## 1 Introduction  
Clay-polymer nanocomposites have received great attention in recent years due to their exceptional properties, such as increased thermal stability, high fire resistance, and enhanced mechanical characteristics, compared with traditional polymer composites [1; 2]. The improvement of many properties of clay-polymer nanocomposites could be achieved using a relatively low clay loading (usually \(<10\) wt %) in a polymer matrix [3]. Among the layered silicates, montmorillonite (MMT) was one of the most widely utilized to fabricate clay-polymer nanocomposite, mainly because the MMT was easily available with a high aspect ratio and high expansion capacity [4]. The unit crystal structure of the MMT consisted of two silicon tetrahedral sheets and a central sheet of alumina octahedrons. The negative charge was generated on the surfaces of the MMT layers due to the isomorphic substitutions of lower valance ions (Mg\({}^{2+}\), Fe\({}^{2+}\)) for the central atoms (Al\({}^{3+}\), Si\({}^{4+}\)) in the interior crystal layers of the MMT [5]. To neutralize the negative charges, hydrated cations (Na\({}^{+}\), Ca\({}^{2+}\)) were electrostatically adsorbed inside the interlayer space and on the surface of the MMT. Therefore, the natural state of the MMT was a hydrophilic character, which was inherently incompatible with the vast majority of organic polymers [6]. The poor affinity between the hydrophilic MMT and the hydrophobic polymer will not result in a clay-polymer nanocomposite with improved properties compared with the pure polymer matrix. To obtain the highest enhancement of properties, the surface modification of the MMT was desirable to increase the interfacial interactions between the MMT and the polymer matrix [7]. The surface treatment not only enhanced the compatibility of the MMT with the organic polymer, but also enlarged the interlayer space between the layers of MMT. The most traditional and convenient method for the MMT modification was ion exchange reaction with quaternary alkylammonium salts [8]. However, the ammonium surfactants could not provide an effective thermal stability for high temperature applications because the decomposition of the ammonium surfactants could be initiated above 200 \({}^{\circ}\)C [9]. Thus, the modification of the MMT with phosphonium surfactants has attracted much interest from researchers. Several research papers found that the thermal stability of organoclays modified with phosphonium surfactants was superior to that of organoclays modified with ammonium surfactants [10; 11]. On the other hand, the anionic surfactant was also used to improve the thermal stability of the organoclays [12]. However, it could be rather difficult to significantly expand the interlayer space of the MMT via the anionic surfactant alone. Therefore, a novel kind of organo-montmorillonite (OMMT) by the synergistic effect between the anionic and cationic surfactants has been prepared in recent years [13]. Another method for the modification of the MMT involved the use of silane coupling agents. These silanes could react with the MMT OH groups to form the chemical bonds with a covalent character between the molecules of the silane and the surface of the MMT [14]. The grafting reaction mainly occurred at the broken edges and at the structural defects situated at the external surfaces of the MMT layers [15]. However, the employment of both a phosphonium surfactant and an anionic surfactant, or both a phosphonium surfactant and a silane coupling agent, to fabricate the OMMT to increase the dispersion degree in the polymer matrix was rarely reported. Polystyrene (PS) is a synthetic thermoplastic that is widely used in industry due to its excellent performance of high abrasion resistance, strong plasticity and load bearing capacity [16]. However, the relatively weak thermal stability and heat resistance of PS has become a significant challenge, hindering its further applications in a variety of areas.