## 4 Results and Discussion
### Fourier Transform NMR Self-Diffusion Coefficient Test  
An advanced molecular investigation was conducted using NMR in order to analyze the chemical ageing processes and microstructural modification. As previously mentioned, Fourier transform (FT) NMR self-diffusion coefficient (SDC) allows insight into the bitumen microstructure by detecting the long-range mobility of the mixture constituents. The determination of motion over long distances, in comparison with ideal micelles, provides a sensitive probe for the state of aggregates [47; 51]. It is crucial to note that the SDC values of asphaltene molecules cannot be detected owing to the short transverse relaxation times of their protons; so, hereafter, the measured SDC values are related to the maltene phase. The observation of self-diffusion is based on the mobility of molecules. The motion of these molecules can be hindered due to the obstruction they face during their mobility in media. These typical data, observed where the motion occurs, can be considered as a fingerprint of the microstructural behavior. In this study, the SDC data for each sample were investigated within the temperature range of 90-130 \({}^{\circ}\)C, increasing in increments of 10 \({}^{\circ}\)C at a time, as per Figure 6. The SDC data are related to the maltene part since at this temperature the asphaltenes are solid, meaning that their signals are not visible on the NMR spectrum after a spin echo sequence. According to this chart, the SDC values for binders decreased with ageing at each specific temperature. This effect is due to the stronger rigidity in aged bitumen which unavoidably involves stronger intermolecular connectivity when compared to the less dense network of the virgin bitumen, where the asphaltene domains are poorly connected to each other. As a result of this aggregation-based process/modification, a progressive shift from the viscoelastic toward the liquid regime dominating the highest temperatures occurs. In this picture, the resin molecules, due to their amphiphilic nature, will tend to reduce the associative interactions between the asphaltene particles by interposing themselves between the asphaltenes and the maltenes. This phenomenon might be like that found in the deactivation of hydrophobic cross-links in hydrophobically modified polymers by surfactants [43]. The interaction of the apolar part of the resin (its apolar moiety) with the maltene phase drags the latter to more-hindered dynamics typical of the stiffened asphaltene-dominated structure. Another mechanism concurrently present may be the formation of direct interactions between the surfactant polar headgroup and polar parts of asphaltene. In fact, it has been recently highlighted that, in addition to polar and apolar interactions, further specific interactions between surfactants themselves with consequent peculiar self-assembly processes [52; 53] dictate the final overall aggregation pattern [54] and the (usually slowed-down) dynamics [55; 56]. As an overall result of the two processes above described, maltene molecules in aged binders showed lower SDC values. What remains to be seen is the reproducibility of this gradual difference between virgin and aged binders at even higher temperatures in future experiments. It is worth noting that the BPAV bitumen's SDC value is a little bit higher than that of BRTFOT-225. This result could be due to the different dimensions of the asphaltenes or different structural network morphology. The BRTFOT-225 asphaltenes are expected to be bigger and to provide a strong obstruction to the maltene molecules' mobility. This difference in behavior is also visible in both ageing techniques (RTFOT and PAV). This is an interesting finding, and it could be hypothesized that the FT-NMR-SDC technique can be used to investigate the effects induced by the ageing phenomenon in bitumen. In order to confirm this hypothesis, AFM morphological imageswere employed.