those which correspond to styrene and butadiene blocks, located at wavelengths equal to 699 cm\({}^{-1}\) and 966 cm\({}^{-1}\), respectively [37]. Results for all PMBs were obtained from samples prepared with 4.0% polymer.  
With respect to the tests carried out on polymers, it can be observed that the two SBR products of different quality showed similar spectra, with an excellent matching of the positions of all peaks. On the contrary, greater differences were found for the PBR products, which exhibited variations in the shape of the spectra and non-negligible misalignments of some of the identified peaks.  
FTIR results obtained on the PMBs indicate that the two types of polymer exhibited different interactions with employed bitumen. This can be appreciated when referring to the butadiene peaks occurring for both types of polymers (regardless of their quality) at a wavelength of 966 cm\({}^{-1}\). In the case of the binders containing SBR or w-SBR, these peaks were clearly visible, whereas they could not be detected in the case of the binders modified with PBR and w-PBR. In particular, it was observed that these binders yielded absorbance spectra were similar to the spectrum of the neat bitumen in the entire range of wavelengths plotted in Figure 2 (between 600 cm\({}^{-1}\) and 1500 cm\({}^{-1}\)).  
The abovementioned findings can be interpreted by referring to the different solubility of the two types of polymer, which most probably led to the formation of different microstructures. In particular, it can be postulated that the employed SBR polymers (regardless of their quality) did not entirely dissolve in the bitumen matrix, thereby resulting in the formation of fine dispersed particles, the presence of which was associated to the previously discussed FTIR peaks. On the contrary, experimental evidence suggests that the PBR polymers were completely dissolved in bitumen, with the corresponding creation of a uniform continuous phase in which no butadiene peaks could be detected by means of FTIR analyses. These outcomes and explanations are consistent with the morphological observations performed in other research projects on PMBs containing SBR and PBR polymers by means of optical and fluorescence microscopes [11; 17; 38]. It should also be mentioned that several researchers observed that when employing high dosages of SBR, as a result of the partial miscibility of the polymer, phase separation phenomena may occur [6; 8; 10; 11; 14].