# Bitumen and Bitumen Modification: A Review on Latest Advances
## 2. Bitumen Polymers
### Thermoplastic Elastomers Modified Bitumen  
Theroplastic elastomers can resist permanent deformation over stretching, and elastically recovers once the load is removed [53; 60]. Generally, thermoplastic elastomers used as modifiers are block copolymers of mono- or di-olefins. As mono-olefins unit the styrene molecule is used as mesomer, whereas as diolefins the butadiene or isoprene unit are normally used. The corresponding elastomers are called styrene butadiene styrene (SBS), styrene isoprene styrene (SIS), styrene-ethylene/butylene-styrene (SEBS). Thermoplastic elastomer structure is generally linear and is characterized by weak inter-chain linkage. However, radial SBS copolymer are widely used as a bitumen modifier. Figure 4 represents the linear and radial SBS structures. As can be seen in Figure 4 the linear polymer is characterized by two polystyrene blocks linked to a two-polybutadiene block, while in the radial structure polystyrene blocks make a radial configuration around the polybutadiene block. This structure distinction is decisive because it characterizes the final performances of the blend. It has been noted that radial structure offers a higher system stability and a better redistribution of the elastic and viscous components of bitumen. The elastic and sturdiness feature of sytrenic elastomers are due to a three-dimensional lattice formed by the physical interlocking of the molecules. The three-dimensional vertexes of the lattice are formed by polystyrene blocks, which form separated domains in a rubber matrix formed by polybutadiene units. A schematic representation is shown in Figure 5.  
Figure 4: Representation of linear and radial SBS (reconstructed from [61] open access). Polystyrene blocks are responsible for the sturdiness of the polymers while polybutadiene blocks give elasticity. At temperatures above the glass transition of polystyrene (Tg = 100 \({}^{\circ}\)C) the latter softens with a consequent weakness of the PS domains. This allows an easier bitumen treatment. Upon cooling to room temperature, styrene blocks reaggregate to re-form rigid domains restoring sturdiness and elasticity. Once SBS polymer is added, some interactions happen between the bitumen and SBS. Polystyrene blocks swell by absorbing light components of bitumen, which causes the hardening of the bitumen [63; 64]. The repeatedly reported excellent properties, relatively good dispersibility in bitumen (dependent on the base bitumen composition), and acceptable cost have made SBS popular as a bitumen modifier. Many studies showed that the addition of SBS to the bitumen increases the softening point, decreases the penetration slightly, lessens the thermal susceptibility, increases the viscosity and decreases the Frass breaking point. It has also been observed that SBS moderates the increasing of stiffness due to the oxidation process simulated by short and long-term aging maintaining almost unaltered virgin bitumen properties also following RTFOT (Running Thin Film Oven Test) treatments. The SBS concentration plays a fundamental role in polymer-added bitumen properties. bitumen modification can be done by adding up to 7% of elastomers and, in some cases, different polymers can be used contemporarily. For soft and medium modifications polymer quantities are less than 4.5% (about 3% for soft modification and 4.2% for medium modification). For hard modifications added quantities are higher than 5%, while at low concentrations, SBS is dispersed as a discrete phase in bitumen, increasing the SBS concentration results in a phase inversion and two interlocked continuous phases are obtained: the bitumen rich phase and SBS rich phase typically shown in Figure 6. The latter is characterized by two sub-phases: one formed by a swollen polybutadiene matrix and polystyrene domains [63]. In spite of many proven benefits regarding the use of SBS copolymers for bitumen modification, they are still far from perfect.