Polymer Nanocomposite Materials. Группа авторов

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Polymer Nanocomposite Materials - Группа авторов

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with traditional conductive materials like metals, conductive polymers (CPs) exhibit excellent flexibility, light weight, low cost, and easy processing characteristics. Therefore, CPs have become the research hotspot in recent decade and been widely used in many fields such as electromagnetic interference (EMI) shielding, sensors, and wearable electronics [1–3]. Generally, CPs can be divided into two categories: (i) structural (intrinsic) structural conductive polymers (SCPs) and (ii) conductive polymers composites (CPCs).

      SCPs refer to a material possessing inherent conductivity provided by the structure carrier. The SCPs were firstly discovered in the 1970s, which aroused the attention of researchers on the chemical structure and the electrical properties of polymers [4, 5]. The common types of SCPs possessing conjugated bonds are polyacetylene, polypyrrole (PPy), polyaniline (PANI), polyphenylacetylene (PPA), and their derivatives. However, the value of electrical conductivity for the conjugated polymer materials is relatively small, usually ranging from 10−10 to 10−5 S cm−1. The electrical conductivity of polymer materials can be enhanced by 104 S cm−1 by doping various molecules like salt ions, hyaluronic acid, and peptides [6, 7]. The doped SCPs show both electrical conducting property from metallic material and many merits like flexibility, lightweight, etc., from polymer material. Therefore, The SCPs have demonstrated their potential applications in flexible electronics, sensors, and EMI shielding [8–10]. However, the SCPs have their inherent deficiencies including poor solubility, mechanical properties, and high cost. Usually, further modification or compounding with other polymers is needed to improve their overall performance [11].

      As known, conductive fillers, especially nano-sized conductive particles, are easy to aggregate in the polymer due to their high-aspect-ratio, resulting in uneven distribution, which may deteriorate the comprehensive performance of CPCs [16]. Therefore, surface modification of nanofillers and special processing technique are required to enhance the dispersion of conductive nanofillers in the polymer, which mainly includes the following methods: (i) The physical blending [17]. The conductive particles are uniformly dispersed into the polymer melt matrix or polymer solution under the ultra-strong external field forces (shear, tensile, etc.). (ii) In situ polymerization [18]. The conductive particles were firstly dispersed in the organic solvent containing the polymer monomer. After the polymerization reaction, the previously evenly dispersed conductive particles were anchored in the polymer matrix in situ. (iii) Chemical modification of conductive filler [19]. After the chemical reaction, the surface of conductive particles is grafted with functional groups such as hydroxyl group, carboxyl group, amino group, etc. These groups have good interactions with the polymer, such as covalent bond and hydrogen bond, which can effectively avoid the nanofillers aggregation and hence improve their dispersion in the polymer matrix. (iv) Introduction of surfactant [20]. The surfactant will wrap around the conductive particles, increasing their compatibility in polymer solution or melt, thus improving the dispersion of the fillers.

      For CPCs, the electrical conductivity is depended on the transportation of charge carriers (current) along the conductive network constructed by conductive fillers in the polymer matrix. Generally, a sudden increase of several orders of magnitude in conductivity (transition from insulator to conductor) can be found as the concentration of conducting phase reaches a critical value in the polymer matrix, which is defined as the percolation threshold. Above this threshold, the concentration dependence of the conductivity of the CPCs (σ) can be described by a scaling law

sigma equals sigma 0 left-bracket left-parenthesis phi minus phi Subscript c Baseline right-parenthesis right-bracket Superscript t

      2.2.1 Melt Blending

      Melt blending is processed by using kneading machine, molding machine, internal mixer or double screw extruder, etc. [31, 32] to evenly mix the polymer matrix and conductive fillers with the processing temperature above the melting point of polymer. Thus, high temperature and high shear force are needed in melt blending process to ensure the homogeneous dispersion of the conductive nanofiller in the melting polymer matrix. During the melting blending, the nanofillers are forced to disperse by the mechanical shear force and at the same time prevented from re-aggregation by the viscous polymer matrix. After the masterbatch is obtained, the final CPCs can be prepared by using different polymer processing technologies like spinning, hot press, and injection molding [33–35]. Melt blending is an environmental-friendly process method and is feasible for large-scale industrial production of the CPCs. Many recent studies have investigated the effect of fillers introduction, dispersion state, and processing factors on the physical and electrical properties of the CPCs prepared by the melt blending [36].

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