## 1 Introduction  
Polymer/clay nanocomposites have attracted much attention for various applications, due to their unprecedented improvement in their performance properties compared to virgin polymers or traditional micro- and macro-composites [1; 2]. Important improvements in the mechanical, barrier, thermal, and fire retardant properties were obtained in the nanocomposites that were prepared with organoclays [3; 4]. The final properties of nanocomposites depend on several factors, such as the properties of each component, contents, dimensions, orientations of the clay, and the interfacial interactions between the matrix and the clay layers [5]. Among them, the interfacial interactions are mainly determined by the chemical compatibility between the polymer matrix and the layered silicates. Among the layered silicates, montmorillonite (Mt) and bentonite are two important clays, and much attention has been paid to Mt to prepare organoclays, due to its excellent properties, such as a high cation exchange capacity, large surface area, and strong adsorption capacity [6]. The surfaces of the Mt layers can have a deficit of positive charges because of the isomorphic substitution, which are neutralized by the adsorption of hydrated cations, such as Na\({}^{+}\) or Ca\({}^{2+}\), inside the interlayer spacing and on the surface of the clay platelets [7; 8]. Thus, the Mt layers are inherently hydrophilic and incompatible with the vast majority of polymers. However, it is known that the homogeneous dispersion of silicate layers, throughout the polymer matrix, is usually desirable for maximum reinforcement properties, compared with the polymer matrix [9]. Therefore, to increase the interfacial interaction and chemical compatibility between the two phases, the hydrophilic Mt surface must be modified to an organophilic surface. The organic treatment of the surface not only increases the interlayer spacing between the silicate layers, but also improves the compatibility between the clay and polymer matrix, which may facilitate the intercalation and exfoliation of clay layers inside the bulk hydrophobic polymers, and thus offers reinforcing effects [10]. The modification of Mt, with various methods, has been investigated previously [11; 12; 13]. The conventional and straightforward method that is used to prepare organophilic Mt is the cation exchange reaction with quaternary ammonium surfactants, which are intercalated into the interlayer spacing and replace the exchangeable cations [14]. However, the alkyl ammonium surfactants that are used in commercially modified Mt production decompose at temperatures above 170-180 \({}^{\circ}\)C, and thus are not suitable for polymers that are processed at higher temperatures [15]. Therefore, to overcome this difficulty, the functionalization of Mt with phosphonium surfactants has attracted great interest in the last few years [16]. Alkyl phosphonium surfactants are typical intercalating agents for the preparation of highly thermally stable organoclays, compared with ammonium surfactants [17]. The intercalation of phosphonium surfactants into the interlayer of Mt increases the interlayer spacing and decreases the surface energy, which results in the improvement of the compatibility with hydrophobic polymers [18]. The activation treatment of clay samples with inorganic acids is commonly used for the production of adsorbents or catalysts that are used in environmental protection. The edges of the crystals are opened, and cations (AI\({}^{3+}\), Mg\({}^{2+}\)) in the octahedral sheets are exposed to acid and become soluble, and the external specific surface area and pore diameter are increased, due to the dissolution of structural ions [19]. Moreover, the acid treatment of Mt can dissolve impurities and replace the exchangeable cations with hydrated protons [20]. In general, the acid treatment of clay is used before the organic modification reactions.