# A Review of the Synthesis and Applications of Polymer-Nanoclay Composites
## 1 Introduction
Moreover, it behaves as a load-bearing constituent when the interfacial adhesion between the filler and the chains is fulfilled [26]. Two crucial challenges of synthesis are to achieve: (1) chemical compatibility between the polymer matrix and the nanofiller at the nanoscale; and (2) homogeneous dispersion of the nanofiller within the polymer matrix. The interfacial interaction between nanoclay fillers and polymer matrix, as well as the quality of nanoclay dispersion, has a significant influence on the performance of polymer/nanoclay composites [27]. These intercorrelated features determine the polymer/nanoclay composites' morphology and, thus, their final bulk properties such as strength, elastic modulus, thermal stability, heat distortion temperature, self-healing, shape memory abilities and gas barrier [28; 29]. The surface functionalization of nanoclays is a practical method to enhance the interfacial interactions between nanoclay fillers and polymeric matrix, enabling the transfer of interfacial stress from the polymer to the nanoclays [30]. For instance, the covalent modification of the outer surfaces of halloysite nanotubes enhances their dispersibility into the polymer matrix, which may improve the thermal stability and tensile properties of the resulting polymer/nanoclay composites [31]. Synthesis approaches may lead to different combinations of polymer matrix and nanoclays, such as an immiscible structure, an intercalated structure and an exfoliated structure [32]. In the immiscible structure, the nanoclay dispersion aggregates within the polymer matrix and the polymers are separated from the clay layers [33]. The polymer chains form an intercalated structure between the clay layers, altering the geometry of the clay layers. This alternation includes variation in the stacking mode of the layers, modification in interlayer spacing, and diminishing the electrostatic forces between the clay layers, which lead to greater enhancement of the mechanical and thermal properties of the composites [34; 35; 36]. Nanoclay stacks are fully separated by polymer chains in the exfoliated structure, providing superior mechanical properties and polymer processability [37]. There are three major synthesis procedures for polymer/nanoclay composites including the melt-blending method, solution-blending method and in-situ polymerization method [6]. Heretofore, the most widely applied synthesis method was the in-situ polymerization method, where the grafted amounts of organics was adjusted and the clay interlayer spacing was controlled by changing the polymerization conditions [38]. The combination of in-situ polymerization with efficient coupling methods, including click chemistry [39], radical-mediated polymerization [40; 41], tandem preparation [42], photopolymerization [38] and miniemulsion [43], has enabled effective dispersion of nanoclays in the form of individual platelets in the polymer matrix, which is a significant challenge inherent to the synthesis of polymer/nanoclay composites. All these methods have been successfully implemented for the chemical modification of clay surfaces with low molecular or polymeric grafts [44]. Polymer/nanoclay composites have been expanded to several hybrid, innovative materials including nanoclay/conductive polymer (polypyrrole (PPy), polyaniline (PANI), polythiophene (PT), and poly(3,4-ethylenedioxythiophene) (PEDOT), biocomposites and organoclay hybrid films with properties superior to conventional composite materials [45]. These novel composites are attained with a lower modified nanoclay filler content in comparison to conventionally filled systems and are, as a result, lighter in weight [46].