# A Review of the Synthesis and Applications of Polymer-Nanoclay Composites
## 1 Introduction  
Based on their chemical composition and particle morphology, clays are organized into several classes such as smectite, chlorite, kaolinite, illite and halloysite [1]. Due to their wide availability, relatively low cost and relatively low environmental impact, nanoclays have been studied and developed for various applications [2]. With the rapid growth of nanotechnology, clay minerals are increasingly used as natural nanomaterials [3]. Nanoclas are nanoparticles of layered mineral silicates with layered structural units that can form complex clay crystallites by stacking these layers [4]. An individual layer unit is composed of octahedral and/or tetrahedral sheets [5]. Octahedral sheets consist of aluminum or magnesium in a six-fold coordination with oxygen from a tetrahedral sheet and with hydroxyl. Tetrahedral sheets consist of silicon-oxygen tetrahedra linked to neighboring tetrahedra, sharing three corners while the fourth corner of each tetrahedron sheet is connected to an adjacent octahedral sheet via a covalent bond [6], see Figure 1. The arrangements of these sheets impact various defining and distinguishing aspects of nanoclays. Based on their mineralogical composition, there are approximately 30 different types of nanoclays, which depending on their properties are used in different applications [7]. As shown in Table 1, three major 1:1, 2:1 and 2:1:1 sheet arrangements are observed in common nanoclay materials. In 1:1 structures, each tetrahedral is connected to one octahedral sheet; in 2:1 structures, each octahedral sheet is connected to two tetrahedral sheets (one sheet on each side); and in 2:1:1 structures, each octahedral sheet is adjacent to another octahedral sheet and connected to two tetrahedral sheets [8; 9]. For example, Halloysite nanoclay is a naturally occurring aluminosilicate nanotube with the average dimensions of 15 nm \(\times\) 1000 nm [10]. Due to the hollow tube structure, halloysite nanoclay is used for medical applications, food packaging andrheology modification [11]. The most common plate-like montmorillonite (MMT) nanoclay (smectite) consists of approximately one nm thick aluminosilicate layers surface-substituted with metal cations and stacked in approximately 10 um-sized multilayer stacks [12]. The stacks can be dispersed in a polymer matrix as fillers/additives to form polymer/nanoclay composites, with applications such as mechanical strength enhancement, flame-resistance material, thickening and gelling agents, waste water treatment and gas permeability modification [12; 13; 14; 15]. MMT nanoclay (2:1 layered silicates) with a high cation exchange capacity has cation exchange sites on the siloxane surface which can be combined with dissimilar substances, such as organic or biological molecules [16]. The MMT nanoclay stacks can be dispersed in a polymer matrix as fillers/additives to form polymer/nanoclay composites, which have been widely studied due to their high cation exchange capacity, swelling behavior, and large surface area [17; 18; 19]. Halloysite nanoclays are readily dispersed in various polymers without the need for exfoliation. This is due to there being less abundant hydroxyl groups on the nanoclay surfaces compared to MMTs. Additionally, tubular halloysite nanoclays are excellent nanocontainers for various chemical molecules [20]. Thus, the functionalized halloysite nanoclays are employed as efficient fillers for polymers to enhance their mechanical and thermal properties. These nanoclays are also used as carriers to achieve a sustained release of active molecules, such as flame-retardants, antioxidants, anticorrosion and antimicrobial agents [21; 22]. The research and development of novel polymer/nanoclay materials has been an advancing field in material chemistry in recent years [12; 13; 14; 15; 23; 24; 9; 25]. Rigid nanoclay may be used as a filler and is able to reinforce polymer structures and impede the free movement of polymer chains neighboring the filler [19].