# Bitumen and Bitumen Modification: A Review on Latest Advances
## 3 Bitumen Chemical Modifiers
### Nanocomposite Modifiers  
Polymer/layered silicate nanocomposite (PLS) (aka polymer/clay nanocomposites) has been, and are still, widely used in polymer science research. Layered silicate can improve the polymer properties, such as mechanical properties, heat resistance, decreasing gas permeability and flammability, and increasing biodegradability [199; 200]. The most commonly used silicates for preparation of PLS nanocomposites are hectorite (M\({}_{\text{x}}\)(Mg\({}_{\text{6-x}}\)Li\({}_{\text{x}}\))\({}_{\text{S}}\)Si\({}_{\text{O}}\)O\({}_{\text{20}}\)(OH)\({}_{\text{4}}\)) and montmorillonite (MMT; M\({}_{\text{x}}\)(Al\({}_{\text{4-x}}\)Mg\({}_{\text{x}}\))\({}_{\text{S}}\)Si\({}_{\text{O}}\)O\({}_{\text{20}}\)(OH)\({}_{\text{4}}\)) that belong to the same general family of phyllosilicates. As shown in Figure 9 they have a crystal structure made up of layers composed of two tetrahedrally-coordinated silicon atoms fused to an edge-shared octahedral sheet of either aluminum or magnesium hydroxide. The layer thickness is around 1 nm, and the lateral dimensions of these layers may vary from 30 nm to several microns or larger, depending on the particular layered silicate. Due to the hydrophilic nature of layered silicates, it is very difficult to disperse them into polymer matrix and in most cases separation into two discrete phases takes place. The most common way to overcome this problem is to replace the interlayer cations (Na\({}^{+}\), K\({}^{+}\), etc.) with cationic surfactants, such as alkyl ammonium or phosphonium, preferably with long alkyl chains. The surfactants have a double role: (1) reducing the surface energy of the inorganic host and (2) improving the wetting characteristics of the polymer matrix. Additionally, their functional groups can react with the polymer matrix, and in some cases initiate the polymerization of monomers to improve the strength of the interface between the inorganic and the polymer matrix [202; 203]. Increasing interactions between the polymer and the layered silicate in PLS nanocomposites led to the dispersion at the nanometer level of organic and inorganic phases. As a result, nanocomposites exhibit unique properties not shared by their micro counterparts or conventionally-filled polymers. Furthermore, depending on the strength of interfacial interactions between the polymer matrix and layered silicate, three different types of PLS nanocomposites are thermodynamically achievable (see Figure 10):  
Figure 9: Structure of a 2:1 phyllosilicate (reprinted from [199] with the permission of Elsevier). * Intercalated nanocomposites: these are obtained by the insertion of few molecular layers of polymer into the layered silicate structure. From the crystallographic point of view this insertion occurs in a regular way. * Flocculated nanocomposites: conceptually, this is the same as intercalated nanocomposites. However, silicate layers are sometimes flocculated due to hydroxylated edge-edge interaction of the silicate layers. * Exfoliated nanocomposites: in these compounds the individual clay layers are separated in a continuous polymer matrix by an average distance that depends on clay loading. The preparative methods of PLS can be divided into three main groups according to the starting materials and processing techniques. 1) From solution: the layered silicate is first swollen in a solvent of the polymer, such as water, chloroform, or toluene. When the polymer and layered silicate solutions are mixed, the polymer chains intercalate and displace the solvent within the interlayer of the silicate. Upon solvent removal, the intercalated structure remains, resulting in PLS nanocomposite. 2) In situ polymerization: the layered silicate is swollen within the liquid monomer or a monomer solution so the polymer formation can occur between the intercalated sheets. Polymerization can be initiated either by heat or radiation, by the diffusion of a suitable initiator, or by an organic initiator or catalyst fixed through cation exchange inside the interlayer before the swelling step.