Applied Science, Faculty of
Engineering, School of (Okanagan)
DSpace
UBCO
Ghaderi, Saeed
Master of Applied Science - MASc
University of British Columbia
3D printing of conducting polymers has been achieved very recently by direct ink writing of poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)-based inks. This field is in its infancy, calling for further research to broaden the application horizon of 3D-printed conductive polymers by fine-tuning their inherent properties. Indeed, engineering PEDOT:PSS inks with customizable electrical properties while keeping their rheological fingerprint within the proper range for high-fidelity 3D printing is an arduous challenge, yet to be achieved. Herein, a range of PEDOT:PSS-based inks were formulated by molecular engineering via co-solvent doping and solvent post-treatment with various solvents for high-resolution (line width and thickness variations less than 20 % from average values) and high-aspect-ratio ( ≥ 25 layers) 3D printing. Dimethyl sulfoxide (DMSO), ethylene glycol (EG), and N,N-dimethylformamide (DMF) were used for co-solvent doping, and DMSO, EG, DMF, methanol, and ethanol were used for solvent post-treatment. The effects of co-solvent doping of high-boiling-point co-solvents on rheological properties and, thus, printability of the inks were explored. Inks’ 3D printability range was defined based on shear viscosity, storage modulus, and yield stress values as a function of PEDOT:PSS concentration. Experimental analyses demonstrated that the strong interactions between solvent molecules and free PSS chains account for the partial removal of non-conductive PSS-rich shells from conductive PEDOT-rich cores. Via coupling solvent treatment and a simple dry-annealing technique, samples featured a wide range of conductivity, i.e., from 0.6 to 858.1 ± 60.8 S cm-1. Taking advantage of solvent treatment, the 3D-printed geometries were wet-transferred onto uneven substrates and complex objects with sharp edges. By exploiting their tunable molecular-scale chemistry and macro-scale geometric features, the 3D-printed parts were used to create electromagnetic interference (EMI) shields with controlled mechanisms. The maximum EMI SE of 39.36 dB for dry annealed and 50.16 dB for freeze-dried were achieved for 10 layers of printed DMSO-doped PEDOT:PSS grid-infilled patterns at the X-band range of the electromagnetic waves (8.2-12.4 GHz). The effects of 3D printing parameters such as the number of printed layers, infill density, and drying technique were evaluated on the EMI shielding effectiveness (EMI SE), specific SE, and absolute SE of the 3D-printed EMI shields.
HIGH-RESOLUTION 3D PRINTING OF ELECTRICALLY CONDUCTIVE INKS BASED ON PEDOT:PSS FOR ELECTROMAGNETIC INTERFERENCE SHIELDING by Saeed Ghaderi M.Sc., Chemical Engineering, Sharif University of Technology, Iran, 2019 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in The College of Graduate Studies (Mechanical Engineering) THE UNIVERSITY OF BRITISH COLUMBIA (Okanagan) February 2023 © Saeed Ghaderi, 2023 ii The following individuals certify that they have read, and recommend to the College of Graduate Studies for acceptance, a thesis entitled: HIGH-RESOLUTION 3D PRINTING OF ELECTRICALLY CONDUCTIVE INKS BASED ON PEDOT:PSS FOR ELECTROMAGNETIC INTERFERENCE SHIELDING submitted by Saeed Ghaderi in partial fulfillment of the requirements for the degree of Master of Applied Science Dr. Mohammad Arjmand, School of Engineering Supervisor Dr. Jian Liu, School of Engineering Supervisory Committee Member Dr. Sina Kheirkhah, School of Engineering Supervisory Committee Member Dr. Mahmudur Fatmi, School of Engineering University Examiner iii Abstract 3D printing of conducting polymers has been achieved very recently by direct ink writing of poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)-based inks. This field is in its infancy, calling for further research to broaden the application horizon of 3D-printed conductive polymers by fine-tuning their inherent properties. Indeed, engineering PEDOT:PSS inks with customizable electrical properties while keeping their rheological fingerprint within the proper range for high-fidelity 3D printing is an arduous challenge, yet to be achieved. Herein, a range of PEDOT:PSS-based inks were formulated by molecular engineering via co-solvent doping and solvent post-treatment with various solvents for high-resolution (line width and thickness variations less than 20 % from average values) and high-aspect-ratio ( ≥ 25 layers) 3D printing. Dimethyl sulfoxide (DMSO), ethylene glycol (EG), and N,N-dimethylformamide (DMF) were used for co-solvent doping, and DMSO, EG, DMF, methanol, and ethanol were used for solvent post-treatment. The effects of co-solvent doping of high-boiling-point co-solvents on rheological properties and, thus, printability of the inks were explored. Inks’ 3D printability range was defined based on shear viscosity, storage modulus, and yield stress values as a function of PEDOT:PSS concentration. Experimental analyses demonstrated that the strong interactions between solvent molecules and free PSS chains account for the partial removal of non-conductive PSS-rich shells from conductive PEDOT-rich cores. Via coupling solvent treatment and a simple dry-annealing technique, samples featured a wide range of conductivity, i.e., from 0.6 to 858.1 ± 60.8 S cm-1. Taking advantage of solvent treatment, the 3D-printed geometries were wet-transferred onto uneven substrates and complex objects with sharp edges. By exploiting their tunable molecular-scale chemistry and macro-scale geometric features, the 3D-printed parts were used to create electromagnetic interference (EMI) shields with controlled mechanisms. The maximum EMI SE of 39.36 dB for dry annealed and 50.16 dB for freeze-dried were achieved for 10 layers of printed DMSO-doped PEDOT:PSS grid-infilled patterns at the X-band range of the electromagnetic waves (8.2-12.4 GHz). The effects of 3D printing parameters such as the number of printed layers, infill density, and drying technique were evaluated on the EMI shielding effectiveness (EMI SE), specific SE, and absolute SE of the 3D-printed EMI shields. iv Lay Summary Electromagnetic interference (EMI) is a coupling of signals from one system to another. In EMI shielding, electromagnetic fields are prevented from leaking into sensitive devices and signals so that they cannot interfere with them. As a protective enclosure or for isolating electromagnetic field sources, EMI shields can be installed on any device that needs protection from electromagnetic fields. EMI or radio frequency interference (RFI) can affect the performance of electronic circuits or even cause them to fail. The electromagnetic field can easily disrupt electronics’ small voltages and currents. Therefore, addressing EMI and RFI is of great importance for electronics. In this research, flexible structures based on intrinsically conductive PEDOT:PSS inks have been developed to fabricate patterned lightweight EMI shields using a direct ink writing (DIW) technique. PEDOT:PSS inks were doped with high-boiling-point co-solvents to achieve highly electrically conductive inks. The formulated inks could form a uniform and continuous filament suitable for manufacturing layer-by-layer deposited self-supported EMI shields. The obtained 3D-printed geometries were used as patterned EMI shields. The EMI shielding performance of the final samples was adjusted using different post processes techniques (dry-annealing and freeze-drying), changing the number of the printed layers (1-10 layers), and using different infill densities. The results validated that the printed lightweight structures based on solvent-treated PEDOT:PSS can be used in high-performance engineered EMI shields at the frequency range of 8.2-12.4 GHz. v Preface 3D laser microscopy analyses were performed at Dr. Mohammad Zarifi’s lab, School of Engineering, University of British Columbia, Okanagan. All chapters in the thesis are based on work conducted at the University of British Columbia in the Nanomaterial and Polymer Nanocomposite Laboratory (NPNL) by Saeed Ghaderi under the supervision of Dr. Mohammad Arjmand. During the whole work and experiments in the thesis, I was responsible for all of the sample preparation, characterization, and data analysis. My supervisor, Dr. Mohammad Arjmand, and the committee members were mentoring and leading me through the thesis direction. • The data and results presented in Chapters 3 and 4 have been submitted in the form of a journal article to the “Journal of Materials Chemistry A”. Chapter 4 has also been presented at different international conferences in the form of oral and poster presentations. A list of conference papers is as follows: • “Extrusion printing of PEDOT:PSS hydrogels: tuning electrical and rheological properties”, S. Ghaderi, A. Ghaffarkhah, M. Kamkar, M. Amini, M. Arjmand, 36th International Conference of Polymer Processing Society (PPS-36), 26-29 September 2021, Montreal, Canada. • “Tuning electrical conductivity of extrusion printed PEDOT:PSS hydrogels”, S. Ghaderi, A. Ghaffarkhah, M. Kamkar, M. Amini, M. Arjmand, 36th International Conference of Polymer Processing Society (PPS-36), 26-29 September 2021, Montreal, Canada. vi Table of Contents Abstract .................................................................................................................................... iii Lay Summary ........................................................................................................................... iv Preface ....................................................................................................................................... v Table of Contents ..................................................................................................................... vi List of Tables ............................................................................................................................ ix List of Figures ........................................................................................................................... x List of Symbols ...................................................................................................................... xiii List of Abbreviations ............................................................................................................. xiv Acknowledgments ................................................................................................................. xvi Dedication ............................................................................................................................. xvii Chapter 1: Introduction ............................................................................................................ 1 1.1 General Background ................................................................................................... 1 1.2 Gaps and challenges .................................................................................................. 1 1.3 Thesis Objectives ....................................................................................................... 1 1.4 Thesis Outline............................................................................................................. 2 Chapter 2: Literature Review .................................................................................................... 3 2.1 Introduction ................................................................................................................. 3 2.2 Additive manufacturing (AM) ....................................................................................... 3 2.3 Electromagnetic wave spectrum ................................................................................. 5 2.4 Electromagnetic interference (EMI) ............................................................................. 6 2.5 Health hazards of electromagnetic radiation ............................................................... 6 2.6 EMI shielding mechanisms ......................................................................................... 7 2.6.1 EMI shielding effectiveness (EMI SE) ............................................................. 8 2.6.2 EMI SE measuring configurations ................................................................... 9 2.7 AC/DC conductivity ....................................................................................................12 2.8 EMI shielding materials ..............................................................................................13 2.8.1 Metals ............................................................................................................13 2.8.2 Conductive polymer composites (CPCs) .......................................................13 2.8.3 Intrinsically conductive polymers (ICPs) ........................................................14 2.9 PEDOT:PSS ..............................................................................................................15 2.9.1 PEDOT:PSS treatment ..................................................................................16 vii 2.9.1.1 Thermal treatment ..................................................................................16 2.9.1.2 Co-solvent doping treatment...................................................................17 2.9.1.3 Solvent post-treatment ...........................................................................18 2.10 Structure design for EMI shielding .............................................................................20 2.10.1 Homogenous polymeric composites ..............................................................20 2.10.2 Porous structures ..........................................................................................20 2.10.2.1 Foams ...............................................................................................20 2.10.2.2 Aerogels ............................................................................................21 2.10.3 Laminated multilayer structures .....................................................................22 2.10.4 Segregated structures ...................................................................................22 2.11 Section summary .......................................................................................................23 Chapter 3: Materials and Methods ......................................................................................... 24 3.1 Materials ....................................................................................................................24 3.2 Ink preparation, co-solvent doping, and solvent post-treatment .................................25 3.3 3D printing .................................................................................................................26 3.4 Techniques and characterization ...............................................................................27 3.4.1 Rheology .......................................................................................................27 3.4.2 Laser microscope ..........................................................................................27 3.4.3 SEM ..............................................................................................................28 3.4.4 XPS ...............................................................................................................28 3.4.5 XRD...............................................................................................................29 3.4.6 Raman spectroscopy .....................................................................................29 3.4.7 Electrical conductivity ....................................................................................29 3.4.8 EMI shielding .................................................................................................29 Chapter 4: 3D-printed PEDOT:PSS-based EMI Shields ........................................................ 32 4.1 Rheology of PEDOT:PSS inks ...................................................................................32 4.2 Direct ink writing of PEDOT:PSS inks ........................................................................39 4.3 Manufacturing free-standing architectures .................................................................41 4.4 Wet-transferring of 3D-printed free-standing structures .............................................42 4.5 Manufacturing of high-aspect-ratio 3D-printed architectures ......................................42 4.6 High-resolution 3D printing ........................................................................................43 4.7 Electrical conductivity measurements ........................................................................51 4.8 XPS results ................................................................................................................51 viii 4.9 XRD results ...............................................................................................................53 4.10 Raman spectroscopy .................................................................................................53 4.11 EMI shielding measurements .....................................................................................55 4.12 Summary ...................................................................................................................62 Chapter 5: Conclusions and Future Works ........................................................................... 63 5.1 Conclusions ...............................................................................................................63 5.2 Challenges and future works .....................................................................................64 Bibliography ............................................................................................................................ 65 Appendices ............................................................................................................................. 79 Appendix A Detailed XPS results ........................................................................................79 ix List of Tables Table 4.1 EMI shielding parameters of printed DMSO-doped PEDOT:PSS structures. .............59 Table 4.2 Comparison of electrical conductivity and EMI shielding parameters for different materials. ..................................................................................................................................60 x List of Figures Figure 2.1 The electromagnetic wave spectrum. ........................................................................ 6 Figure 2.2 Schematic illustration of propagation of EMWs with multiple reflections for a thin EMI shield (a) and internal interfaces (b) through an inhomogeneous EMI shield. ............................. 7 Figure 2.3 Schematic illustration of measuring complex scattering parameters from a two-port VNA of an EMI shielding material. .............................................................................................10 Figure 2.4 Chemical structures of conductive polymers. ...........................................................15 Figure 2.5 Chemical structures of PEDOT:PSS. .......................................................................16 Figure 3.1 Chemical structure of PEDOT, PSS, DMSO, DMF, EG, MeOH, and EtOH. .............24 Figure 3.2 Co-solvent doping and solvent post-treatment steps. ...............................................25 Figure 3.3 3D bioprinter used for DIW of the printed structures. ................................................26 Figure 3.4 MCR 102 Rheometer used for rheology tests. ..........................................................27 Figure 3.5 3D measuring laser microscope set-up used for topographical analyses. ................28 Figure 3.6 Electrical conductivity measuring equipment. ...........................................................29 Figure 3.7 VNA set up for electromagnetic Interference (EMI) Shielding measurements. ..........31 Figure 4.1 Chemical structure of PEDOT:PSS. .........................................................................32 Figure 4.2 Freeze-dried PEDOT:PSS nanofibrils and 6 wt.% ink of redispersed freeze-dried PEDOT:PSS nanofibrils in water. The redispersion of high concentrations of PEDOT:PSS in water results in strong flow-resistant inks (gels) that maintain their free-state shape. These inks are favorable for 3D direct ink writing. .............................................................................................33 Figure 4.3 Shear viscosity-shear rate curves of PEDOT:PSS inks with and without co-solvents. .................................................................................................................................................35 Figure 4.4 Shear stress-shear rate experimental data for pristine and co-solvent-doped PEDOT:PSS inks. .....................................................................................................................36 Figure 4.5 Storage and loss moduli as a function of shear strain for pristine (a1, a2), EG-doped (b1, b2), DMSO-doped (c1, c2), and DMF-doped (d1, d2) PEDOT:PSS inks. ...............................37 Figure 4.6 Storage and loss moduli as a function of shear stress curves for pristine (a), EG-doped (b), DMSO-doped (c), and DMF-doped (d) PEDOT:PSS inks. ..................................................38 Figure 4.7 3D printable range based on the shear viscosity (a), storage and moduli (b), and yield stress (c) as a function of PEDOT:PSS concentration. ..............................................................39 xi Figure 4.8 Digital images of filament formation for various concentrations of PEDOT:PSS (a). High-resolution one-layer printed (b) and high-aspect-ratio (c) 3D direct ink writing of DMSO-doped 6 wt.% PEDOT:PSS ink. ................................................................................................40 Figure 4.9 Direct ink wiring of DMSO-doped 6 wt.% PEDOT:PSS ink on glass, polystyrene, cellulose acetate, and polyethylene terephthalate substrates (i). ...............................................41 Figure 4.10 Digital images of a flexible free-standing printed triangle-patterned structure of DMSO-doped 6 wt.% PEDOT:PSS on a PDMS substrate. ........................................................42 Figure 4.11 Digital images of the wet-transferring of free-standing 3D-printed DMSO-doped PEDOT:PSS hydrogels on the edge of a glass slide. ................................................................42 Figure 4.12 Digital images of an ultra-lightweight freeze-dried 3D-printed geometry hovering over delicate cactus hair (a), microscope image of a triangle-patterned structure (b), SEM micrographs of a freeze-dried triangle-infilled 3D-printed structure (c), and digital images and SEM micrographs of an as-printed and freeze-dried grid-infilled 3D-printed structure (d). All geometries were printed using the 7 vol/vol% DMSO-doped 6 wt.% PEDOT:PSS ink. ....................................................43 Figure 4.13 Digital images of various printed infill-patterned architectures of 7 vol/vol % DMSO-doped 6 wt.% PEDOT:PSS ink. ................................................................................................44 Figure 4.14 Topographical micrographs of the printed line-, grid-, and triangle-patterned of 7 vol/vol % DMSO-doped 6 wt.% PEDOT:PSS ink. ......................................................................45 Figure 4.15 Profile (a) and surface roughness (b) of the printed lines of 7 vol/vol% DMSO-doped 6 wt.% PEDOT:PSS ink. ..........................................................................................................46 Figure 4.16 Line thickness variation (a) and line width variation (b) for the printed 7 vol/vol % DMSO-doped PEDOT:PSS 6 wt.% ink (f). ................................................................................47 Figure 4.17 Average line thickness (a) and line width (b) of different PEDOT:PSS-based inks. 47 Figure 4.18 Topographical micrographs of line-, grid-, and triangle-patterned structures of pristine 6 wt.% PEDOT:PSS ink (a), statistical analysis of line width variation (b) and line thickness variation (c) of the printed pristine 6 wt.% PEDOT:PSS lines. ...................................................48 Figure 4.19 Topographical micrographs of line-, grid-, and triangle-patterned structures of EG-doped 6 wt.% PEDOT:PSS ink (a), statistical analysis of line width variation (b) and line thickness variation (c) of the printed EG-doped 6 wt.% PEDOT:PSS lines. ..............................................49 Figure 4.20 Topographical micrographs of line-, grid-, and triangle-patterned structures of DMF-doped 6 wt.% PEDOT:PSS ink (a), statistical analysis of line width variation (b) and line thickness variation (c) of the printed DMF-doped 6 wt.% PEDOT:PSS lines. ............................................50 xii Figure 4.21 Electrical conductivity of co-solvent doped (a) and solvent post-treated (b) PEDOT:PSS printed films. ........................................................................................................51 Figure 4.22 Narrow-scan XPS spectra of the S2p signal for co-solvent doped (a) and solvent post-treated (b) PEDOT:PSS films. PEDOT/PSS ratio (c), calculated from the surface areas under the XPS spectra for PEDOT (~165 eV) and PSS (~169 eV) characteristic peaks. ..........................52 Figure 4.23 XRD spectra for co-solvent doped (a) and solvent post-treated (b) PEDOT:PSS films. .................................................................................................................................................53 Figure 4.24 Raman spectroscopy results for co-solvent doped (a) and solvent post-treated (b) PEDOT:PSS films. ....................................................................................................................54 Figure 4.25 Schematic illustration of PEDOT:PSS gel particles’ structural changes after the solvent treatment and drying techniques. ..................................................................................54 Figure 4.26 DIW printed visually transparent structures with different infill densities. ................55 Figure 4.27 Digital and microscopic images of the printed structures with different infill patterns and the number of printed layers. ..............................................................................................56 Figure 4.28 Total EMI SE of the printed pristine PEDOT:PSS structures. .................................56 Figure 4.29 Total EMI SE of dry-annealed DMSO-doped (a), and freeze-dried DMSO-doped (b) PEDOT:PSS structures. Total EMI SE of the printed dry-annealed and freeze-dried DMSO-doped PEDOT:PSS structures as a function of the number of printed layers (c). .................................58 Figure 4.30 Schematic illustration of EMI shielding mechanism of a 3D-printed grid-infilled structure. ...................................................................................................................................61 Figure 4.31 EMI shielding coefficients for DMSO-doped PEDOT: PSS structures. ....................61 xiii List of Symbols e Euler’s number 𝛿 Skin depth σr Relative electrical conductivity μr Relative magnetic permeability f Electromagnetic wave frequency 𝜀 r Relative complex permittivity 𝜀′ Dielectric constant 𝜀" Dielectric loss 𝜆 Wavelength Z0 Impedance of free space ε* Complex permittivity µ* Complex permeability E Electric field H Magnetic field S Complex scattering parameters S11 Ratio of reflected power to incident power in port 1 S12 Ratio of transmitted power from port 1 to port 2 to incident power in port 1 S21 Ratio of transmitted power from port 2 to port 1 to incident power in port 1 S22 Ratio of transmitted power from port 2 to port 1 to incident power in port 1 R Reflectance T Transmittance A Absorption P0 Power of incident electromagnetic wave PR Power of the reflected electromagnetic wave PT Power of transmitted electromagnetic wave 𝛼 Electromagnetic wave attenuation constant dB Decibel Hz Hertz J Current density SSE/t Specific shielding effectiveness τ Shear stress 𝜎 Shear stress 𝜎𝑦 Yield stress ?̇? Shear rate ?̇?𝑐 Critical shear rate n Flow index LVR Linear viscoelastic region G′ Storage modulus G′′ Loss modulus xiv List of Abbreviations PEDOT:PSS Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate AM Additive manufacturing EMI Electromagnetic interference DIW Direct ink writing DMSO Dimethyl sulfoxide EG Ethylene glycol DMF Dimethyl formamide RFI Radio frequency interference ICPs Intrinsically conductive polymers FMS Flexible manufacturing systems 1D One-dimensional 2D Two-dimensional 3D Three-dimensional PANI Polyaniline FDM Fused deposition modeling IEEE Institute of Electrical and Electronics Engineers EM Electromagnetic EMWs Electromagnetic waves EMI SE EMI shielding effectiveness FT-IR Fourier-Transform Infrared SEM Scanning Electron Microscopy NA Network analyzer VNA Vector Network Analyzer MUT Materials under test TRL Thru-reflect-line LRL Line-reflect-line LRM Line-reflect-match TRM Thru-reflect-match DC Direct current AC Alternating current CPCs Conductive polymer composites s-CPCs Segregated conductive polymer composites PMCs Polymer matrix composites PVA Polyvinyl alcohol PVDF Polyvinylidene fluoride PLA Polylactic acid PU Polyurethanes PEI Polyethyleneimine OLEDs Organic light-emitting diodes OTFTs Organic thin film transistors OPVs Organic photovoltaics PPy Polypyrrole PTP polythiophene ITO Pndium tin oxide DMF N, N-dimethyl formamide THF Tetrahydrofuran IPA Isopropyl alcohol ACN Acetonitrile xv FPL Fountain pen lithography DCB 1,2-dichlorobenzene HZ Hydrazine MEA Ethanolamine PSVA Polar-solvent vapor annealing CNTs Carbon nanotubes CB Carbon black MWCNTs Multiwall carbon nanotubes PS Polystyrene PMMA Poly(methyl methacrylate) WPU Waterborne polyurethane GO Graphene oxide GNP Graphene nanoplatelets SCCO2 Subcritical CO2 WVIPS Water vapor-induced phase separation NWF Nonwoven fabric PDMS Polydimethylsiloxane CAD Computer-aided design XPS X-ray Photoelectron Spectroscopy XRD X-ray diffraction analysis xvi Acknowledgments My sincere gratitude goes to my supervisor, Dr. Mohammad Arjmand, for his support and guidance throughout this challenging process. I would like to thank the Syilx Okanagan Nation for the use of their unceded territory, the land on which the research was conducted. I also would like to acknowledge Zentek Ltd. especially Dr. Colin van der Kuur and UBC Okanagan for providing financial support. I am also grateful to Dr. Mohammad Hossein Zarifi (Okanagan Microelectronics and Gigahertz Applications (OMEGA) Lab) for sharing the 3D laser microscope equipment for topography analyses. I am thankful to Dr. Jian Liu and Dr. Sina Kheirkhah for their guidance throughout the program. I am so appreciative to have the supportive and caring family that anyone could ever wish for. Above all, I would like to appreciate my wife, Dr. Somayeh Heidari, for her all support and patience during this tough time. Thank you for accepting me for who I am and where I am at right now. It is a huge source of comfort to know how much I am genuinely loved. xvii Dedication To Hanar For all her love and support along the way 1 Chapter 1: Introduction 1.1 General Background Intrinsically conductive polymers (ICPs) are a class of organic materials showing an elevated level of electrical conductivity, which makes them suitable for manufacturing organic electronics [1]. Despite the common conductive metallic materials, ICPs are lightweight, flexible, and corrosion-resistant. ICPs can be used to fabricate printed electronics, wearable sensors, and electromagnetic interference (EMI) shields. Among ICPs, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) has shown great possibilities for manufacturing printed electronics [2]. Due to its outstanding properties, such as high electrical conductivity, water processibility, exceptional film-forming properties, and chemical stability, PEDOT:PSS has attracted the attention of researchers to develop new classes of flexible organic electronics [3]. Additive manufacturing of PEDOT:PSS dispersions and inks are still challenging. Therefore, it is essential to introduce reproducible and reliable manufacturing techniques to fabricate high-performance EMI shields and printed organic electronics. 1.2 Gaps and challenges Despite extensive existing literature in the field of EMI shielding, there are still unexplored areas in two aspects of materials and processing. The available materials are mostly reflection-based, which can cause secondary electromagnetic pollution in the environment. Additionally, these materials are rigid and substrate-dependent, and thus cannot be used in developing flexible and free-standing EMI shields. Furthermore, the current processing techniques suffer from materials waste and their design and geometries are limited to molds and specific structures like thin films. This study aims to contribute to the growing field of flexible organic EMI shields and electronics by preparing, characterizing, and manufacturing highly electrically conductive patterned PEDOT:PSS architectures for EMI shielding applications as well as disclosing the mechanism of electrical enhancement by solvent treatment. 1.3 Thesis Objectives This thesis aimed to prepare, characterize, and employ solvent-treated PEDOT:PSS inks for EMI shielding applications, where the 3D printing process is performed by direct ink writing technique. This thesis contributes to: 2 • Drawing a rheological map for preparation of pristine and solvent-treated PEDOT:PSS inks with optimized shear viscosity, storage modulus, loss modulus, and yield stress values for developing 3D printable inks. • Printing flexible, self-standing, patterned, and highly electrically conductive structures for EMI shielding. • Producing hydrogels based on DMSO-doped PEDOT:PSS and wet-transferring them onto unusual substrates, such as edges, uneven substrates, and complex objects. • Fabricating lightweight and flexible patterned absorption-based EMI shields with high EMI SE. 1.4 Thesis Outline This thesis is focused on using additive manufacturing of PEDOT:PSS (an outstanding intrinsically conductive polymer) and solvent treatment (an efficient technique to enhance the electrical conductivity of PEDOT:PSS) to manufacture engineered high-performance EMI shields. To fulfill this, chapter 2 is compiled with a literature review on EMI shielding principles in addition to the state-of-the-art materials and technologies that are applied to polymeric EMI shields. Additionally, it provides a background on processing conductive polymers, including fundamentals, technologies, and applications. Chapter 3 encompasses the methodology and experimental procedures used in this thesis, including sample preparation, 3D printing, and characterization techniques and analyses. In chapter 4, the achievements of the current work, including rheological data, 3D printing of support-free geometries, and high-resolution micron-sized 3D printing are provided. In addition, chapter 4 contains the electrical conductivity results for co-solvent doped PEDOT:PSS using DMSO, EG, and DMF, and solvent post-treated PEDOT:PSS using DMSO, EG, DMF, methanol, and ethanol are reported. In the end, in this chapter the effects of infill density, the number of printed layers, and the drying method on the EMI shielding performance of the manufactured DMSO-doped PEDOT:PSS EMI shields are studied. 3 Chapter 2: Literature Review 2.1 Introduction In the following chapter, first, direct ink writing with its processing details is introduced as a beneficial additive manufacturing technique for processing polymeric materials. Then, the concept of EMI shielding along with its mechanisms, materials, and testing methods is discussed in detail. Next, the physical and chemical properties of electrically conductive poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) are described in detail. Finally, diverse forms and structures of materials feasible for manufacturing EMI shields are explored. 2.2 Additive manufacturing (AM) Additive manufacturing (AM) technology has recently been highlighted for its potential to spark a revolution in manufacturing by extending the features of conventional technologies [4,5]. In AM, materials are joined layer by layer to make objects based on 3D models, also referred to as “3D printing”. One of the main advantages of AM technologies is that it enables the versatile production of customized parts and products at lower costs. This is accomplished by using direct digital manufacturing techniques to convert 3D models directly into physical parts without requiring tools or molds [6]. As well as producing functional parts in a single manufacturing step. Layer-by-layer manufacturing can also reduce assembly requirements. In this way, AM technologies affect capital costs, marginal production costs, flexibility, and individualization costs significantly [4]. Although AM offers many advantages, there are also some limitations to it, such as the available materials do not always match those used in conventional manufacturing processes, production speeds are rather slow, and most manufacturers still require an additional surface finish [4]. It is also critical to note that AM technology affects market structure in a variety of ways beyond the direct effects it has on the production process. In recent years, a growing community of developers has developed and shared 3D models, sold 3D printed products on marketplaces, and even developed and provided 3D printers for home use [6,7]. A significant advancement in printed electronics would be the fabrication of structures of electrically conductive polymers using printing techniques. Various laboratory-based techniques have been investigated for the development of electrically conductive thin films, including spray coating, spin coating, doctor blade coating, and slot-die coating [8,9]. In these techniques, thin 4 films are produced that uniformly coat substrates without allowing for patterning, but emerging applications in electronics and bioelectronics require the ability to create two-dimensional (2D) or three-dimensional (3D) structures of a conductive material. Until now, lithography has been the most common method for patterning thin films. In addition, conductive hydrogels can also be fabricated by using customized molds made by lithography [10,11]. This technique has been used to form conductive polyaniline (PANI) and PEDOT:PSS hydrogel patterns when their precursors are cast in molds and the hydrogels are transferred to the target substrates after gelation [12]. Its scalability is limited due to the multiple processing steps involved in this technique. There is a great need for developing a simple method for preparing PEDOT:PSS films or PEDOT:PSS hydrogels that require the least amount of pre- or post-processing steps [8]. Inkjet printing has become increasingly popular for the deposition of multiple materials in printed electronics [13]. As a cost-effective and rapid method for patterning functional materials, inkjet printing eliminates the need for lithography or molding. Inkjet printing has been used to produce patterned depositions, including printed circuits, organic solar cells, transistors, light-emitting diodes, and bioelectronic applications [14–16]. However, it produces thin films and requires materials with low fluid viscosity (<40 cP) [17]. 2.2.1 Direct ink writing (DIW) A significant amount of attention has been given to fine-tuning processing techniques in organic electronics to make them more beneficial. Common manufacturing methods are time-consuming, costly, and produce low-quality end products, which often hinder their effectiveness. As mentioned above, 3D printing has gained rapid adoption in recent years due to several interesting advantages, such as the ability to customize object geometry, cost-effectiveness, and rapid on-demand fabrication [18,19]. It is common to print flexible substrates with a variety of printing techniques, including contact printing (flexography, gravure, offset, and screen printing) and non-contact printing (inkjet and aerosol). Since direct ink writing (DIW) consumes less material, costs less, and allows for easy adjustments, it is gaining much attention for ink printing. However, the viscoelastic properties of inks limit their use [20,21]. Inks must flow to pass through the printing nozzle yet must be viscous enough to deposit 3D patterns after printing and retain their structure. Moreover, they should possess properties such as shear-thinning or stimuli-responsiveness to enable 3D printing. A shear-thinning ink can be printed under shear force and then extruded to regain its mechanical properties [22]. 5 For DIW to be successful, the ink needs to possess appropriate rheological properties to extrude continuous filament and maintain its shape. Rheological parameters are the cornerstone of successful manufacturing for forming self-supporting layers during and after printing. During printing, intense shear-thinning properties result in a rapid transition from a fluid-like to a solid-like behavior and vice versa. It has been shown that the rheological properties of inks containing 7 wt.% PEDOT:PSS is desirable for 3D printing implantable conductive patches [23]. Furthermore, it has been reported that adding a 2D nanofiller to PEDOT:PSS improved its rheological properties and led to high-resolution printed lines [24]. Direct ink writing involves applying a viscous fluid (ink) to extrude through a printing nozzle to create or add functionality. Inks that are polymerized by heat or light also can be printed with direct ink writing. As a result, metals [25], ceramics [26], or wood [27] can be deposited with properties similar to those of solid materials inside the ink. In DIW using a pneumatic extruders, the pressure inside a syringe is controlled with high-precision pressure generators. Syringes are connected to dispensers through flexible tubes, but only the syringe moves [28,29]. Pressure should be adjusted according to ink’s viscosity and nozzle’s hydraulic resistance to adjust the required volumetric flow rate for 3D printing. 2.3 Electromagnetic wave spectrum The electromagnetic waves (EMWs) spectrum (Figure 2.1) includes diverse types of waves for different applications, among which the microwave range is used for various applications ranging from paging and telecommunications covering low- and high-speed data and video transmission. As part of the electromagnetic spectrum, the X-band is a band of frequencies of 8.2–12.4 GHz, based on the specifications of the Institute of Electrical and Electronics Engineers (IEEE). X-band is used for telecommunications and radars. 6 Figure 2.1 The electromagnetic wave spectrum. 2.4 Electromagnetic interference (EMI) The rapid progress in technology and the widespread use of electronics have caused electromagnetic interference (EMI) to become a major challenge [30]. In electromagnetic interference, electromagnetic noise is generated either by natural sources (solar flares, lighting, etc.) or by manmade electronic devices. It affects the performance or degradation of other electronic devices/electrical circuits and losses stored data over a frequency range that varies depending on the source [31]. EMI may result from electromagnetic coupling, electromagnetic induction, or electromagnetic conduction. It has also been found that human health is adversely affected by anthropogenic electromagnetic noise [32]. Due to these major concerns, researchers have developed materials for EMI shielding/attenuation, which have been applied to both electronic and biological systems [33]. 2.5 Health hazards of electromagnetic radiation EMWs are not only reducing the performance of other devices, but they can also harm living organisms and biodiversity as well as pose health risks to humans. EM radiation can cause nervousness, insomnia, headaches, languidness, tumors, infertility, and other symptoms, in addition to damaging tissues and cells [34]. Electronic devices can be protected by shields, but humans, animals, or living organisms cannot be protected and will lose contact with their surroundings. Therefore, biodiversity and humans must both be protected and shielded [35]. 7 2.6 EMI shielding mechanisms To address EMI, EMI shielding has been introduced as a mechanism to reduce, lock, or filter the EMWs. In EMI shielding, reflection, absorption, and multiple reflections within the shield are controlled to minimize the transmission of EMWs. The primary mechanism of EMI shielding is reflection, which because of the existing impedance mismatch between air (first phase environment of the incident EMWs) and the shield (second phase), some incident EMWs on the shield's surface are reflected [36]. The second shielding mechanism is absorption loss. EMWs that do not reflect across the front surface of the shield propagate within it. The propagation of EMWs is exponentially dissipated as the shield’s thickness increases [37] By absorbing electromagnetic waves, absorption losses are caused by Ohmic, dielectric, and magnetic losses. As a portion of the EMWs reaches the back surface of the shield, they are also reflected, and only quite a small of them are eventually transmitted [38]. In addition, multiple reflections within the shield contribute to the shielding efficiency. Multiple internal reflections happen when there is a scattering of EMWs at internal interfaces within very thin shields and shields with a porous, foam, or inhomogeneous structures [39]. The EMI shielding mechanisms for homogenous and porous materials are schematically shown in Figure 2.2. Figure 2.2 Schematic illustration of propagation of EMWs with multiple reflections for a thin EMI shield (a) and internal interfaces (b) through an inhomogeneous EMI shield. Losses caused by the Ohmic properties of materials are called Ohmic losses. An Ohmic loss is a voltage drop across a circuit. A circuit's voltage drop is directly proportional to its current flow according to Ohm's law [40,41]. Additionally, the dielectric loss of a dielectric material quantifies the power of the material in the dissipation of electromagnetic energy. The molecules and atoms of a dielectric material form dipoles when an electric field is applied. Dielectric permittivity is a 8 dimensionless quantity that describes how well dipoles line up in the direction of an electric field [42]. Polarization occurs at a variety of different frequencies due to electric, atomic, dipolar, space charge, and ionic sources [43]. Measuring dielectric (relative complex permittivity) and magnetic (relative complex permeability) properties can reveal EMI shielding mechanisms. Furthermore, the real part of complex permittivity (ε′) is also known as the dielectric constant and the imaginary part (ε′′) as dielectric loss or loss factor, and the imaginary part is defined as a dielectric loss (𝜀") as shown in Equation 2.1: 𝜀 = 𝜀′ + 𝑖𝜀" (2.1) The real part ε′ represents the electric energy storage, whereas ε′′ is the Ohmic loss (related to the conductivity of a material). The temperature and frequency of the electric field affect both parts of complex permittivity. The permittivity of a material also depends on its composition and its lattice flexibility [43]. 2.6.1 EMI shielding effectiveness (EMI SE) EMI shielding effectiveness (EMI SE) refers to the ability of a material to attenuate or reduce EM signals. As shown in Equations 2.2-2.4, EMI SE is a ratio of power, magnetic field, or electric field of the incident waves to those of the transmitted waves [44]. 𝐸𝑀𝐼 𝑆𝐸𝑃 = 10 log(𝑃𝑖𝑛𝑃𝑜𝑢𝑡) (2.2) 𝐸𝑀𝐼 𝑆𝐸𝐸 = 20 log(𝐸𝑖𝑛𝐸𝑜𝑢𝑡) (2.3) 𝐸𝑀𝐼 𝑆𝐸𝐻 = 20 log(𝐻𝑖𝑛𝐻𝑜𝑢𝑡) (2.4) where P, E, and H are the power of the plane wave, electric field, and magnetic field of the incoming EM wave, respectively. The index in and out refer to the incident and transmitted EMWs through the EMI shielding material, respectively. The unit for describing EMI SE is decibels (dB). An electromagnetic wave is composed of electric (E) and magnetic (H) fields perpendicular to each other moving in the direction of the wave propagation. The characteristics of an EM wave depend on its frequency and associated photon energies, and it propagates at a right angle to a plane containing an electric field and magnetic field. In electromagnetic fields, wave impedance 9 is the ratio between the magnitude of the electric and magnetic fields. Based on the distance (r) of an EMI shielding material from an EM wave source, EMI shielding can be divided into far-field or near-field regions. A far-field region is defined as the distance (r) between the shielding material and the EMW source where it exceeds 𝜆/2𝜋 causing the EM wave impedance (E/H ratio) to be equal to the intrinsic impedance of free space (Z0 = 377Ω). As a result, plane waves exist in the far-field region, and SEE = SEH exists [45]. In the near-field region, where r less is than 𝜆/2𝜋, the EMW impedance unequal the intrinsic impedance of the free space. Depending on the source and distance, an EMW is either electrically or magnetically dominant (large EMW impedance). In case the impedance value of an EMW is greater than Z0, then it's an electric field dominant wave and vice versa. Therefore, SEE ≠ SEH. Distances less than 𝜆/2𝜋 mark the transition point. At approximately 𝜆/2𝜋, r represents the region of a transition [45]. The total EMI SE is the sum of reflection SE (SER), absorption SE (SEA), and multiple internal reflections SE (SEM). SETotal = SEA + SER + SEM (2.5) Reflection losses and multiple-internal reflection losses are dependent on the impedance, so vary with different electric fields, magnetic fields, and plane waves. The absorption phenomena, however, do not vary with impedance, and therefore the absorption loss will remain constant for all three fields. When the value of SEA is larger than 10 dB, SEM is negligible. 2.6.2 EMI SE measuring configurations To measure the EMI SE of an EMI shield, a network analyzer (NA) is commonly used, which is classified into scalar network analyzers (SNAs) and vector network analyzers (VNAs). SNAs can only measure amplitudes of electrical signals, while VNAs can measure both phase and magnitude of signals. Although VNAs are expensive, they are the most used equipment due to the inability of SNA to measure complex permittivity (ε*) and complex permeability (µ*). For this aim, as illustrated in Figure 2.3, network analyzers send a single frequency signal from port one (S1) to the EMI shield, and receivers measure the reflection (S11) and transmission (S21) waves as well as their phase and magnitude. The same process is repeated for port two, and the same signal (S2) is emitted into port two, and the reflected (S22) and transmitted (S12) waves are recognized by the receiver. For a uniform and homogeneous shield, S11 and S22, as well as S12 and S21, should be of equal magnitude since the frequency and EMI shield are the same [46]. Repeated measurement of each frequency is performed for the entire spectrum. In this case, the 10 complex scattering parameters (S) are utilized to calculate the reflection and transmission of EMWs. Figure 2.3 Schematic illustration of measuring complex scattering parameters from a two-port VNA of an EMI shielding material. The electromagnetic wave absorption (A) of an EMI shield is defined based on the achieved reflectance (R) and transmittance (T) from the scattering parameters, as follows: A = 1 - R - T (2.6) 𝑅 = |𝑃𝑅𝑃0| = |𝑆11|2 = |𝑆22|2 (2.7) 𝑅 = |𝑃𝑇𝑃0| = |𝑆12|2 = |𝑆21|2 (2.8) Where PR, PT, and P0 are the power of the reflected, transmitted, and incoming wave, respectively. The EMI shield’s performance is calculated by the EMI shielding effectiveness (EMI SE) of a shield, as follows: 𝑆𝐸𝑅 = 10 log (11 − 𝑅) = log (11 − |𝑆11|2) = log (11 − |𝑆22|2) (2.9) 𝑆𝐸𝐴 = 10 log (1 − |𝑆11|2|𝑆11|2) = 10 log (1 − |𝑆22|2|𝑆21|2) = log (1 − 𝑅𝑇) (2.10) By using the phasor form of the electric and magnetic fields, the EMI SE parameters can be rewritten as follows: 11 𝑆𝐸𝑅 = 39.5 + log [𝜎2𝜋𝑓𝜇] = 50 + 10 log [𝜎𝑓] (2.11) 𝑆𝐸𝐴 = 20 log 𝑒𝛼𝑑 = 20𝛼𝑑 log 𝑒 = 8.7 𝑑𝛿= 8.7𝑑 √𝜋𝑓𝜇𝜎 = 1.7𝑑 √𝑓𝜎 (2.12) 𝑆𝐸𝑀 = 20 log(1 − 𝑒−2𝛼𝑑) = 20 log(1 − 𝑒−2𝑑𝛿 ) (2.13) 𝜎 : the electrical conductivity (S m-1) of the shield, 𝑓 : is the frequency (Hz) of the wave, 𝜇 ∶ is the magnetic permeability (H m-1) of the shield, 𝛼 : is the attenuation constant (Np m-1), d : is the thickness of the shield (m), and 𝛿 ∶ is the skin depth of the shield (m). Therefore, the total EMI shielding effectiveness (EMI SETotal) is the summation of these three components. The value for SEM significantly changes based on the thickness of the EMI shield and becomes negligible when the thickness equals to or greater than the skin depth of the material or when the SEA value is greater than 10 dB. Therefore, the SETotal can be rewritten as: 𝑆𝐸𝑇𝑜𝑡𝑎𝑙 = 50 + 10 log [𝜎𝑓] + 1.7𝑑√𝑓𝜎 (2.14) By using the S parameters, a network analyzer (NA) provides an overview of electrical network characteristics, including reflection and transmission of incident EMWs (from a few KHz to 110 GHz). NAs with two ports are capable of emitting EM radiation in the specific frequency region from both ports as well as measuring the reflected and transmitted radiations from materials under test (MUT) [47]. Transmission-reflection lines such as coaxial airlines & waveguides are the most used measurement approaches for solid samples, along with free-space measurements such as reverberation chambers and arch reflectivity systems [48]. In the waveguide method, the EMI shield is usually introduced in a rectangular section. The dimensions of the waveguide relies on the frequency range being measured, and with increasing the frequencies, waveguide size decreases. A major benefit of this method is the simple sample preparation process, while a major drawback is a need for many waveguides since the measuring frequencies are narrow [47,48]. 12 In the coaxial airline method, the EMI shield is placed at a precise distance from both ends of the internal and external conductors. In this technique, the MUT is prepared in a rectangular toroidal shape. The main advantage of this method is that it can be used to measure any frequency range with the same sample. However, the downside is that it requires a very thin ring-shaped sample and must be prepared with great care to eliminate air gaps, which can lead to unfavorable measurement errors [47,48]. The free space method is a non-contact method characterized by two antennae on either side of the MUT. One of the advantages of this method is the possibility of measuring a wide frequency range. In addition, it is possible to change the angle of incidence of electromagnetic radiation on the MUT. Its major drawback is the larger MUT required compared to the MUT used in the previous methods. To measure EMI SE at frequencies of 0-15 GHz, the MUT should have a dimension range of 10-15 cm, while at frequencies of 20 GHz, it should have a dimension range of 30–40 cm [47,48]. In noncoaxial systems, such as waveguides, wafer probing, and fixtures, the thru-reflect-line (TRL) is a well-known two-port calibration standard. An interesting advantage of TRL calibration is its ease of design, fabrication, and characterization. The TRL calibration standards of a four-port VNA can be executed while the TRL calibration standards of a three-port VNA analyzer are commonly called TRL*. A TRL calibration can involve several variations, including line-reflect-line (LRL), line-reflect-match (LRM), and thru-reflect-match (TRM). They all use error models analogous to that of TRL calibrations [47,48]. 2.7 AC/DC conductivity The electrical conductivity of materials depends on their nature, which can vary from non-conductive plastics and glasses to highly conductive metals. Accurate measuring conductivity is often challenging, especially in polycrystalline materials. Direct current (DC) conductivity refers to the flow of charges under a unidirectional unit field. DC conductivity measurements are necessary to guarantee that the conductivity values are primarily dependent upon charge migration over long distances, and not on dielectric losses associated with localized or limited fast-moving ions [49]. DC is known as the constant ratio in the linear relationship between current density (J) and the applied electric field gradient (E) as follows: 𝐽 = 𝜎 𝐸 (2.15) 13 Conductivity measurements in a spectrum of frequencies can also be done using alternating current (AC) techniques. DC conductivity values can also be derived from AC data for the material. [49]. 2.8 EMI shielding materials 2.8.1 Metals The magnetic material can shield electromagnetic waves largely by resonance, hysteresis loss, eddy current loss, ferromagnetic resonance, and domain wall resonance. A magnetic shield material can be classified as either iron alloy material, cobalt alloy material, nickel alloy material, or metal oxides like carbonyl nickel, ferrite, and garnet [50]. Metals are shown to be the best electrical conductors so far, they are capable of reflecting, absorbing, and transmitting EM waves. Because of their excellent thermal and electrical conductivity, metals are highly preferred for a variety of applications. For years, metal closures and shields have been used to prevent electromagnetic waves from penetrating [51]. Rubber and plastics are both transparent for EM and electrically insulating to EM waves. The most used metallic EMI shield is mumetal, an alloy made from iron (14%), copper (5%), chromium (1.5%), and nickel (79.5%) [52]. As shielding materials, silver, aluminum, nickel, brass, metal-filled plastics, and conductive carbon/graphite materials are also used. Because of their district characteristics, the application of these metals is limited. For instance, carbon/graphite is brittle, stainless steel is highly dense, and aluminum is not very impact-resistant [53]. The corrosion of the metallic shields is the main challenge that still prevents the wide usage of these materials. Galvanic corrosion of distinct metals can affect the EMI shield, which leads to non-linear and thus, reduces the total EMI SE [54]. For reflection-dependent EMI shielding applications, magnesium (Mg) enclosures are highly preferred due to active response over the entire frequency range. 2.8.2 Conductive polymer composites (CPCs) It is highly advantageous to use conductive composites (CPCs) containing conductive fillers in EMI shielding applications due to their facile processing and their ability to minimize seams in shields or housings. Seams in metal sheets make them inefficient shielding materials because they leak EM radiation, reducing their overall shielding efficiency. Because of their low density and fair EMI shielding characteristics, CPCs are a proper candidate for EMI shielding. The 14 processability of composite materials also greatly depends on the type of polymer matrix, which can be insulating or conducting polymers. A range of metal-based fillers can be added to non-conductive polymers such as polyurethanes (PU), polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), polyethyleneimine (PEI), and polylactic acid (PLA). 2.8.3 Intrinsically conductive polymers (ICPs) ICPs have both polymeric and metallic properties. In addition to electrons and holes, conductive polymers can conduct ions. Some substances exhibit both metals’ and semiconductors’ electrical and optical properties due to their conjugated structure. The presence of a delocalized π-system, generated from p-orbitals of the double bond, provides the path for charge carriers' movement. It is possible to increase electrical conductivity by forming long chains of conjugated carbon atoms because this reduces the band gap between the highest occupied and lowest unoccupied molecular orbitals. Due to their low cost, ease of synthesis, and tunable conductivity, ICPs are used in many applications, including flexible organic electronics [55], optoelectronics and storage devices [56], soft actuators [57], organic light emitting diodes (OLEDs), thermoelectric devices [58], organic thin film transistors (OTFTs), electromagnetic interference (EMI) shields, and organic photovoltaics (OPVs). Due to their long-term stability, better mechanical compatibility, and increased signaling sensitivity, conductive polymers are increasingly being used for diverse biomedical applications, such as sensors, bioelectronics, and tissue engineering [2,55,59]. Conducting polymers are capable of recording more sensitive results/signals and showing more stability over time than metals. Polyacetylene doped with iodine has a comparable to metals’ electrical conductivity (1.7 × 105 S cm-1). Diverse conductive polymers have been studied for EMI shielding applications because of their excellent electrical conductivity, including polyaniline (PANI), polyacetylene, polypyrrole (PPy), poly (3,4-ethylenedioxythiophene) (PEDOT), and polythiophene (PTh), as their chemical structures are shown in Figure 2.4 [60]. Compared to metals, ICPs have low density (1.1–1.3 g cm-3) and corrosion-resistant properties. As opposed to metals and carbon-based materials where reflection dominates shielding mechanisms, ICPs usually exhibit both absorption and reflection mechanisms. These conjugated polymers possess inherent conductivity in the microwave and radio wave frequency ranges (100 MHz–20 GHz), which makes them excellent EMI shielding materials. 15 Figure 2.4 Chemical structures of conductive polymers. 2.9 PEDOT:PSS Poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) is a newly developed intrinsically electrically conductive polymer [61]. A wide variety of electronic devices are fabricated using PEDOT:PSS, including flexible electronics, soft actuators, optoelectronics, storage devices, thermoelectric devices, supercapacitors, solar cells, transparent electrodes, electromagnetic interference shields, and sensors. PEDOT:PSS is an effective material for advanced organic electronics due to its advantageous properties, including water processability, tunable electrical conductivity, chemical stability, and superior flexibility [62]. In its stable and bipolaronic conductive state, PEDOT is a conductive polymer with a low band gap. Short PEDOT oligomers (~6-18 repeating units) form a highly crystalline state with high electrical conductivity [63–65]. Oxidative polymerization of PEDOT with polystyrene sulfonate (PSS) forms a stable gel dispersion with conductive PEDOT grains and a nonconductive PSS matrix in water. In the form of water dispersion gel particles, PSS serves as a charge-balancing counterion for PEDOT cations [64,66]. PEDOT:PSS has exceptional film-forming properties that have made it a widely available commercial conductive polymer. The conductivity of PEDOT:PSS is largely affected by the size of the gel particles, with a larger gel particle size corresponding to higher conductivity. Gels of smaller size have more boundaries when they are dried, resulting in a greater resistance [63,65]. A high-pressure shear rate can be applied during manufacturing to diminish the number of boundaries, or a secondary dopant can be added during processing to increase the intermingling 16 of individual gel particles, which increases electrical conductivity [65]. The chemical structure of PEDOT:PSS is exhibited in Figure 2.5. Figure 2.5 Chemical structures of PEDOT:PSS. 2.9.1 PEDOT:PSS treatment In addition to its excellent film-forming properties, PEDOT:PSS has excellent optical transparency, excellent thermal stability, and high conductivity using secondary doping [67,68]. Former studies have reported that the conductivity of PEDOT:PSS can be remarkably improved using various treatment strategies including using dopants, such as organic solvents, ionic liquids, surfactants, salt solutions, zwitterion, acidic, and alkaline solutions, as well as thermal and light treatments [69–76]. In comparison with pristine PEDOT:PSS, the electrical conductivity of treated PEDOT:PSS films can be enhanced by four orders of magnitude via sulfuric acid post-treatment [77]. In organic electronics, highly electrically conductive and transparent PEDOT:PSS films can replace indium tin oxide (ITO) as a charge transport layer [78,79]. 2.9.1.1 Thermal treatment A variety of post-treatments can dramatically alter the electrical properties of PEDOT:PSS by altering its morphology, chemical structure, and physical structure. PEDOT:PSS films are known for their thermal stability; they can be heated at 100°C for over 1000 hours without substantially changing their electrical conductivity [64]. As reported by Huang et al. [69], spin-coated PEDOT:PSS films have demonstrated a remarkable improvement in electrical conductivity 17 appeared when thermally treated at temperatures from 100 °C to 250 °C and during different treatment times at 200 °C, which was well aligned with the surface roughness, which indicates a significant improvement in conductivity after thermal treatment. Consequently, thermal treatment could be a useful tool for improving PEDOT:PSS conductivity [62]. 2.9.1.2 Co-solvent doping treatment For the first time, Kim et al. showed that DMSO, N,N-dimethyl formamide (DMF), and tetrahydrofuran (THF) can enhance the electrical conductivity of PEDOT:PSS films [72]. By adding DMSO, the conductivity was enhanced by about two orders of magnitude, while by adding DMF or THF, the enhancement was less notable. Several polar organic solvents have been employed to improve the conductivity of PEDOT:PSS films, including DMSO, DMF, THF, EG, diethylene glycol, methoxyethanol, glycerol, sorbitol, dimethyl sulfate, meso-erythritol, and xylitol, showing the final electrical conductivity ranging from 0.61 to 677 S cm−1 [62]. By addition of ethylene glycol (EG), meso-erythritol, and 2-nitroethanol solvents to the PEDOT:PSS aqueous solution, Ouyang and coworkers improved the conductivity of the resulting film by over two orders of magnitude [80,81]. Xu et al. have explored the electrical conductivity of treated PEDOT:PSS with DMSO and EG, and reported a DMSO-doped PEDOT:PSS nanofilm as an efficient organic electrode for electrochemical polymerization [82–85]. In some cases, DMSO or EG could be used as a co-solvent in PEDOT:PSS aqueous solutions. Wei et al. confirmed that the increase in electrical conductivity of PEDOT:PSS was mainly because of the increased carrier mobility and density [86]. The electrical conductivity of PEDOT:PSS has also been enhanced using glycerol and sorbitol as dopants [87]. Makinen and co-workers found that PEDOT:PSS films became more electrically conductive upon glycerol doping [88]. A concentration-dependent increase in PEDOT:PSS film conductivity was also found by Lai et al [89]. As a solvent-free electric glue, sorbitol mixed PEDOT:PSS was reported by Ouyang et al. to demonstrate an electrical conductivity of 102 S cm−1 [80]. In a research by Huang et al. [90] and Nardes et al. [91] as well. Onorato et al. found that sorbitol-treated PEDOT:PSS films are subjected to cyclodehydration to form 1,6-anhydrosorbitol in the presence of polymer and demonstrated that sorbitol and 1,6-anhydrosorbitol remain in the structure, improving its conductivity [92]. 18 2.9.1.3 Solvent post-treatment In addition to co-solvents doping of the PEDOT:PSS aqueous solution (co-solvent doping), Ouyang et al. [93] reported that EG or DMSO post-treated PEDOT:PSS films have higher electrical conductivity. When the solvents were changed to pyridine, cyclohexone, methyl alcohol, and acetonitrile, the enhancement was not apparent. Using inkjet printing and line patterning approaches, Sankir et al. [94] fabricated PEDOT:PSS thin films and post-treated them with EG, resulting in 350- and 900-times improvement in conductivity. According to Okuzaki and co-workers [95], by wet-spinning PEDOT:PSS microfibers and then dipping them in EG, the conductivity of the PEDOT:PSS microfibers improved largely from 74 to 467 S cm−1. Ely et al. [96] used a spray deposition technique to obtain smooth PEDOT:PSS films, and when the films were treated with EG, the film's electrical conductivity peaked at 610.1 S cm−1. In a study by Luo and colleagues, DMSO post-treatment of PEDOT:PSS significantly improved conductivity compared with DMSO addition. However, Unsworth et al. reported that the electrical conductivity of PEDOT:PSS was improved by both DMSO doping and DMSO post-treatment approaches [97]. Solvents with low boiling points due to the rapid evaporation rate, evaporate fast before they undergo any necessary change to achieve high electrical conductivity. The post-treatment method using low boiling point solvents is shown to be efficient in enhancing electrical conductivity of PEDOT:PSS. Alemu et al. [98] used methanol post-treatment for PEDOT:PSS films and the conductivity rose from 0.3 S cm−1 to 1362 S cm−1. In addition to enhancing the electrical conductivity by four orders of magnitude, methanol treatment was also fast. Compared to methanol, other alcohols such as ethanol and propanol, enhanced the conductivity of PEDOT:PSS films less efficiently [93]. The intrinsic characteristics of the alcohols, such as hydrophilicity and dielectric constant were considered major factors in enhancing conductivity. As a result of treatment with water or with organic solvents such as water, acetone, ethanol, acetonitrile (ACN), isopropyl alcohol (IPA) or tetrahydrofuran (THF), PEDOT:PSS films didn't exhibit significant changes in conductivity. Nonetheless, it was found that co-solvents such as water and organic solvents largely enhance the electrical conductivity of PEDOT:PSS films. PEDOT:PSS’s electrical conductivity enhanced from 0.2 S cm-1 to more than 100 S cm-1 [99]. Additionally, Kim et al. [100] presented an inexpensive and simple fountain pen lithography (FPL) technique to fabricate PEDOT:PSS polymer arched nanowires. Electrical conductivity increased from 0.02 S cm−1 to 15.7 S cm−1 without any significant changes of the resulting wires. After 19 treating the PEDOT:PSS films with hydrophilic methanol and hydrophobic 1,2-dichlorobenzene (DCB), three orders of magnitude improvement in electrical conductivity were achieved. PEDOT and PSS have hydrophobic and hydrophilic properties, respectively, which can be combined to selectively de-dope PSS with hydrophilic solvents such as EG. The PEDOT:PSS aqueous solution was treated with DMSO (or EG) and then the polymer film was etched in an EG bath, according to Kim et al [101], this boosted the conductivity up to 1418 S cm−1. Additionally, Pipe et al. [102] employed a similar approach to investigate the enhancement of conductivity in PEDOT:PSS (DMSO-mixed) after treatment with an EG bath. The de-doping of PEDOT:PSS (DMSO or EG-mixed) films with ethanolamine (MEA), hydrazine (HZ), and ammonia solutions was unsuccessful due to the decreased conductivity. A notable conductivity was achieved for PEDOT:PSS (glycerol-mixed) films after EG treatment [103–105]. DeLongchamp et al. [106] rinsed PEDOT:PSS films with water. A reduction of 35% in film thickness is achieved when PSS was removed, and a 50% increase in conductivity was achieved. A combination of solvent and thermal treatments is used by Dimitriev and colleagues to assess the electrical conductivity of PEDOT:PSS films. As an alternative method of increasing the electrical conductivity, polar-solvent vapor annealing (PSVA), which involves exposing PEDOT:PSS films to solvents’ vapor, has been used [107]. Yeo et al. [108] have reported that DMSO-treated PEDOT:PSS films by PSVA method showed a much higher electrical conductivity than conventional solvent additives, peaking at 1057 S cm−1. Therefore, PSVA is a promising candidate for developing polymeric blend films and more effective ITO-free organic electronics. According to Jiang et al. [109], PEDOT:PSS nanofilms exhibit an improved conductivity with PSVA using DMSO, EG, DMF, NMP, and H2O, with a maximum conductivity of 496 S cm−1 for EG-treated PEDOT:PSS. Interestingly, H2O was able to significantly increase the conductivity of PEDOT:PSS film from 0.2 S cm−1 to 57 S cm−1. It was shown that, in PEDOT:PSS films, the carrier mobility, and carrier concentration were simultaneously increased. As a result of these technological advances, thermal and organic solvent treatments have shown promising results, with less environmental impact and easier processing. As a secondary dopant or as a post-treatment agent, polar or high-dielectric co-solvents alter the electrical conductivity of PEDOT:PSS. PEDOT:PSS conductivity enhancement in the presence of solvents has been attributed to a variety of factors, including PEDOT grain size, PEDOT crystallinity, Columbic screening effect, and phase separation between PEDOT-rich grains and PSS-rich segments. This leads to the partial removal of nonconductive PSS from conductive PEDOT-rich cores and the extension of the PEDOT:PSS coil conformation. 20 2.10 Structure design for EMI shielding Polymeric EMI shielding materials can be formulated in a wide range of architectures like polymer composites and are abundantly available as precursors to polymer derivatives. Polymer-based shields also satisfy the mass market's need, which spawns a variety of products, such as conductive coatings, pastes, adhesives, fabrics, foams, gaskets, transparent films, etc. A surge of EMI shielding development and production is occurring regarding 5G internet [110]. 2.10.1 Homogenous polymeric composites For EMI shielding, homogeneous structures are designed to ensure uniform distribution of fillers. A percolation network is formed by fillers dispersed throughout the polymer matrix, which is affected by higher loadings. The uniform dispersion of fillers is ideal when the materials are isotropic. In mixed filler composites, fillers may have varied sizes and different dimensions. Filler contents are frequently high (greater than 5 wt.%), and this may increase the risk of agglomeration in the matrix and therefore affect the composite's performance adversely. Through both external force intervention and modification of the interior filler, dispersibility could be improved. Due to their straightforward operation, homogenous structures have gained popularity in commercial sectors. A homogenous structure can be obtained in a variety of methods, including situ polymerization, solution blending, and melt blending. 2.10.2 Porous structures Lightweight designs are required for EMI shielding applications. To attain realistic shielding performance, these criteria must balance EMI SE with lightweight. The terms specific shielding effectiveness (SSE) and absolute shielding effectiveness (SSE/t) describe these criteria. In addition to reducing density, pores also improves the multiple internal reflections of EMWs, thereby increasing SE [111]. 2.10.2.1 Foams Porous materials such as foams and aerogels are both well-known for their porous architectures. Chemical and physical foaming agents have been used in industrial foam production [112]. The chemical blowing agent used by Gupta et al. [113], 2,2′-azoisobutyronitrile (AIBN), was developed for fabricating the composite carbon nanotube/polystyrene (CNT-PS) foams. When heated, AIBN decomposes and releases nitrogen gas, which improves EMI shielding. A mixture of graphene 21 nanosheets and PMMA was blended and then foamed with subcritical CO2 (SCCO2). Multi-interface microporous structures can be used to improve EMI SE by multiple scattering and attenuating EMWs, thus resolving the ubiquitous fragility of nanocomposites. As an example, Zhang et al. [114] reported lightweight and flexible graphene-based foams produced by SCCO2 foaming [31]. At low graphene loading (1.8 vol %), the resulting nanocomposite showed a high electrical conductivity of 3.11 S m-1 and EMI SE of 13−19 dB at frequencies from 8 to 12 GHz. It is much easier to operate phase separation or phase inversion without using high-pressure equipment or an uncontrollable foaming process compared to chemical and physical foaming [110]. By a modified water vapor-induced phase separation (WVIPS) process, Shen et al. [115] produced porous PVDF/MWCNTs/graphene composites (PMGC). PVDF/MWCNTs/graphene solution was used to coat a nonwoven fabric (NWF) substrate and exposed it to water vapor. At a thickness of 2 mm, the composite foam demonstrated an EMI SE of 28.5 dB. Additionally, syntactic foam is another technique to prepare foam composites using hollow particles. By adding conductive hollow particles as a filler, the syntactic foam can be achieved [116–118]. Foam-based shielding materials can be made using templates due to their facile operation, controllable structure, and varying modification possibilities. In one study, Tang et al. achieved high EMI SE of 37.2 dB and SSE/t of 3410 dB cm3 g−1 at 0.105 vol% graphene loading by immersing melamine foam in the graphene oxide (GO) suspension [119]. For shielding composite foams, the polymeric composition can be layered on pre-constructed conductive foam in reverse. Using conductive Ti3C2Tx (MXene) sheets, Wu et al. [120]5] developed a three-dimensional (3D) compressible foam [38], which was then coated with elastic polydimethylsiloxane (PDMS) to enhance mechanical stability. This PDMS-coated foam produced superior EMI SE after 500 compression-release cycles, demonstrating its potential as a durable and compressible EMI shielding gasket. 2.10.2.2 Aerogels Recently, accomplishments have been revealed in fabricating 3D aerogels of conductive 2D or 1D nanomaterials, e.g., graphene, CNTs, and MXene aerogels [121–126]1]. Ultralightweight and highly porous aerogels offer great potential for EMI shielding applications. Because the skeletons of inorganic aerogels are relatively weak, it is difficult to construct a robust, freestanding aerogel. Polymers are used as bridging materials to strengthen interfacial interactions to improve mechanical properties. Graphene sheets were bridged with polyimide macromolecules to create a 3D composite aerogel [127]. Mixtures of graphene (13 wt.%) and polyimide precursors were 22 successively freeze-dried and thermally imidized. The achieved low-density composite aerogels (0.076 g cm−3) showed a high SSE/t of 1373–1518 dB cm2 g−1. 2.10.3 Laminated multilayer structures Laminated structures are composed of alternatively arranged layers. Laminated structures can be used to create a conductive pathways perpendicular to the stacking direction for EMI shielding materials [128,129]. In addition, varying the layer numbers and thicknesses can indirectly adjust the filler content [44,130]. The layer-by-layer technique is the most common method of making the layers. To prepare a layered PVDF/GNP composite, Qi et al. [131] used hot pressing of layered PVDF/Ni and PVDF/CNT composites to assemble three layers of each layer to make a three-layered sandwich. By increasing the number of layers, it was possible to further improve the shielding performance of the EMI SE of the three-layered composites owing to its multilayered design with electric and magnetic losses. 2.10.4 Segregated structures High electrical conductivity can be achieved by adding conductive filler to the polymer matrix in substantial amounts. Segregated conductive polymer composites (s-CPCs) are examples of architectures with low electrical percolation thresholds. In a composite with high EMI SE, almost all the fillers form a continuous conductive network, which is segregated into individual polymeric bulks for interconnected frameworks of conductive fillers [132,133]. To prepare s-CPCs, three approaches have been developed. In the first method, the decorated polymer granules with conductive fillers are compressed via dry or solution mixing to create segregated conductive networks [134]. External surfaces of polymeric particles can be decorated with various conductive fillers using easy processing techniques that include mixing and compaction. This allows proceeding using hot compression to form bulk materials with segregated structures [135]. At high temperatures compression molding process, the polymers must possesses a relatively high melt viscosity to maintain the segregated conductive networks. Moreover, because of processing difficulties, the filler loadings are typically low (less than 10 wt.%) [136]. In a second method, conductive fillers are dispersed within polymeric latex and the fillers are maintained within the interstitial spaces within the latex particles while freeze-drying the polymer emulsion. Despite the relatively complex manufacturing technology, there are obvious benefits regarding this technique, including conductive fillers being dispersed satisfactorily at the surface 23 of latex particles, (ii) a low-cost and environmentally friendly process involving only distilled water, and the possibility of mixing polymer-filler mixtures with any composition, without limits due to melting viscosities [137]. In the third strategy, conductive fillers are selectively distributed at the interfaces of immiscible polymer blends using melt blending [138]. Since melt blending is a simple method, it is the method of choice when producing s-CPC materials. Nevertheless, forming a stable segregated conductive network at polymer blend interfaces is far more complex. Due to the many influencing factors involved in this method, such as thermodynamic coefficients (interface energy between polymer matrixes and conductive fillers) and kinetic parameters (mixing procedures and sequence, blending time, and shear strength) [136]. 2.11 Section summary EMI has made severe problems for electronic devices and human beings as well. EMI shielding is the solution to reduce the destructive effects of EMI and dissipate the radiation of EM. Manufacturing high-performance flexible lightweight EMI shields compatible with the rapid pace of technological advancement is important. AM techniques are material saving approaches, which allow for fabricating design-customized architecture. PEDOT:PSS is an ICP with facile processability and tunable electrical conductivity which is appropriate for formulating 3D printable inks. PEDOT:PSS inks can be used for scale-up manufacturing printed lightweight EMI shields using the DIW technique. 24 Chapter 3: Materials and Methods This chapter describes the properties and information related to the employed raw materials in this project, including commercial PEDOT:PSS, diverse types of low- and high-boiling point solvents. The employed methodologies for co-solvent doping, 3D printing, solvent post-treatment, and drying processes are described in detail. At the end, the characterization techniques in this research are presented in detail. 3.1 Materials PEDOT:PSS (1:2.5) (Clevios PH1000, Heraeus) aqueous dispersion (1–1.3 wt.%) was ultrasonicated for 10 min followed by filtration with a 0.45 μm syringe filter (Sterlitech) to remove possible agglomerations. Organic solvents including dimethyl sulfoxide (DMSO), ethylene glycol (EG), dimethylformamide (DMF), methanol, and ethanol (Sigma Aldrich, ≥ 99.5%,) were used for co-solvent doping and solvent post-treatment processes. The chemical structures of the used materials are shown in Figure 3.1. Figure 3.1 Chemical structure of PEDOT, PSS, DMSO, DMF, EG, MeOH, and EtOH. 25 3.2 Ink preparation, co-solvent doping, and solvent post-treatment To prepare the PEDOT:PSS inks, first, the freeze-dried foam-like PEDO:PSS nanofibrils were re-dispersed in deionized water with and without different concentrations of various co-solvents, followed by stirring for 12 hours (1000 rpm, 25 °C) using a mechanical dispersing propeller stirring blade mixer. The final homogenized dispersions of PEDOT:PSS inks were kept at 4 °C for further characterization and printing. The resulting dispersions without co-solvents were used for the solvent post-treatment process. To do this, the printed pristine PEDOT:PSS architectures were first dry-annealed (at 140 °C for 2 hr), and then fully immersed in different low- and high-boiling-point solvents (DMSO, EG, DMF, methanol, and ethanol) for 30 min at room temperature followed by washing out with deionized water (three times) and then drying in an oven at 140 °C for 2 hrs. It is worth mentioning that in co-solvent doping, PEDOT:PSS dispersion contains water as the primary solvent and secondary solvents (DMSO, EG, or DMF) as co-solvents. However, for solvent post-treatment, the dried PEDOT:PSS was immersed in pure solvents (DMSO, EG, DMF, methanol, or ethanol) without water. The co-solvent doping and solvent post-treatment steps are illustrated in Figure 3.2. Figure 3.2 Co-solvent doping and solvent post-treatment steps. 26 3.3 3D printing Before printing, first, the prepared inks were homogenized by a high-speed homogenizer and then loaded into 10 mL syringes. Then, the syringes were centrifuged (2000 rpm, 5 min) to remove all the bubbles. Finally, the resulting homogeneous inks were printed using an extrusion-based Allevi 2 bioprinter, 2-axis micro-positioning stage (Allevi, Philadelphia, PA, USA) in patterned structures designed by CAD models. The printer was driven by a compressed air system to pressurize the syringe barrel (up to 110 psi) and control the printing flow rate. The 3D bioprinter equipment and its parts are shown in Figure 3.3. Figure 3.3 3D bioprinter used for DIW of the printed structures. Different infill patterns were printed on glass, flexible substrates, and spin-coated thin films of polydimethylsiloxane (PDMS) substrates using printing speed, layer thickness, and nozzle diameter for the inks were 6-10 mm s–1, 0.2-0.3 mm, and 280 μm, respectively. To manufacture free-standing PEDOT:PSS structures, the prepared inks were printed on the PDMS thin films followed by dry-annealing treatment and then peeling off from the substrate. PDMS elastomer Sylgard 184 (Dow Corning Corporation, Midland, MI, USA) and its hardener mixed (10:1) spin-coated (300 rpm, 30 s) on glass slides followed by curing at 90 °C for 2 hours. For post-treatment of the printed pristine PEDOT:PSS structures, the printed pristine PEDOT:PSS architectures were dry-annealed (at 140 °C for 2 hrs) and then fully immersed in different low- and high-boiling-point solvents (DMSO, EG, DMF, methanol, and ethanol) for 30 min, followed by three times washing out with deionized water and then dried in an oven at 140 °C for 2 hrs. 27 3.4 Techniques and characterization 3.4.1 Rheology Rheological measurements were carried out using an MCR 102 Anton Paar rheometer (Anton Paar, Austria) equipped with a 25 mm parallel plate geometry with a gap distance of 0.8 mm, as shown in Figure 3.4. Apparent shear viscosity (η) as a function of shear rate (γ̇) was attained by steady-state flow tests with a logarithmic sweep of shear rate from 0.01 to 1000 s-1. Dynamic viscoelastic moduli including storage modulus (G') and loss modulus (G'') were measured as a function of shear stress (τ) and shear strain (γ) via dynamic oscillation tests strain sweep at a shear frequency of 1 Hz and logarithmic shear stress sweep at a strain of 0.1 %. The printability regions of the different inks were identified based on shear viscosity and dynamic moduli as a function of PEDOT:PSS concentration. All rheological measurements were carried out at 25 °C with a preliminary equilibration time of 45 s. Figure 3.4 MCR 102 Rheometer used for rheology tests. 3.4.2 Laser microscope The topography of the printed lines and the quality of printing were studied using a 3D laser microscopic scanning (Olympus LEXT OLS5000), as shown in Figure 3.5. To achieve the line’s width and thickness, 20 cross-sectional laser paths were determined on the printed lines for each sample. To obtain the line surface roughness profile, the laser paths were defined on the printed for all lines in each sample. The maximum width and thickness are considered as the line width and thickness values. 28 Figure 3.5 3D measuring laser microscope set-up used for topographical analyses. 3.4.3 SEM Tescan Mira 3 XMU Scanning Electron Microscope (SEM) was used to evaluate microscale images of the 3D-printed PEDOT:PSS-based structures. 3.4.4 XPS X-ray Photoelectron Spectroscopy (XPS) (Thermo Scientific K-Alpha, USA) was employed to evaluate the chemical structure of pristine PEDOT:PSS and solvent-treated PEDOT:PSS using a Monochromated Al Ka X-ray source. 29 3.4.5 XRD Wide range X-ray diffraction analysis (XRD) spectra of pristine, co-solvent doped, and solvent post-treated PEDOT:PSS inks and structures were obtained using (Rigaku ULTIMA III) in conventional 2theta geometry (3°- 60°), with Cu-K α radiation (λ = 1.5406 Å) at 40 kV and 100 mA. The XPS spectra (Kratos AXIS Ultra) were achieved using Al Kα radiation as the X-ray source. 3.4.6 Raman spectroscopy Raman spectra were obtained using a Bruker Senterra Raman microscope with a He-Ne laser source at 632.8 nm and a 50x lens (NA = 0.51) and a laser power of 0.2 mW. 3.4.7 Electrical conductivity To measure the electrical conductivity, the printed thin films on glass slides were dry-annealed and a linear four-point probe (Loresta GP, MCP-T610, Mitsubishi Chemical Co., Japan) set-up was used at the applied voltage of 10 V, as shown in Figure 3.6. The thickness of the formed uniform films on the surface of the glass slides was measured using the laser microscope setup. For each sample, the conductivity measurements were repeated three times and the average values were reported as the final conductivity of the sample. Figure 3.6 Electrical conductivity measuring equipment. 3.4.8 EMI shielding EMI shielding parameters of the manufactured shields were measured in the X-band frequencies (8.2 GHz to 12.4 GHz) using a WR-90 rectangular waveguide and a Keysight P9374A vector 30 network analyzer, as depicted in Figure 3.7. A vector network analyzer (VNA) is one of the main tools for testing the performance of a wireless material under test (MUT) during different phases of product development. The main VNA configuration consists of the source and a set of receivers. Signals generated by the source are altered as they pass through MUTs. Using receivers, changes can be determined by comparing the receiving signals with the incident signals. Measurements of microwave transmission and reflection can be made with a VNA. The transmission measurement involves emitting the signal into MUT and measuring changes in the signal at a single frequency at the receiver on the output side. Reflection measurements, on the other hand, monitor the signal reflected from MUTs. It will be possible to collect magnitude and phase data for each signal. Afterward, the measurement is repeated at a new frequency. A complex scattering parameter (S) can be used to represent the results mathematically, and these values indicate the input-output ratio between the two ports of the system. The network analyzer sends a single frequency signal from port one (S1) to the sandwich sample, and the receiver detects the reflected (S11) and transmitted (S21) waves along with their phase and magnitude. The same process occurs by port two and the same single frequency signal (S2) is emitted from port two and the reflected (S22) and transmitted (S12) wave is detected by the receiver. Since the frequency and EMI shield for these two measurements are the same, for a uniform and homogenous shield the magnitude of S11 and S22, as well as S12 and S21 should be equal. Then, the same measurements are repeated for the whole spectrum of the frequencies. The reflection and transmission of the waves were reported as complex scattering (S) parameters. The electromagnetic wave absorption of a shield is defined based on the measured reflectance (R) and transmittance (T) as follows: A = 1 - R - T (3-1) 𝑅 = |𝑃𝑅𝑃0| = |𝑆11|2 = |𝑆22|2 (3-2) 𝑅 = |𝑃𝑇𝑃0| = |𝑆12|2 = |𝑆21|2 (3-3) Where PR, PT, and P0 are the power of the reflected, transmitted, and incident wave, respectively. S_11 is the reflected voltage magnitude over the incident voltage magnitude in port S1, and S_21 is the transmitted voltage magnitude from port 1 to port 2 over the incident voltage magnitude in port 1. 31 The EMI shielding efficiency is quantified by the EMI shielding effectiveness (EMI SE) of a shield, as shown as follows: 𝑆𝐸𝑅 = 10 log (11 − 𝑅) = log (11 − |𝑆11|2) = log (11 − |𝑆22|2) (3-4) 𝑆𝐸𝐴 = 10 log (1 − |𝑆11|2|𝑆11|2) = 10 log (1 − |𝑆22|2|𝑆21|2) = log (1 − 𝑅𝑇) (3-5) 𝑆𝐸𝑇𝑜𝑡𝑎𝑙 = 𝑆𝐸𝑅 + 𝑆𝐸𝐴 (3-6) Figure 3.7 VNA set up for electromagnetic Interference (EMI) Shielding measurements. 32 Chapter 4: 3D-printed PEDOT:PSS-based EMI Shields In this chapter, first, the achieved results for rheology at steady and dynamic mode are discussed. Then, the 3D printing of free-standing structures, 3D printing on various rigid and flexible substrates, wet-transferring process, and 3D printing of high-aspect-ratio DMSO-doped PEDOT:PSS inks are provided in detail. Next, the high-resolution topography of the printed lines, including the average width and thickness with their variations from the average values for pristine and different co-solvent-doped PEDOT:PSS inks are elucidated. Next, electrical conductivity results for different 3D-printed PEDOT:PSS inks doped with co-solvent or post-treated with solvents are presented. The mechanism of electrical conductivity enhancement is propounded using XPS, XRD, and Raman spectroscopy analyses. Finally, the EMI shielding performance of the 3D-printed pristine PEDOT:PSS and DMSO-doped PEDOT:PSS inks is reported. The effects of infill density, number of printing layers, and drying technique on the EMI shielding effectiveness as well as specific and absolute shielding effectiveness are studied. 4.1 Rheology of PEDOT:PSS inks The chemical structure of the PEDOT and negatively charged PSS (PSS-) are shown in Figure 4.1. PEDOT is a hydrophobic conductive polymer when doped with hydrophilic PSS, water-dispersible PEDOT:PSS is formed. Figure 4.1 Chemical structure of PEDOT:PSS. Commercial water dispersion of PEDOT:PSS (1-1.3 wt.%) possesses low viscosity (0.039 Pa.s at a shear rate of 100 s-1). However, its highly concentrated water dispersions (inks) maintain 33 flow-resistant gels with high viscosity which makes them suitable for micro- and mesoscale 3D DIW (Figure 4.2). Figure 4.2 Freeze-dried PEDOT:PSS nanofibrils and 6 wt.% ink of redispersed freeze-dried PEDOT:PSS nanofibrils in water. The redispersion of high concentrations of PEDOT:PSS in water results in strong flow-resistant inks (gels) that maintain their free-state shape. These inks are favorable for 3D direct ink writing. Fundamental to a successful DIW and extrusion-based printing of inks is demonstrating an intense shear-thinning behavior, which allows extruding through a narrow printing nozzle at lower pressures and avoiding spreading after depositing the filaments [139]. To achieve uniform printed geometries, the extruded filament must maintain its continuity during printing without particle jamming to avoid nozzle clogging. To this end, the ink should possess shear-thinning behavior at variable applied flow rates [140,141]. PEDOT:PSS dispersion in water establishes a deep-blue dispersion of colloidal polymeric micro-gel particles, consisting of a weak physically crosslinked conductive PEDOT-rich core encased with a non-conductive PSS-rich shell. PEDOT:PSS aerogels are easily achievable by the freeze-drying technique. The gel particle size of the developed suspension dramatically affects the electrical conductivity and viscosity [2]. The smaller the gel particle size, the lower conductivity and viscosity [63]. By increasing the concentration of the PEDOT:PSS micro-gels, the viscosity of the resulting dispersion changes significantly. Inks based on PEDOT:PSS demonstrate a distinct shear-thinning behavior, and the shear-thinning response is intensified by the addition of co-solvents due to the breakage of the solvent-PSS linkages. By increasing the concentration of PEDOT:PSS from 1 to 8 wt.%, the shear viscosity and thus, the shear stresses increased by three orders of magnitude (see Figure 4.3 and Figure 4.4). This strongly depends on the increase in the gel particle size and consequently the formation of particle-particle junctions, floc-floc interactions, and the rise of possible entanglements. The results represent the shear-thinning behavior by increasing the shear rate for all inks. By increasing the concentration of PEDOT:PSS, the shear viscosity increases due to the chain entanglements and forming a 3D physical network. By the addition of co-solvents, the shear viscosities increase due to the interaction of co-solvents' 34 polar molecules with the PEDOT:PSS. Additionally, the addition of co-solvents improves the shear-thinning response of the inks. As shown in Figure 4.3, the addition of 7 v/v% (solvent/water) of organic co-solvents of DMSO, EG, and DMF into the pristine PEDOT:PSS inks change the flow responses, that is, intensifies the shear-thinning properties. Theoretically, the resulting inks with intensified shear-thinning behavior would be more suitable for 3D printing due to the wider viscosity changes during applying shear rates. The strong co-solvent/PSS interfacial interactions remove the undoped PSS (free PSS) chains from the shell, leading to an increase in the gel particle size by concatenating the hydrophobic PEDOT chains together. The more hydrophobic nature of the resulting polymer gel particles gives rise to a higher viscosity due to the formation of strong bonding between PEDOT chains. Based on this, the pristine PEDOT:PSS inks show the lowest shear viscosities compared to the co-solvent-doped inks. Figure 4.3 illustrates that the shear viscosity curves of DMF-doped PEDOT:PSS inks have higher values compared to the other ones. This may lay down based on the interfacial interactions between DMF and the polymeric chains of PEDOT and PSS, which change the morphology of the gel particles by interacting with the PEDOT and PSS segments. 35 Figure 4.3 Shear viscosity-shear rate curves of PEDOT:PSS inks with and without co-solvents. Similar to solid materials, in which yield stress is identified as the transition from an elastic state to a plastic one, the yield stress is defined for fluids as the minimum stress required to disrupt the integrated network throughout the media and transform the ink from a solid-like form to a liquid-like gel [140]. Yield stress plays a key role in DIW since it determines the resistance of the ink against the flow, assuring the shape-maintaining characteristics in extrusion-based 3D printing techniques. The shear stress data as a function of shear rate for PEDOT:PSS inks are demonstrated in Figure 4.4 By increasing the concentration of PEDOT:PSS, the shear stress increases, showing higher resistance against flowing, and thus higher capacity to maintain their shapes after printing. The shear-thinning behavior is intensified by the addition of solvents. This confirms that by increasing the concentration, PEDOT:PSS nanofibrils entanglement leads to forming of a reversible 3D physical network within the solvent, enabling the inks to exhibit a solid-like behavior at stresses lower than the yield stress. Results showed that solvent-doped inks represent higher 36 values for the estimated yield stresses, which were mostly higher for DMF-doped PEDOT:PSS inks compared to the other inks. Figure 4.4 Shear stress-shear rate experimental data for pristine and co-solvent-doped PEDOT:PSS inks. To further study the rheological properties, the viscoelastic properties were analyzed to characterize the dynamical performance of the inks for 3D direct ink writing. As shown in Figure 4.5, almost at shear strains lower than 1 %, a plateau region is observable, indicating the linear viscoelastic region (LVR) in which the dynamic moduli are independent of the applied shear rate. Therefore, a lower strain (0.1 %) was selected as a safe shear strain to conduct dynamic rheology analyses precisely. All the prepared inks show LVR at low concentrations (< 1%). 0.1 % and lower strains can be selected as the LVR for further rheological measurements. The concentration of co-solvents is 7 vol/vol to water. 37 Figure 4.5 Storage and loss moduli as a function of shear strain for pristine (a1, a2), EG-doped (b1, b2), DMSO-doped (c1, c2), and DMF-doped (d1, d2) PEDOT:PSS inks. 38 Shear-yielding response in viscoelastic fluids is defined by the yield stress value at which the loss modulus (G′′) value crossovers the storage modulus (G′) value, meaning that at shear stresses lower than yield stress (σy), the ink shows a solid-like behavior and is capable to maintain its printed shape. The shape fidelity of a printed structure is determined by the ratio of G′′ to G′. A viscoelastic ink showing a shear-yielding behavior can maintain high shape fidelity. Inks based on PEDOT:PSS represent a shear-yielding behavior, in which by increasing the applied shear stress, the viscoelastic moduli of the inks drop sharply, indicating the breakage of the gel network upon increasing the shear stress. As shown in Figure 4.6, by increasing the concentration of PEDOT:PSS as well as by the addition of co-solvents, the yield stress (stress at which G′ and G′′ curves cross) increases. The concentration of co-solvents was selected based on the electrical conductivity values, where PEDOT:PSS inks doped with 7 vol/vol % DMSO, 5 vol/vol % EG, and 5 vol/vol % DMF showed the maximum conductivity (refer to Figure 4.21). Figure 4.6 Storage and loss moduli as a function of shear stress curves for pristine (a), EG-doped (b), DMSO-doped (c), and DMF-doped (d) PEDOT:PSS inks. 39 4.2 Direct ink writing of PEDOT:PSS inks Before rheological measurements, the electrical conductivity measurements showed that the DMSO-doped PEDOT:PSS inks possess the highest electrical conductivity (refer to Figure 4.21); therefore, the DMSO-doped PEDOT:PSS inks were used for further 3D printing and processes. Principally, inks should have appropriate rheological responses after printing to allow filament formation and then, prevent the filament from sagging because of its weight and avoid lateral spreading upon deposition on the bed. The summarized results for the shear viscosity and storage and loss moduli coupled with the filament formation testing (Figure 4.7 and Figure 4.8) showed that at concentrations lower than 4 wt.% of PEDOT:PSS, inks because of low rheological properties are more likely to form droplets rather than filaments. However, in the range of 4-7 wt.%, extruding a uniform and continuous filament is possible. Figure 4.7 3D printable range based on the shear viscosity (a), storage and moduli (b), and yield stress (c) as a function of PEDOT:PSS concentration. At concentrations higher than 7 wt.%, extruding a uniform filament was found to be challenging due to the high shear viscosity and yield stress. At concentrations higher than 8 wt.%, the nozzle clogging hinders successful printing. By taking the rheological properties and extrusion printability into account, the printability region could be defined as a domain of PEDOT:PSS concentration 40 within which, a uniform and continuous filament can be extruded from a printing nozzle (see the highlighted areas in Figure 4.7). By increasing the number of printed layers, the weight of the deposited layers increases, leading to lateral spreading and, thus, the collapse of the structure. However, proper viscoelastic properties provide shape retention. This combined with higher printing resolution suggests that manufacturing high-performance miniaturized printed devices and EMI shields is feasible with our designed inks. All in all, the proper rheological properties lead to high shape fidelity, high-resolution micro-thickness, and high aspect ratio 3D printing of deposited filaments, as shown in Figure 4.8b (one layer) and Figure 4.8c (25 layers ). Figure 4.8 Digital images of filament formation for various concentrations of PEDOT:PSS (a). High-resolution one-layer printed (b) and high-aspect-ratio (c) 3D direct ink writing of DMSO-doped 6 wt.% PEDOT:PSS ink. In addition to shape fidelity, PEDOT:PSS inks showed printability on various rigid (glass, polystyrene) and flexible (cellulose acetate, and polyethylene terephthalate) substrates, as depicted in Figure 4.9, which further expands the horizon of their application. 41 Figure 4.9 Direct ink wiring of DMSO-doped 6 wt.% PEDOT:PSS ink on glass, polystyrene, cellulose acetate, and polyethylene terephthalate substrates (i). 4.3 Manufacturing free-standing architectures One of the most outstanding properties of PEDOT:PSS inks is the capability of manufacturing flexible self-standing geometries. As depicted in Figure 4.10, a self-standing printed DMSO-doped 6 wt.% PEDOT:PSS ink was achieved by printing on spin-coated PDMS substrates, followed by dry-annealing (solvent and co-solvent evaporating at elevated temperatures) and then, peeling off from the substrate. The thickness of the obtained structures is adjustable by changing the number of deposited layers. The electrical conductivity of the self-standing structures can be tuned using both co-solvent doping and/or solvent post-treatment approaches. Showing high flexibility and visual transparency, free-standing patterned structures of solvent-treated PEDOT:PSS are versatile candidates for compact flexible organic electronics, specifically, EMI shields and human motion detection sensors. 42 Figure 4.10 Digital images of a flexible free-standing printed triangle-patterned structure of DMSO-doped 6 wt.% PEDOT:PSS on a PDMS substrate. 4.4 Wet-transferring of 3D-printed free-standing structures Common processing techniques are unable to manufacture patterned structures on sharp and uneven substrates as well as on the edges. Direct ink writing assists with fabricating patterned self-standing PEDOT:PSS EMI shields, which can be transferred on unusual sharp edges and uneven complex objects using a wet-transfer technique (Figure 4.11). The partial PSS removal due to the co-solvent treatment leads to the formation of long-term (3 months) stable treated PEDOT:PSS hydrogels [142]. Taking advantage of this outstanding attribute, as shown in Figure 4.11, the printed self-standing hydrogels were easily transferred on a glass substrate using a wet transfer technique. Figure 4.11 Digital images of the wet-transferring of free-standing 3D-printed DMSO-doped PEDOT:PSS hydrogels on the edge of a glass slide. 4.5 Manufacturing of high-aspect-ratio 3D-printed architectures ultra-lightweight aerogels (with a density of 0.1915-0.3871 g cm-3) of 3D-printed DMSO-doped PDOT:PSS 6 wt.% with high thickness were obtained using a freeze-drying technique, as depicted in Figure 4.12a. Due to the low solid content of the prepared inks, the ultra-lightweight large-size structure hovers over the delicate hairs of a cactus. Electrically high conductive aerogels are proper candidates for lightweight high-performance EMI shields. As aforementioned, due to the appropriate rheological properties of the prepared inks, high-resolution micro/macro-scale 3D printed aerogels with various infill patterns can be obtained. Figure 4.12b shows a high-resolution and uniform microscopic image of a 3D-printed geometry with a triangle infill pattern for the DMSO-doped 6 wt.% PEDOT:PSS ink. These printed structures were easily turned into aerogels without observable deformation or shrinkage as demonstrated in SEM images (Figure 4.12c and Figure 4.12d). This confirms the capability of the PEDOT:PSS inks for manufacturing reliable and uniform 3D architectures with different infill patterns (triangle and grid). 43 Figure 4.12 Digital images of an ultra-lightweight freeze-dried 3D-printed geometry hovering over delicate cactus hair (a), microscope image of a triangle-patterned structure (b), SEM micrographs of a freeze-dried triangle-infilled 3D-printed structure (c), and digital images and SEM micrographs of an as-printed and freeze-dried grid-infilled 3D-printed structure (d). All geometries were printed using the 7 vol/vol% DMSO-doped 6 wt.% PEDOT:PSS ink. 4.6 High-resolution 3D printing High zero-shear viscosity (viscosity at very low shear rates) and high storage modulus of the PEDOT:PSS inks allow reliable and reproducible high-resolution printing of micro- and macro-scale printed electronics and EMI shields. The 3D-printed micrometer-thick lines of the PEDOT:PSS inks indicate a high-resolution DIW for architectures featuring various infill patterns and densities. The digital images of the dry-annealed single-layer printed lines and various infill patterns of the 7 vol/vol % DMSO-doped 6 wt.% PEDOT:PSS inks are depicted in Figure 4.13, unveiling high-resolution and high-quality printing. 44 Figure 4.13 Digital images of various printed infill-patterned architectures of 7 vol/vol % DMSO-doped 6 wt.% PEDOT:PSS ink. The achieved 2D and 3D topographical results from the laser microscope analysis confirmed the uniform width and thickness of the printed lines. The 3D microscopic images captured from the 3D laser microscope are depicted in Figure 4.14. The printed structures with different infill patterns represent a uniform and homogenous topography. This gives rise to manufacturing reproducible and reliable microscale printed electronics, particularly EMI shields. 45 Figure 4.14 Topographical micrographs of the printed line-, grid-, and triangle-patterned of 7 vol/vol % DMSO-doped 6 wt.% PEDOT:PSS ink. The line and surface roughness profiles of the printed lines of the 7 vol/vol % DMSO-doped 6 wt.% PEDOT:PSS ink is shown in Figure 4.15, maintaining a uniform printed line profile with low surface roughness. 46 Figure 4.15 Profile (a) and surface roughness (b) of the printed lines of 7 vol/vol% DMSO-doped 6 wt.% PEDOT:PSS ink. As illustrated in Figure 4.16, the micrometer-thick printed lines of the 7 vol/vol % DMSO-doped PEDOT:PSS 6 wt.% ink were achieved with an average width and thickness of 306.9 μm and 14.9 μm, respectively. A narrow distribution with less than 20 % variation from the average values was achieved for both width and thickness using a printing nozzle with a 280 μm inner diameter. The deviation of the line average width (306.9 μm) from the inner diameter of the printing nozzle (280 μm) is negligible (~10 %). To the best of our knowledge, there is no similar evaluation of the quality of printing of PEDOT:PSS inks, processed using a direct ink writing technique. According to the literature, the best-reported variation of the line’s average width from the inner diameter of the printing nozzles was in the range of 20-120 % for the inks based on PEDOT:PSS containing 2D MXenes nanosheets [143–146]. The printed line width and thickness can be adjusted by adjusting the rheological properties of the ink (concertation of PEDOT:PSS and the type of co-solvents) and printing parameters (e.g., the injection speed, layer thickness, and inner diameter of the printing nozzle). The small line width deviation from the inner diameter of the printing nozzle facilitates the printing process for large-scale production of high-resolution printed electronics and EMI shields with accurate and reproducible homogeneity. 47 Figure 4.16 Line thickness variation (a) and line width variation (b) for the printed 7 vol/vol % DMSO-doped PEDOT:PSS 6 wt.% ink (f). Due to the different rheological properties, the thickness and width of the printed lines of the prepared PEDOT:PSS-based inks may vary. As shown in Figure 4.17, the average line thickness values for pristine, EG-doped, DMSO-doped, and DMF-doped PEDOT:PSS inks were 13.92, 14.89, 14.94, and 13.87 µm. The obtained average line width for the printed lines for pristine, EG-doped, DMSO-doped, and DMF-doped were 343.67, 322.47, 306.68, and 295.59 µm, respectively. Figure 4.17 Average line thickness (a) and line width (b) of different PEDOT:PSS-based inks. The obtained topographical results for the printed lines and geometries of the pristine 6 wt.% PEDOT:PSS ink (Figure 4.18) indicate less than 25 % lines width variation from the average 48 value (343.7 μm) and lines thickness variation of less than 25 % from the average value (13.92 μm). Figure 4.18 Topographical micrographs of line-, grid-, and triangle-patterned structures of pristine 6 wt.% PEDOT:PSS ink (a), statistical analysis of line width variation (b) and line thickness variation (c) of the printed pristine 6 wt.% PEDOT:PSS lines. The higher average line width and lower average thickness are due to the inferior rheological properties of the pristine PEDOT:PSS ink (refer to rheology section) compared to co-solvent-doped PEDOT:PSS inks. The topographical findings for the 7 vol/vol % EG-doped 6 wt.% PEDOT:PSS ink (Figure 4.19) showed a lines width variation of less than 25 % from the width average value (322.5 μm) and a lines thickness variation of less than 15 % from the average value (14.89 μm). 49 Figure 4.19 Topographical micrographs of line-, grid-, and triangle-patterned structures of EG-doped 6 wt.% PEDOT:PSS ink (a), statistical analysis of line width variation (b) and line thickness variation (c) of the printed EG-doped 6 wt.% PEDOT:PSS lines. The same examinations revealed that the printed lines of 7 vol/vol % DMF-doped 6 wt.%PEDOT:PSS inks displayed the smallest line width average (295.6 μm) with a line width variation of less than 15 % and the smallest line average thickness (13.87 μm) with a line thickness variation of less than 20 % (Figure 4.20). According to rheological data, the DMF-doped PEDOT:PSS inks exhibited the highest shear viscosity and viscoelastic moduli compared to other inks, offering lower flowability and lower spreading after printing. Therefore, the DMF-doped inks with slightly improved rheological properties indicated smaller width and thickness deviation from the average values. However, the higher shear viscosity and yield stress demand higher working pressures. 50 Figure 4.20 Topographical micrographs of line-, grid-, and triangle-patterned structures of DMF-doped 6 wt.% PEDOT:PSS ink (a), statistical analysis of line width variation (b) and line thickness variation (c) of the printed DMF-doped 6 wt.% PEDOT:PSS lines. Overall, by dry annealing at a high temperature (140 °C) right after printing, the evaporation of the solvent accelerates solidification, resulting in uniformly printed lines with minimal spreading. The high-temperature dry-annealing process reduces the activation energy of the PSS's partial phase separation and aids in the molecular penetration of PEDOT-rich grains [99,147]. Solvent and thermal treatments together enhance the electrical conductivity of the resulting inks as well as guarantee high-resolution printed structures, therefore boosting the final EMI shields' performance. These findings manifest the importance of ink formulation and confirm the reliability of the direct ink writing technique for manufacturing micro-scale printed electronics and EMI shields, provided that inks with proper rheological properties are designed. 51 4.7 Electrical conductivity measurements By increasing the concentration of co-solvents, the electrical conductivity rose to the maximum values and then declined (Figure 4.21a). The electrical conductivity increased from ~0.6 S cm-1 for the pristine PEDOT:PSS to maximum values of 858.1 ± 60.8, 625.31 ± 39.98, and 492.81 ± 42 S cm-1 for the PEDOT:PSS inks doped with 7 vol/vol % DMSO, 5 vol/vol % EG, and 5 vol/vol % DMF, respectively. Additionally, the solvent-post treatment of the printed pristine PEDOT:PSS films with various high- and low-boiling-point solvents (methanol and ethanol) showed similar enhancement trends for electrical conductivity. The highest electrical conductivity of 967 ± 97 S cm-1 was obtained for the DMSO post-treated PEDOT:PSS ink compared to those for EG (801.76 ± 86 S cm-1), DMF (828.63 ± 75.44 S cm-1), methanol (813.78 ± 59 S cm-1), and ethanol (444.96 ± 98 S cm-1) post-treated PEDOT:PSS inks (Figure 4.21b). Figure 4.21 Electrical conductivity of co-solvent doped (a) and solvent post-treated (b) PEDOT:PSS printed films. 4.8 XPS results A solvent treatment process followed by dry annealing led to more than three orders of magnitude increment in the electrical conductivity. The mechanism of this significant enhancement lies behind the modifications of the PEDOT:PSS structure due to the partial PSS-rich removal from the surrounding of PEDOT-rich cores of the PEDOT:PSS gel particles. The ratio of PSS to PEDOT is usually 2.5:1 and a proportion of the PSS chains (excess PSS) are free and undoped with PEDOT oligomers. The presence of free PSS chains impedes electrical conductivity by forming non-conductive boundaries [148]. The smaller gel particle size results in a greater number of gel particle boundaries when dried, blocking the electron transfer between conductive PEDOT 52 segments. Hence, the larger the gel particles, the fewer boundaries, which gives rise to a higher electrical conductivity [63]. The addition of co-solvents leads to the appearance of strong electrostatic interactions between co-solvent molecules and PSS chains. Consequently, the partial PSS removal phenomenon occurs due to the high hydrophilicity of PSS. As shown in narrow-scan XPS graphs (Figure 4.22a and Figure 4.22b), values of the PEDOT/PSS ratio were increased by adding co-solvents. The calculated surface areas under the characteristic peaks of PEDOT (~165 eV) and PSS (~169 eV) in the achieved XPS spectra of co-solvent doped and solvent post-treated PEDOT:PSS inks proved that the presence of co-solvents increases the ratio of PEDOT/PSS. It means that undoped PSS chains have been removed from the PEDOT:PSS gel particles to some extent (Figure 4.22c). The higher PSS removal induces a higher electrical conductivity because of increasing the pathways for transferring charge carriers. The XPS survey and deconvoluted XPS spectra are provided in Appendix A. Figure 4.22 Narrow-scan XPS spectra of the S2p signal for co-solvent doped (a) and solvent post-treated (b) PEDOT:PSS films. PEDOT/PSS ratio (c), calculated from the surface areas under the XPS spectra for PEDOT (~165 eV) and PSS (~169 eV) characteristic peaks. 53 4.9 XRD results XRD plots (Figure 4.23) revealed that the small shifting in the characteristic peaks at 2θ = 5 ° to lower values represents an increase in the distance of the in-plane PSS and PEDOT chains (lamella stacking distance) [148]. The slight slide of the peaks at 2θ = 26 ° to higher values, due to co-solvent doping and solvent post-treatment implies the reduction of the π-π stacking distance of PEDOT aromatic groups. It is clear from Figure 4.23 that the presence of co-solvents intensifies the sharpness of the characteristic peaks at 2θ = 26 °, which is due to a decrease in the π-π stacking distance and, thus, an increase in the degree of molecular packing [148]. These structural changes in PEDOT:PSS on account of the incorporation of co-solvents indicate that the PEDOT π-π interchain couplings are intensified. Therefore, the π-π interchain charge transfer pathways were improved by increasing the π-electronic overlaps [149]. Figure 4.23 XRD spectra for co-solvent doped (a) and solvent post-treated (b) PEDOT:PSS films. 4.10 Raman spectroscopy In addition to the PSS-removal due to the incorporation of co-solvents/solvent post-treatment, the conformational transformation from a coiled structure of PEDOT to an extended structure leads to improved charge transport among PEDOT chains [150]. As demonstrated in Figure 4.24, the Raman shifts between the 1400 and 1500 cm-1 range are assigned to the stretching vibration of Cα = Cβ on the fifth-member ring of PEDOT, exhibiting a red shift from 1437 cm-1 to lower values after co-solvent doping and solvent post-treatment. This Raman shift represents a structural resonant change in PEDOT chains from a benzoid structure to a quinoid one. This indicates that 54 the former conformation of PSS chains is altered from a coil conformation to a linear or extended structure, which is desirable for improving electrical conductivity. Figure 4.24 Raman spectroscopy results for co-solvent doped (a) and solvent post-treated (b) PEDOT:PSS films. This indicates that the former conformation of PSS chains is altered from a coil conformation to a linear or expanded-coil structure, which is desirable for improving electrical conductivity [93]. Structural changes in the PEDOT:PSS networks after the solvent treatment are schematically illustrated in Figure 4.25. Figure 4.25 Schematic illustration of PEDOT:PSS gel particles’ structural changes after the solvent treatment and drying techniques. 55 4.11 EMI shielding measurements Inks with high electrical conductivity can be used to fabricate flexible, lightweight, and highly reproducible printed EMI shields with arbitrary geometries and sizes. The primary EMI shielding mechanism of highly conductive materials is the reflection of the incident wave, due to the impedance mismatch between free space and the shielding material [130,151]. For instance, in metallic EMI shields, the low impedance gives rise to a high ratio of reflection, which causes secondary EM wave (EMW) pollution. However, intrinsically conductive polymers have shown enhanced performance in absorbing the incident wave through the interaction of the EMW radiation with charge carriers, which induces polarization, Ohmic loss, scattering, and multiple internal reflections (in multiple-phase or porous EMI shields) [152,153]. Various inherent properties of the shield’s constituents such as electrical conductivity, and geometrical features of the shield, e.g., thickness, structure, and density, affect the total shielding effectiveness (SETotal) in a specific frequency range of the incident EMW. Interestingly, DIW enables the manufacturing of visually transparent patterned EMI shields with competitive performance compared to compact laminated structures (Figure 4.26). Figure 4.26 DIW printed visually transparent structures with different infill densities. As shown in Figure 4.27, different geometries including laminated compact layers (100 % infill density), grid-infilled structures (with 1- and 2-mm line spacing, namely G1 and G2, respectively), and different numbers of deposited layers were 3D printed to correlate the macro-scale design of the shields to their shielding performance. 56 Figure 4.27 Digital and microscopic images of the printed structures with different infill patterns and the number of printed layers. By increasing the number of printed layers of the dry-annealed (DA) pristine PEDOT:PSS EMI shields with 100 % infill density (Full) from one layer (11.66 µm) to ten layers (273.54 µm), the SETotal increased from 16.61 dB to 28.35 dB (Figure 4.28) within the X-band (8.2-12.4 GHz). Expectedly, the grid-infilled printed EMI shields featured a lower SETotal compared to the compact laminated films, which is a direct consequence of the reduced effective surface area. However, by increasing the number of deposited layers, the EMI SE of the grid-infilled structures increased dramatically. Findings revealed that the printed grid-infilled EMI shields of pristine PEDOT:PSS inks with at least five printed layers exhibited a SETotal greater than 20 dB, which is suitable for a wide range of industry applications. A SETotal of 20 dB can be translated to the shielding of 99 % of the incident waves, and a SETotal shield with 30 dB blocks 99.9 % of the incident waves. It is worth mentioning that the grid-infilled patterns with a line spacing of 3 mm were also fabricated and tested. However, due to their weak EMI shielding performance, these samples were discontinued from further testing. Figure 4.28 Total EMI SE of the printed pristine PEDOT:PSS structures. 57 With the advancement in technology, the requirement for advanced high-performance EMI shields is fundamental. As shown in Equation (4-1), the electrical conductivity of the shield affects the EMI SETotal directly. 𝑆𝐸𝑇 = 50 + 10 log [𝜎𝑓] + 1.7𝑑√𝑓𝜎 (4-1) The solvent treatment enhances the electrical conductivity of the prepared inks, and as a result of that, the EMI SE increases remarkably. Among the used co-solvents and solvent post-treatments, DMSO has presented the greatest influence on electrical conductivity. Solvent doping is of more importance due to the lower steps and facile ink preparation; therefore, the EMI shielding performance of the prepared DMSO-doped PEDO:PSS shields (858.1 ± 60.8 S cm-1) was investigated. As shown in Figure 4.29a, for the dry annealed printed 100 % infill density DMSO-doped PEDOT:PSS shields, the SET reached 22.78 dB (1-layered, 9.4 µm), 27.49 dB (2-layered, 27.1 µm), 31.84 dB (5-layered, 43.3 µm), and 39.36 dB (10-layered, 196.7 µm). As indicated in Figure 4.29a and Table 4.1, the dry-annealed printed grid-infilled DMSO-doped PEDOT:PSS shields with 1 mm line spacing (G1) show SETotal of 23.50 dB (1-layer, 8 µm thickness), 26.14 dB (2-layer, 14.9 µm thickness), 30.55 (5-layer, 33.1 µm thickness), and 36.91 dB (10-layer, 184.2 µm thickness). By fabricating the grid-patterned structures, the empty spaces (openings) in the EMI shield give rise to the impedance mismatch between the free space and the shield. This leads to high EMI SE values despite the use of less material. Additionally, the drying technique affects the final properties of the printed DMSO-doped PEDOT:PSS EMI shields. As illustrated in Figure 4.29b and Table 4.1, employing a freeze-drying technique to fabricate EMI shields improves the EMI SE. This technique allowed the printed shields to maintain their micro- and macro-scale structures with larger thicknesses. Due to the increase in thickness as well as the increased micro-scale porosity of the freeze-dried samples, the incident electromagnetic waves are dissipated at higher rates due to the change in shielding mechanisms and improved multiple internal reflections. According to Figure S13, freeze-drying samples showed increments of 19.55 %, 20.18 %, and 11.00 % in EMI SE values for full, G1 grid-infilled, and G2 grid-infilled structures compared to dry-annealed samples. 58 Figure 4.29 Total EMI SE of dry-annealed DMSO-doped (a), and freeze-dried DMSO-doped (b) PEDOT:PSS structures. Total EMI SE of the printed dry-annealed and freeze-dried DMSO-doped PEDOT:PSS structures as a function of the number of printed layers (c). Since the thickness of different printed samples varies, the specific shielding effectiveness (SSE) is defined as the normalized SETotal over the thickness of the shield (SETotal/t) to present a better comparison. As listed in Table 4.1, printed shields with only one printed layer (1L) possess significant SSE values of 24126.37, 27040.09, and 23729.66 dB cm-1 for Full, G1, and G2 dry-annealed DMSO-doped PEDOT:PSS structures, respectively. The SSE values decreased to 10127.20, 17497.59, and 17200.11 dB cm-1 for 2-layered 100 % infill, G1, and G2 EMI shields. The freeze-dried EMI shields have lower SSE values than the dry-annealed samples due to their larger thickness. Due to the structural differences between the dry-annealed and freeze-dried structures, normalizing SSE by the density of the printed EMI shields (absolute SE or SSE/d) provides a lucid comparison. Accordingly, the dry-annealed EMI shields with the lowest printed layers (1L) maintain the highest values among the absolute SE values. Using DIW to print patterned EMI shields compensates for the impedance mismatch between free space and the EMI shields and reduces the required material for fabricating the shield. Therefore, engineering 59 the structure of EMI shields leads to lower apparent density values and, thus, higher absolute SE values. Table 4.1 EMI shielding parameters of printed DMSO-doped PEDOT:PSS structures. EMI Shield SER SEA SETotal Thickness (mm) SSE (dB cm-1) SSE/d (dB cm2 g-1) Dry Annealed Shields 1L Full 9.41 13.37 22.78 0.0094 24126.37 69251.05 1L G1 8.94 12.56 21.50 0.0080 27040.09 209558.95 1L G2 3.32 15.55 18.87 0.0080 23729.66 306505.52 2L Full 7.91 19.58 27.49 0.0271 10127.20 27587.05 2L G1 5.34 20.80 26.14 0.0149 17497.59 126564.86 2L G2 5.08 20.61 25.70 0.0149 17200.11 213279.60 5L Full 4.70 27.14 31.84 0.0433 7360.58 19239.11 5L G1 6.89 23.66 30.55 0.0331 9226.88 64356.92 5L G2 6.18 24.36 30.54 0.0331 9223.26 111984.78 10L Full 6.04 33.32 39.36 0.1967 2000.77 4876.05 10L G1 7.37 29.54 36.91 0.1842 2003.77 13458.56 10L G2 7.40 28.59 35.99 0.1842 1953.68 23216.06 Freeze-dried Shields 1L Full 6.07 22.87 28.95 0.0167 17343.08 90564.40 1L G1 7.34 17.14 24.48 0.0113 21701.59 339972.76 1L G2 4.81 16.01 20.81 0.0113 18451.18 481754.05 2L Full 5.07 27.49 32.57 0.0453 7184.67 36782.22 2L G1 6.37 25.93 32.31 0.0331 9754.46 142623.97 2L G2 6.88 19.49 26.38 0.0331 7963.86 199616.42 5L Full 4.95 29.39 34.34 0.3908 878.68 4411.93 5L G1 8.14 24.40 32.53 0.3523 923.39 13019.08 5L G2 6.68 26.23 32.90 0.3523 933.91 22921.02 10L Full 6.89 46.40 53.30 0.6128 869.76 4284.72 10L G1 7.11 43.05 50.16 0.5014 1000.35 13581.82 10L G2 7.45 36.92 44.37 0.5014 884.97 21257.80 As listed in Table 4.1, 1-layered patterned DMSO-doped G1 and G2 EMI shields showed extraordinary absolute SE values of 209558.95 and 306505.52 dB cm2 g-1, respectively, compared to 1-layered full film (69251.05 dB cm2 g-1). For EMI shields with 2 printed layers, the absolute SE values were 27587.05, 126564.86, and 213279.60 dB cm2 g-1 for Full, G1, and G2 samples, respectively. These outstanding achieved absolute SE values for different printed samples are comparable to previous works in the literature in which high levels of conductivity 60 and EMI SSE were achieved using nanocomposites of PEDOT:PSS and different conductive nanofillers such as graphene [154,155], MXenes [144,156–159], and Ag NWs [160] to PEDOT:PSS (Table 4.2). The freeze-dried 1-layered EMI shields showed remarkable absolute SE values for Full (90564.40 dB cm2 g-1), G1 (339972.76 dB cm2 g-1), G2 (481754.05 dB cm2 g-1). By increasing the number of printed layers, the absolute SE values decrease, whereas for 10-layered printed EMI shields, absolute SE values decreased to 4284.72, 13581.82, and 21257.80 dB cm2 g-1 for Full, G1, and G2 samples, respectively. These results are superior to those of the reported EMI shields in the literature (Error! Reference source not found.). Table 4.2 Comparison of electrical conductivity and EMI shielding parameters for different materials. EMI Shielding Material Thickness (mm) Conductivity (S cm-1) SETotal (dB) SSE/d (dB cm2 g-1) Reference PEDOT:PSS/ Graphene 0.8 6.84 70 841 [154] 1.5 22.3 69.1 8040 [155] 1.5 35.2 91.9 20800 [155] PEDOT:PSS/ Ti2C3Tx 0.011 340.5 42.1 19498 [156] 0.06 388 41 89924 [157] 0.007 2900 55.4 38079 [158] 5 0.02 32.5 5000 [159] 5 0.03 59 10841 [159] PEDOT:PSS/ Ti2C3 0.021 1600 28.06 32240 [144] 0.024 100 16.26 20246 [144] PEDOT:PSS/PVA/Ag NWs 0.020 3.82 33.6 16800 [160] PEDOT:PSS/WPU 0.15 77 62 4590 [161] 61 Figure 4.30 Schematic illustration of EMI shielding mechanism of a 3D-printed grid-infilled structure. Other than the total SE, the shielding mechanism can be interpreted using the EMI shielding coefficients. Interestingly, as shown in Figure 8e, the reflection coefficient values decreased by decreasing the printed EMI shields' infill density. This indicates that by controlling the macro-scale design of the manufactured EMI shields, the mechanism of EMI shielding switches from reflection to adsorption. The printing of customized patterned structures not only uses less material but also improves the absorption mechanism of the engineered shields by changing the mechanism of shielding (see Figure 8f). Figure 4.31 EMI shielding coefficients for DMSO-doped PEDOT: PSS structures. 62 4.12 Summary In summary, analyses validated that a combination of solvent treatment and thermal treatment is an efficient approach to enhance the electrical conductivity of PEDOT:PSS by three orders of magnitudes. The reason for this enhancement is the partial PSS removal as well as the conformational changes in the PEDOT structure. It was shown that 6 wt.% PEDOT:PSS inks in water and the mixture of water and co-solvent maintain high viscoelastic properties suitable for layer-by-layer 3D printing. The printed structures retain their shape without lateral spreading. The achieved printed structures are proper for patterned and design-customized EMI shields with high electrical conductivity, EMI shielding effectiveness, and specific and absolute shielding effectiveness. Using spin-coated polydimethylsiloxane on glass slides, we were able to manufacture flexible free-standing 3D-printed structures, which can be transferred onto any kind of unusual substrates, such as edges and curved geometries. 63 Chapter 5: Conclusions and Future Works 5.1 Conclusions High-performance engineered flexible EMI shields were manufactured using a DIW technique by formulating DIW 3D printable highly electrically conductive PEDOT:PSS-based inks. EMI shielding characterizations indicated that by engineering the micro- and macro-scale design of the printed shields, the mechanism of shielding shifts from reflection to absorption. The highest EMI SE of 39.36 dB for dry-annealed and 50.16 dB for freeze-dried were achieved for 10 layers of the printed DMSO-doped PEDOT:PSS grid-infilled patterns. The electrical conductivity was enhanced by three orders of magnitude from ~0.6 S cm-1 for pristine PEDOT:PSS to 858.1 ± 60.8, 625.31 ± 39.98, and 492.81 ± 42 S cm-1 for the DMSO-doped, EG-doped, and DMF-doped PEDOT:PSS inks, respectively. The rheological properties of the PEDOT:PSS inks were fine-tuned by varying polymer concentrations and changing the type of co-solvent. The results indicated that the 6 wt.% of pristine PEDOT:PSS inks feature proper viscoelastic properties, which are improved by the addition of co-solvents, rendering them suitable for additive manufacturing of advanced flexible self-standing patterned EMI shields and printed electronics. Topographical analyses confirmed the high resolution of the printed structures. Pristine and EG-doped PEDOT:PSS inks showed lower printing resolution (higher spreading after printing) due to lower viscoelastic properties. However, DMSO- and DMF-doped PEDOT:PSS inks exhibited improved topographical profiles. The ink with the highest electrical conductivity (6 wt.% PEDOT:PSS doped with water:DMSO 93:7 vol/vol) showed an average line width of 306.9 µm, an average line thickness of 14.9 µm, a line width/thickness variation of less than 20 % from the average values, and line width with a 10 % deviation from the inner diameter (280 µm) of the printing nozzle. This approves the reliability and reproducibility of the inks for the fabrication of pattern-infilled structures via the DIW technique. Electrical conductivity measurements showed that solvent post-treatment can be as effective as solvent doping. The maximum electrical conductivity of 967 ± 97 S cm-1 was obtained for the DMSO post-treated pristine PEDOT:PSS film compared to those for EG (801.76 ± 86), DMF (828.63 ± 75.44), methanol (813.78 ± 59), and ethanol (444.96 ± 98) post-treated PEDOT:PSS ones. The results demonstrated the partial removal of non-conductive PSS from the conductive PEDOT cores, as well as PEDOT structural transformations, thereby leading to an enhancement in electrical conductivity. 64 5.2 Challenges and future works In this project, inks based on different concentrations of PEDOT:PSS doped with various concentrations of high-boiling-point solvents were prepared and used to manufacture high-performance EMI shields using DIW. To assess the performance of the doping process, solvent post-treatment was employed to improve the electrical conductivity of the printed pristine PEDOT:PSS architectures. A few challenges and future pathways are introduced as follows: • Achieving uniform and homogenous inks In highly concentrated inks, achieving a uniform and homogenized mixture is challenging due to the increased viscosity. PEDOT:PSS agglomeration can cause nozzle clogging as well as affect the electrical conductivity, in turn, EMI shielding performance adversely. Therefore, different ink preparation techniques can be a topic of research to obtain high-quality inks and printed structures. • Changing the type of co-solvent In this research, three high-boiling-point co-solvents (DMSO, EG, and DMF) were used to enhance the electrical conductivity of PEDOT:PSS. There is a wide range of different high-boiling-point solvents that can be used as a co-solvent and dopant of PEDOT:PSS. 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Thesis/Dissertation
eng
Mechanical Engineering
Vancouver : University of British Columbia Library
Attribution-NonCommercial-NoDerivatives 4.0 International
Graduate
Arjmand, Mohammad
High-resolution 3D printing of electrically conductive inks based on pedot:pss for electromagnetic interference shielding
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