## 1 Introduction  
Recently, polymer-clay nanocomposites have attracted considerable interest from researchers due to their tremendous properties and broad applications [1; 2; 3; 4; 5]. By introducing a low clay content (usually 1-10 wt %) into a polymer matrix, these nanocomposites can achieve many unique properties, such as reduced thermal diffusion coefficients, increased barrier characteristics, high heat-distortion temperatures, decreased gas permeability and enhanced mechanical properties, compared with conventional composites or pure polymers [6; 7; 8; 9]. The factors impacting the efficiency of these nanocomposite materials include the aspect ratio of the clay layers, the dispersion quality and the interfacial adhesion between the polymer matrix and the clay layers [10; 11]. Unique and improved properties are often observed when the dispersed clay layers were less than 100 nm thick. Thus, many techniques have been evaluated to achieve homogeneous dispersions of ultrafine silicate clay layers inside polymer matrices. Layered silicates such as sodium-montmorillonite (MMT), which is an aluminosilicate mineral with sodium counterions between its layers, are one of the types of materialsmost commonly used for making polymer-based nanocomposite inorganic materials because of their high aspect ratios, large specific surface areas, high cation exchange capacities and excellent swelling capacities [12; 13]. In an MMT crystal structure, the two-dimensional layers are formed by the fusion of a central octahedral sheet of alumina with the tips of two external silica tetrahedra, and the sheet thickness is approximately 1.00 nm [14]. Due to the incompatibility of the hydrophilic MMT surface with the hydrophobic polymer matrix, the individual layers are not easily separated and dispersed in most organic polymers [15]. Therefore, to improve the affinity of these clay layers for organic polymers, the clay layers are usually treated with surfactants to increase their hydrophobicity for the synthesis of clay-based nanocomposites [12]. The surfactants can not only improve the compatibility between the clay layers and the polymer matrix but also enlarge the interlayer gallery spaces of the clay, which is significant for allowing monomers to penetrate into the clay galleries and to polymerize. Conventional MMT modifiers include quaternary ammonium cationic surfactants, from which many products have been commercialized over the past few decades. [PERSON] et al. prepared an organic montmorillonite with cetyltrimethylammonium bromide (CTAB) by a cationic exchange reaction and synthesized exfoliated PS-MMT nanocomposites containing 5 wt % CTAB-MMT [16]. [PERSON] and coworkers employed a series of quaternary ammonium surfactants as clay modifiers to investigate the impact of the chemical structure of the surfactant on the morphology of the PS-MMT nanocomposites prepared via bulk polymerization [17]. [PERSON] et al. prepared organic montmorillonite by anchoring two functionalized ammonium salts inside the MMT interlayer galleries to investigate the fire resistance of the respective intercalated and exfoliated PS-clay nanocomposites [18]. However, MMT modified with conventional cationic surfactants has a fundamental problem: the quaternary ammonium surfactants decompose at temperatures above 170-180 \({}^{\circ}\)C. Therefore, MMT modified by traditional cationic ammonium surfactants is not suitable for high-temperature processes and applications. For these reasons, MMT modified by an anionic surfactant was designed and prepared due to the excellent thermal stability of anionic surfactants compared to traditional cationic surfactants [19]. Furthermore, the swelling, thixotropy and dispersion properties of MMT modified by an anionic surfactant are superior to those of CTAB-modified MMT [19]. Nevertheless, reports on anionic surfactant-modified MMT account for <20% of all reported MMTs, and only a few reports have considered MMT modification using anionic surfactants for the synthesis of polymer-clay nanocomposites [20].