At present, a widely used group of materials in road construction is polymer–bitumen compositions, also known as polymer-modified binders (PMB) [1,2,3]. In general, it is a composition consisting of a rationally selected ratio of petroleum bitumen, styrene–butadiene–styrene polymer, and plasticizer, if necessary. Depending on the specified requirements for polymer-modified binders, they may contain surfactants, antioxidants, and nano- and micro-dimensional additives [4,5,6,7]. All these substances are aimed at improving the quality of the polymer-modified binder. But at the same time, they significantly complicate the system, making it less stable and, as a result, more susceptible to the processes of delamination and aging [8].

The term “bitumen aging” in a generalized form combines the whole set of reversible and irreversible changes in its chemical composition, physical transformations, and changes in structural and mechanical parameters. These changes occur during the production of bitumen, its storage, transportation, technological processing, and operation, i.e., during the entire lifecycle of the bitumen binder. In modern studies, it is customary to separate short-term and long-term aging. The short-term aging process of bitumen takes place in a thin film of bitumen at high temperatures (150–200 °C). This short-term aging occurs when bitumen is connected to mineral material. Long-term or operational aging occurs at lower temperatures (less than 80 °C) but for a longer time during the operation of asphalt–concrete pavement. The short-term and long-term aging processes have different characteristics and differ in the rate of flow.

In the process of the short-term aging of bitumen, its group composition changes [9]. The number of oil fractions decreases, and the number of resinous–asphaltene fractions increases. These transformations occur because of oxidative reactions and the polymerization of light fractions, including their partial evaporation. It is possible to describe the ongoing processes in more detail using Semenov’s theory of chain reactions [10]. In the initial period of the short-term aging of bitumen, because of the interaction of hydrocarbons and oxygen in the air, peroxide and hyperoxide compounds are formed. These compounds are unstable, so they break down into free radicals. These trigger new chains of oxidative reactions. In the process of oxidative reactions, oxygen molecules are absorbed (embedded in chains), which leads to the destruction of high-molecular hydrocarbons (asphalt–resinous complexes). The oxidation reaction proceeds until the asphalt–resinous complexes turn into unsaturated chemical compounds, which are further polymerized, i.e., compacted, forming high-carbon compounds. Many studies [11,12,13,14] have reported that the short-term aging of bitumen leads to a natural change in its structure and properties. Viscosity, heat resistance, stiffness, and elasticity increase, but at the same time, there is a decrease in plasticity, which leads to an increase in fragility.

In the long-term aging of bitumen, in addition to chemical processes (oxidation) at low operating temperatures, physical processes also flow. These physical processes also change properties. These physical processes are associated with the formation of equilibrium supramolecular structures, leading to the hardening (solidification) of bitumen. There are several different theoretical descriptions of this process. Traxler and Coombs [15] reported that the physical aging of bitumen is associated with the manifestation of colloidal properties, i.e., the transition of bitumen from a sol structure to a gel structure. Gussfeldt [16] reported that the physical aging of bitumen during operation is associated with a change in the state of peptization of asphaltenes by maltenes. After that, a new adsorption equilibrium is established between the polar components of bitumen. The paraffin included in the bitumen crystallizes and, therefore, affects the hardening processes. In the process of spreading (remelting) bitumen, all these processes become reversible, but in the conditions of the operation process, these processes are irreversible.

The term “aging of polymer-modified binder”, as well as the term “bitumen aging” in a generalized form, combines the whole set of reversible and irreversible changes in its chemical composition, physical transformations, and changes in structural and mechanical properties occurring during the production of polymer-modified binders, its storage/transportation, technological processing, and operation, i.e., during the entire lifecycle of the polymer-modified binder. Polymer-modified binders are complex multicomponent systems in which polymer and other additives, if available, make a significant contribution to the aging process. Therefore, despite numerous studies, at present, there is no reliable mechanism for the aging of polymer-modified binders. Due to the different nature and chemical properties of polymers, complex mechanisms of interaction occur between bitumen with polymers and modified bitumen with mineral aggregate [17]. It is worth noting that there are many studies focused on the study of aging on the structure and properties of polymer-modified binders [18,19,20,21,22,23,24,25,26,27,28,29].

Rheological methods are widely used to establish the thermal properties of polymer-modified binders [21,22]. Fourier transform infrared (FTIR) spectroscopy is also widely used to determine the polymer content in the modified binder and the effect of the thermo-oxidative aging process on the bitumen and SBS copolymers [23,24,25,26]. Fluorescence microscopy (FM) is used to assess the degree of dispersion of polymer, as well as its effect on the aging processes and morpho-structural features of the modified binder [27,28,29]. Therefore, the authors in [18] studied the elemental composition of polymer-modified binders (with several varieties of polymer) aged by the RTFOT method. They found that the content of the carbon component increased in all the aged samples studied, but the authors reported that they received mixed results in the presence of hydrogen, sulfur, and nitrogen. No clear trends were found for carbon, hydrogen, nitrogen, and sulfur due to polymer modification. Therefore, the authors also concluded that the aging process of polymer-modified binders largely depends on the type and level of modification [20]. The researchers in [19] found that there is not enough polymer in bitumen to form a strong polymer mesh, so it is necessary to introduce additional crosslinking additives. It was also established that an excess of polymer and crosslinking additives will not always improve the properties and resistance of modified bitumen to short-term aging.

Currently, the use of nanoscale modifiers (carbon nanomaterials) as a structuring component for polymer-modified binders is widespread [5,7,30]. Carbon nanotubes (CNTS), fullerenes, and graphene are most often used to modify polymer–bitumen composites [5,31,32]. The aspect ratio of the length to the diameter of the carbon nanotube can be unusually high, in the order of 1·10⁷:1 or more. This ratio is the reason that all the properties of carbon nanotubes are extremely anisotropic (i.e., they depend on direction) and can be varied directionally, and is of great interest since it opens the possibility of directional formation of the structure of the material [33,34,35,36]. They are also used because they have unique physical and mechanical characteristics for the directional structure formation of building composites. Their use makes it possible to significantly increase the strength and structural stability of modified binders [5], but despite all the advantages of using carbon nanomaterials, they are characterized by a feature that prevents industrial application, namely that despite the achievement of the nanoscale parameter of carbon nanotubes and distribution uniformity in the binder, they tend to aggregate over time. This feature minimizes the initial effect of nano-modification. Therefore, studying the influence of carbon nanotubes on the aging of polymer-modified binders is an urgent task, especially in terms of the effect of the long-term aging of polymer-modified binders.

The aging process of polymer-nanomodified binders is complicated by the presence of SBS polymer and carbon nanotubes. When the bitumen is joined with the SBS polymer, its elastomeric phase swells due to the absorption of the maltenes fractions (oil fractions) [37,38,39], which results in two phases: a bitumen matrix (bitumen phase) and a polymer phase. Earlier, the authors of [5], using electron microscopy to study asphalt–resin complexes with nanotubes, installed the physical barrier effect, which is conducted as follows: the introduction of carbon nanotubes into bitumen leads to an increase in the dispersion of asphalt–resin complexes, resulting in the formation of structural elements (physical barriers) that prevent the coagulation of asphalt–resin complexes.