Development and Characterization of High Performance Electrically Conductive Polymer Nanocomposite for Electromagnetic Interference (EMI) Shielding Applications

Abstract

The fast-growing pace and massive utilization of compact portable electronic and wireless telecommunication systems provided an easier life for humans. However, this positive progress has come at the expense of significant electromagnetic interference (EMI) pollution, which requires the development of highly efficient shielding materials to impede the malfunction of electronic devices and reduce its deleterious impact on human health. Although metals have been used traditionally as an excellent EMI shielding material, high density, high cost, susceptibility to corrosion, and reflection shielding mechanism limited their applications. Recently, multifunctional conductive polymer nanocomposites (CPnC) have demonstrated great promise as next-generation materials for energy management and EMI shielding components in electronic industries. Such highly desirable and multifunctional properties in CPnCs are achievable by a combination of lightweight, easy to process and chemically stable polymer matrices and a large array of nanosized conductive fillers. In this Ph.D. research, we aimed to develop multifunctional CPnCs for EMI shielding applications. Our goal was enhancing the EMI shielding effectiveness (SE) of the CPnCs, concentrating on amplifying the electromagnetic (EM) wave absorption mechanism as opposed to reflection which could potentially lead to secondary EMI pollutions. First, narrow elongated strips of graphene with high aspect ratio and abundant edges, namely reduced graphene nanoribbons (GNR), were synthesized from multiwalled carbon nanotubes (MWCNT) through a chemically oxidative unzipping, and subsequent thermal reduction method. The purpose of this study was to gain a comprehensive understanding of how the structural modifications of MWCNTs influence the end characteristics of polymer nanocomposites, especially electrical properties and EMI shielding. GNR and parent MWCNT were added to thermoplastic polyurethane (TPU) and the electrical conductivity, the dielectric property, the EMI SE and the mechanical properties of the nanocomposites were thoroughly investigated and compared. It was found that the electrical conductivity, the EMI SE and the mechanical properties of GNR/TPU nanocomposites were far superior to those of MWCNT/TPU nanocomposites. This greatly heightened performance of GNR added nanocomposites is mainly attributed to enhanced filler interconnections between GNRs. The individual filler particle flexibility was significantly increased when tubular multi-layered MWCNTs were opened and transformed into long, thin strips. This allows geometrical conversion from line-to-line to sheet-to-sheet contact interfaces, which considerably increases the contact area. The increased flexibility also increased the chances of forming a percolating network between fillers. In addition, the unzipping and exfoliation of MWCNTs increased the number concentration of filler particles, leading to improved electrical and mechanical performance. To further enhance the EMI SE of the CPnCs, we synthesized Ti3C2Tx MXene nanoflakes via the minimally intensive layer delamination (MILD) method. Then, conductive polyaniline (PA) nanofibers were grafted on the surface of the large and low-defect MXene nanoflakes via oxidant free oxidative polymerization at two different MXene to monomer ratios. To investigate the impact of MXene functionalization on electrical properties, EMI SE and mechanical properties of the CPnCs, the synthesized nanomaterials were incorporated in polyvinylidene fluoride (PVDF) via solution blending method. The surface modification of MXene nanoflakes resulted in the outstanding enhancement of EMI SE and absorption coefficient of the PVDF based nanocomposites. The higher EMI SE of the modified nanocomposites at comparable electrical conductivity was attributed to three main reasons: (i) exfoliation of the MXene nanoflakes by the intercalation of PA nanofibers in the inter-gallery spaces, (ii) induction of abundant capacitor-like structures in the interfaces between PVDF chains and nanofiller, especially after increased specific surface area with PA modification, and (iii) the contribution of PA conducting chains in the electron transfer mechanisms and responses to the EM field. Moreover, the mechanical properties of the PVDF nanocomposites showed higher stiffness for PA-grafted MXene nanocomposites due to the sufficient exfoliation of nanoflakes by PA nanofibers’ intercalation inside the MXene nanoflakes and the better polymer−filler interactions, which resulted in facilitating load transfer between the nanoflakes and the PVDF chains. In a follow-up study, we hybridized the reduced graphene oxide nanoribbons (rGOnR) and MXene nanoflakes and used an engineering design to enhance the EMI SE of a thin but highly electrically conductive MXene sheet. In this research, we took the advantage of excellent hydrophilicity of the GONR and MXene nanofillers to construct a three-dimensional (3D) conductive percolated network employing freeze casting. The constructed GOnR/MXene 3D networks at different GOnR to MXene ratios were then heat treated and infiltrated with a polydimethylsiloxane (PDMS) matrix. This unique approach significantly improves the state of filler dispersion, achieving a high electrical conductivity at lower nanofiller loadings. Electrical properties and EMI SE and shielding mechanism of the nanocomposites were systematically investigated as a function of the specimens' thickness. Furthermore, the influence of the rGOnR/MXene hybridization on enhancing the EMI SE and absorption coefficient of a highly reflective EMI shielding MXene sheet was investigated. It is revealed that the rGOnR/MXene hybridization not only enhances synergistically the EMI SE of the MXene sheet, also plays as an EM waves absorption layer which impedes the environment from the secondary pollutions originating from the reflective dominant shielding mechanism of the highly conductive MXene sheet. It is worth noting that the highly conductive MXene sheet in this study was considered a model material which could be substituted with any conductive coating that possesses efficient EMI SE yet reflection dominant shielding.