Polyurethane based Self-healing Nanocomposites for Electromagnetic Interference Shielding Applications

Abstract

An augmentation in the growth of modern electronics and telecommunication has led to an increase in electromagnetic interference (EMI) as its aftermath. Device malfunctioning, unwanted noise and radio leakage are the direct consequences of EMI. Modern nanotechnology seeks to integrate the demands of the ever-growing telecommunication industry through the miniaturisation of devices which brings crucial electronic circuitry in close proximity of one another, thereby causing them to cross talk and interfere among each other. In the search for a good EMI shield, nanotechnology has come a long way from metal-based EM wave reflector to now lightweight polymeric EM wave absorbers. The advantage of polymeric materials lies in their ability to be tuned to achieve the desired level of shielding performance by incorporation of suitable fillers. Polymer nanocomposites are however, prone to mechanical damage over a period of time. Therefore, development of self-healable EMI shields is necessary to extend its life-time. This thesis entitled “Polyurethane based self-healing nanocomposites for electromagnetic interference shielding applications” aims to develop polyurethane (PU) based self-healable EMI shielding nanocomposites by combining the self-healing property of suitably functionalised PU and electronic properties of multi-walled carbon nanotubes (MWNTs) and hybrid functional nanoparticles composed of reduced graphene oxide (rGO), MoS2 and Fe3O4. The thesis comprises of 7 chapters. Chapter 1 serves as an introductory note on self-healing polyurethane and EMI shielding. It discusses the existing strategies (both extrinsic and intrinsic type of healing) which have been used till date for the development of self-healing PUs. It also discusses recent literatures and existing state of art in the field of PU based EMI shielding materials. It also discusses the motivation behind development of EMI shielding materials which are self-healable. Chapter 2 discusses the road map of the thesis. It discusses the rationale behind choosing various functional nanoparticles. It also discusses the progression of various chapters and key aspects of each chapter in the thesis. In Chapter 3, self-healing PU was synthesised by tethering Diels-Alder chemistry which has the capability to heal by heating the polymer to 65 °C. The hybrid rGO@Fe3O4 (5 wt %) nanoparticles used in tandem with MWNTs (3 wt %) resulted in excellent EMI shielding efficiency of -31 dB at 18 GHz with up to 90 % absorption of incoming EM waves at a thickness of 5 mm. Additionally, the incorporation of microwave absorbing nanoparticles iv helped in development of an EM shield which could heal using microwaves from a commercial microwave oven. Moving forward in Chapter 4 self-healing ultrathin EMI shielding films were developed. In this case the self-healing disulphide bonds were harnessed on the nanoparticle (MWNTs) itself through suitable chemistry. The disulphide linked MWNTs were composited along with hybrid rGO@MoS2@Fe3O4 in commercial TPU in 80/20 (wt % / wt %) ratio which resulted in high EMI shielding efficiency of -65 dB at 18 GHz with up to 81 % of EM wave being absorbed at a thickness of only 200 μm. The damaged films could be repaired by heating to a temperature of 50 °C. However, it was realised that in spite of the high EMI shielding efficiency of these ultra-thin self-healing shields their flexibility was compromised due to high filler loading. Therefore, in Chapter 5 the disulphide chemistry was harnessed (used in Chapter 4 for self-healing MWNTs) in the PU chain itself which resulted in a polymer which could heal at room temperature, eliminating the need to provide a trigger like temperature such as used in Chapter 3 and 4. MWNTs in tandem with hybrid rGO@MoS2@Fe3O4 resulted in high EMI shielding efficiency of -43 dB at 18 GHz with up to 96 % of EM waves being attenuated by absorption. In Chapter 6 the self-healing effect was combined with shape memory property to design shield with multifunctionality. A mussel-inspired PU was synthesised by incorporating dopamine moiety in the PU chain. In previous chapters the crack closure was initiated manually. However, in this chapter, the polymer exhibited shape memory assisted self-healing (SMASH) behaviour whereby the crack closure in case of damages could be achieved through shape memory property of the polymer by heating it above the Tg of the polymer (50 °C). Additionally, the hydroxyl functionality in dopamine could be crosslinked with Fe3+ ions which could also exhibit moisture triggered healing effect. The hybrid nanoparticles Fe3O4@MoS2 in synergism with MWNTs resulted in high EMI shielding efficiency of -37 dB at 18 GHz with up to 96 % of EM waves being attenuated by absorption. Finally, in Chapter 7 a consolidated summary of the results obtained in the thesis has been provided. The chapter also discussed the possible extension of this work which could be adopted to develop novel self-healing EMI shielding polymer nanocomposites.