Preparation of Poly(acrylic acid-_co_-acrylamide)-_Grafted_ Deproteinized Natural Rubber and Its Effect on the Properties of Natural Rubber/Silica Composites  
used for the fabrication of high-performance tires with wet grip ability and fuel saving efficiency [15]. According to the chemical structure of NR, it contains cis-1,4-polyisoprene, which is nonpolar. Therefore, it is incompatible with silica, resulting in the undesired properties of the products. This is one of the main drawbacks of the preparation of silica-based NR composites. Therefore, to overcome the incompatibility of silica with natural rubber, the polarity of natural rubber may be improved by introducing polar components. Many attempts have been reported so far to enhance the polarity of NR and improve its compatibility with reinforcing fillers [16; 17]. The modification of NR was accomplished by introducing polar functional components to its structure. Graft copolymerization is one of the most common methods for modification, in which the other polymer can be chemically bonded to the polymer backbone. From this method, the free radicals on the main chain are generated. The monomer is then added and undergoes polymerization to form a grafted copolymer. The graft copolymerization on the NR particles can be performed via surface functionalization in the latex stage, which allows for the additional functional groups on their particle surface. NR consists of polyisoprene chains and nonrubber components, including proteins, phospholipids, and fatty acids. The nonrubber components are in the aqueous medium and surface of the NR particles. The deproteinized natural rubber (DPNR) is employed in order to suppress the side reactions from the adsorbed proteins and can improve grafting efficiency [18]. [PERSON] et al. prepared DPNR grafted poly(dimethylaminoethyl methacrylate) (PDMAEMA) through emulsion polymerization to perform grafting modifications and followed this by blending it with poly(vinyl alcohol) (PVA) to fabricate membranes [19]. The polarity of the DPNR was increased after grafting, leading to a strong interfacial interaction with PVA. The enhancement of the mechanical properties was achieved by showing the increase in tensile strength and modulus. These membranes were used for the loading and releasing of model drugs, which could be interesting in biomedical fields. Therefore, it can introduce a variety of functional groups to natural rubber from this technique by the use of functional monomers. When increasing the polarity of natural rubber, the natural rubber should have additional functional groups, such as carboxylic acid, amine, amide, and hydroxyl. Acrylic acid, a carboxylic acid-containing monomer, can play an important role in improving the hydrophilicity of materials. [PERSON] et al. successfully prepared poly(acrylic acid)-_grafted_ natural rubber (PAA-_g_-NR) via radical solution polymerization [20]. However, the organic solvent was used as a medium for this preparation system. These are the drawbacks in terms of cost and environmental considerations. The hydrophilicity of the resulting product increased after grafting with PAA. The grafted product was applied as a coating material for controlled release purposes. Moreover, the formed poly(acrylic acid) chains can also act as electrosteric stabilizers that improve colloidal stability for particles dispersed in an aqueous phase [21]. In addition, the copolymerization of acrylic acid and acrylamide has been studied to prepare efficient polyelectrolyte-based materials. The combination of polyacrylic acid and polyacrylamide provides a homogeneous structure with hydrophilic properties and hydrogen bonding formation, as well as enhancing its mechanical properties [22; 23]. In this work, to improve the compatibility of natural rubber with silica, DPNR was modified by emulsion graft copolymerization with acrylic acid and acrylamide using an environmentally friendly process. The polymerization is carried out in latex stage, which is a water-based system.