Nonthermal Plasmas for Materials Processing

This unique book covers the physical and chemical aspects of plasma chemistry with polymers and gives new insights into the interaction of physics and chemistry of nonthermal plasmas and their applications in materials science for physicists and chemists.

The properties and characteristics of plasmas, elementary (collision) processes in the gas phase, plasma surface interactions, gas discharge plasmas and technical plasma sources, atmospheric plasmas, plasma diagnostics, polymers and plasmas, plasma polymerization, post-plasma processes, plasma, and wet-chemical processing, plasma-induced generation of functional groups, and the chemical reactions on these groups along with a few exemplary applications are discussed in this comprehensive but condensed state-of-the-art book on plasma chemistry and its dependence on plasma physics.

While plasma physics, plasma chemistry, and polymer science are often handled separately, the aim of the authors is to harmoniously join the physics and chemistry of low-pressure and atmospheric-pressure plasmas with polymer surface chemistry and polymerization and to compare such chemistry with classic chemistry.

Readers will find in these chapters

• Interaction of plasma physics and chemistry in plasmas and at the surface of polymers;
• Explanation and interpretation of physical and chemical mechanisms on plasma polymerization and polymer surface modification;
• Introduction of modern techniques in plasma diagnostics, surface analysis of solids, and special behavior of polymers on exposure to plasmas;
• Discussion of the conflict of energy-rich plasma species with permanent energy supply and the much lower binding energies in polymers and alternatives to avoid random polymer decomposition
• Technical applications such as adhesion, cleaning, wettability, textile modification, coatings, films, etc. New perspectives are explained about how to use selective and mild processes to allow post-plasma chemistry on non-degraded polymer surfaces.

Audience

Physicists, polymer chemists, materials scientists, industrial engineers in biomedicine, coatings, printing, etc.

Professor Jörg Florian Friedrich is a chemist at the at the Technical University of Berlin. He has worked in the in the fields of plasma chemistry, polymer chemistry, polymer and surface analytics since 1972 and has published several books on these topics. His special interest is the clarification and explanation of chemistry in plasma and at the surface of polymers.

Professor Jürgen Meichsner is a physicist at the University of Greifswald. He has worked in the field of low temperature plasma physics and plasma surface interaction, e.g., plasma polymerization and polymer surface modification/functionalization since 1977. He has published numerous articles in peer-reviewed journals, and few book contributions. His special interest is the diagnostics of molecular low temperature plasmas and the analysis of thin organic films and surfaces.

Preface xiii

1 Introduction 1

References 15

2 Basic Principles of the Plasma State of Matter 17

2.1 Characteristics and Physical Properties of Plasmas 17

2.1.1 Ionization Degree, Energy Content and Classification 17

2.1.2 Quasi-Neutrality, Debye Shielding Length, Plasma Frequency 19

2.1.3 Ambipolar Diffusion 24

2.1.4 High-Frequency Conductivity and Permittivity of Non-Thermal Plasmas 26

2.1.5 Charged Particles in External Magnetic Field 30

2.1.6 Thermal and Non-Thermal Plasmas 34

2.1.7 Plasma Kinetics and Transport Equations 40

References 56

2.2 Elementary Processes and Collision Cross Section 57

2.2.1 Classification of Collision Processes in Non-Thermal Plasmas 57

2.2.2 The Collision Cross Section 64

References 77

2.3 Interaction of Non-Thermal Plasmas with Condensed Matter 79

2.3.1 Stationary Plasma Boundary Sheath and Bohm Criterion 80

2.3.2 Plasma Boundary Sheath in Front of the Floating Surface 83

2.3.3 Generalized Bohm Sheath Criterion 84

2.3.4 High-Voltage Plasma Sheath 84

2.3.5 Non-Stationary Plasma Sheaths 88

References 93

2.4 Non-Thermal Plasmas of Electric Gas Discharges 94

2.4.1 Overview 94

2.4.2 The Electric Breakdown in Gases 95

2.4.3 The Glow Discharge 101

2.4.4 Glow Discharges at Harmonic Electric Fields, RF and MW Plasmas 109

2.4.5 High-Voltage Breakdown at Atmospheric Pressure, Corona and Barrier Discharge 115

References 118

3 Plasma Diagnostics 119

3.1 Introduction 119

3.2 Overview of Diagnostic Methods Used for the Characterization of Non-Thermal Plasmas 119

3.3 Analysis of Charged and Neutral Plasma Particles in Non-Thermal Plasmas 119

3.3.1 Electric Probe Measurements 119

3.3.2 Special Case for Single Electric Probe Measurements in Radio-Frequency (RF) Plasmas 133

3.4 Microwave Interferometry 136

3.4.1 Microwave Propagation in Non-Magnetic Plasmas 136

3.4.2 Heterodyne Microwave Interferometry at 160 GHz 138

3.4.3 Electron Density Analysis in CCP and ICP with Argon and Oxygen as Processing Gas 140

3.5 Mass Spectrometry 143

3.5.1 Principle of Mass Spectrometry 143

3.5.2 Quadrupole Mass Spectrometry 143

3.5.3 Analysis of Low-Pressure Plasmas by Quadrupole Mass Spectrometry 145

References 155

3.6 Plasma and Laser-Induced Optical Emission Spectroscopy 157

3.6.1 Spectral Analysis of Plasma Emission (VUV, UV-vis-NIR) 157

3.6.1.1 Optical Emission Spectroscopy (OES) of Low-Pressure Plasmas – Examples 159

3.6.1.2 Determination of the Rotation Temperature from Atmospheric O2 A Band, PP and PQ Branch 161

3.6.1.3 Determination of Ground State Particle Density from Plasma Emission Spectrum 164

3.6.1.4 Abel Inversion 165

3.6.1.5 Phase Resolved Optical Emission Spectroscopy (PROES) of RF Plasmas 166

3.6.2 Laser-Induced Fluorescence (LIF) Spectroscopy 169

3.7 IR Broadband and IR Laser Absorption Spectroscopy 172

3.7.1 Fourier Transform Infrared (FTIR) Spectroscopy for Gas Phase Analysis 172

3.7.1.1 Principle of FTIR Spectroscopy 172

3.7.1.2 FTIR Gas Phase Spectroscopy of RF Plasma with Precursor Ethylenediamine and Argon 178

3.7.2 Infrared Tunable Diode Laser Absorption Spectroscopy (IR-TDLAS) 180

3.7.2.1 Configuration of the IR-TDLAS Experiment 180

3.7.2.2 Principle Procedure for Measuring Single Absorption Lines 181

3.7.2.3 IR-TDLAS of Fluorocarbon Radicals and Reaction Products in CF4 or CF4+H2 RF Plasmas 183

References 185

4 Methods of Polymer and Polymer Surface Analysis 187

4.1 Introductory Remarks 187

4.2 Photoelectron Spectroscopy (XPS) or Electron Spectroscopy for Chemical Analysis (ESCA) 188

4.3 Secondary Ion Mass Spectrometry 193

4.4 NEXAFS – Use of Synchrotron Radiation 194

4.5 Infrared Reflection Absorption Spectroscopy (IRRAS) 195

4.6 Size-Exclusion Chromatography (SEC)/Gel Permeation Chromatography (GPC) and Field-Flow-Fractionation (FFF) 196

4.7 Matrix-Assisted Laser/Desorption Ionization Time-of-Flight Mass Spectrometry (MALDI-ToF-MS) 197

4.8 Electrospray Ionization Time-of-Flight Mass Spectrometry (ESI-ToF-MS) 199

4.9 Overview of Methods 200

References 202

5 Chemical Interactions Between Polymer and Plasma 203

5.1 Introduction 203

5.2 General Conflict Between High Plasma Energies and Low Dissociation Energies of Bonds in Polymers 203

5.3 Chemical Bonds and Functional Groups in Polymers 206

5.4 Response of Different Types of Polymers to Plasma Exposure 208

References 214

6 Polymer Surface Functionalization 217

6.1 Important Properties of Polymers 217

6.2 Why Pretreatment? 217

6.3 Chemical and Structural Problems of Polymers Provoked by Plasma Pretreatment 220

6.4 Inevitability of Simultaneous Functionalization and Polymer Degradation 221

6.5 Physical and Chemical Attacks of the Plasma to Polyolefin Surfaces 223

6.6 Chemical Grafting onto Plasma-Exposed Polymer Surfaces 224

6.7 Oxidation of Polymers by Exposure to the Oxygen Low-Pressure Plasma 225

6.7.1 Introduction of O-Functional Groups Onto Polymer Surfaces 225

6.7.2 Nature of Oxygen-Plasma Introduced Functional Groups 226

6.7.3 Identification of O-Functional Groups Bonded Onto the Topmost Polymer Surface Layer 226

6.7.4 Fit Strategy of O-Functional Groups as Introduced by D. T. Clark 232

6.7.5 Other Surface-Sensitive Analytical Methods 233

6.7.6 Derivatization of O-Functional Groups 234

6.7.7 Identification of Radicals by Chemical Labeling or ESR Spectroscopy 236

6.7.8 Physical Characterization of Oxygen Plasma 237

6.7.9 Use of Plasma Afterglow for Polymer Modification 238

6.7.10 Surface Oxidation and Etching (see also the special section on etching) 239

6.7.11 Changes in Supermolecular Structure in Subsurface Layers Upon Exposure to Oxygen Plasma 240

6.7.12 Changes in Polymer Structure Generated by Exposure to the Vacuum UV Radiation of the Oxygen Plasma 245

6.7.13 Depth of Modification 248

6.7.14 Accelerated Artificial Aging of Polymers by Exposure to Low-Pressure Oxygen Plasma 251

6.7.15 Kinetics of Crosslinking 253

6.7.16 Time-Dependence of Oxygen Introduction 257

6.7.17 Reaction Details of Poly(ethylene terephthalate) Upon Exposure to Oxygen Plasma 264

6.7.18 Optimum Time of Exposure to Oxygen Plasma for Formation of O-Functional Groups and Preventive Avoidance of Structural Degradation and Decomposition 269

6.7.19 Dependence of Oxygen Introduction on Plasma Parameters 272

6.7.20 Behavior of Molecular Orientation and Chain Structure Upon Exposure to Oxygen Plasma 272

References 279

7 Sensitivity of Polymer Units and Functional Groups Towards Exposure to Oxygen Plasma 291

7.1 Introductory Remarks 291

7.2 Behavior of Polymer Structure Upon Exposure to Oxygen Plasma 291

7.3 Etching Behavior of Polymers Upon Exposure to Oxygen Plasma 294

7.4 Classification of Polymers with Similar Degradation Behavior on Exposure to Oxygen Plasma 299

7.5 Stability of Surface Functionalization and Superposition with Post-Plasma Effects upon Exposure to Air 302

7.6 Surface Oxidation of Polyolefins Using Atmospheric-Pressure Plasmas (DBD, APGD or Corona Discharge, Spark Jet, etc.) 308

7.6.1 Dielectric Barrier Discharge 308

7.6.2 Plasma-Assisted and Plasma-Less Spraying of Intact High-Molecular-Weight Polymers at Atmospheric Pressure 314

7.7 Oxidation of Carbon Nanomaterials 320

7.7.1 Graphene 320

7.7.2 Oxidation of Carbon Fibers 321

7.8 Generation of Monosort O-Functional Groups at Polyolefin Surfaces as Anchor Points for Grafting of Molecules 323

7.8.1 OH Groups 323

7.8.2 COOH Groups 332

7.8.3 CHO Groups 333

7.8.4 Super-Acidic Groups via Oxyfluorination 334

7.8.5 Functionalization of Fluorine-Containing Polymers with O-Functional Groups 337

7.9 Post-Plasma Chemical Grafting of Molecules, Oligomers or Polymers Onto OH-Groups 339

7.10 Course of Oxidation from Virgin Polymer to Oxidized Polymer and Finally to CO2 342

7.10.1 Problems of Depth Profiling of Oxidation at Polymer Surface 342

7.10.2 Binding Energies of Covalent Bonds in Polyolefins 343

7.10.3 Analogy Between Thermal Oxidation and Auto-Oxidation of Paraffins 344

7.10.4 Decarboxylation and Emission of CO2 345

7.10.5 Formation of Gaseous Low-Molecular-Weight Etch Products by Oxygen Plasma Treatment 345

7.10.6 Introduction of Oxygen-Containing Groups at Surface of Polyolefins as a Forerunner of Gasification/Etching 347

7.10.7 Formation and Characterization of Low-Molecular-Weight Oxidized Material (LMWOM) 350

7.10.8 LMWOM Formation by Re-Deposition of Etched Fragments 351

7.10.9 Depth Profiling of O/C from Surface to Bulk 352

7.10.9.1 Angle-Resolved XPS 353

7.10.9.2 Dynamic SIMS 354

7.10.9.3 Sputtering 354

7.10.9.4 Post-Plasma Oxidation 354

7.10.10 Tentative Mechanism 355

References 359

8 Ammonia and Bromine Plasmas 371

8.1 Generation of Monosort NH2 Groups 371

8.1.1 Brief History of Plasma-Induced Introduction of Primary Amino Groups Into the Surface of Polyolefins 371

8.1.2 Ways to Produce Amino Groups at Polymer Surfaces 372

8.1.3 Ammonia, Nitrogen-Hydrogen and Hydrazine Plasmas 373

8.1.4 Carbon Fibers Exposed to Ammonia Plasma 376

8.1.5 Oxygen Post-Plasma Introduction After Ammonia Plasma Exposure 380

8.1.6 Invalidity of Le Chatelier’s Principle in Low-Pressure Plasma 381

8.1.7 Time Dependence of N and NH2 Introduction on Exposure of the Ammonia Plasma into Polyolefin Surfaces 383

8.1.8 Hydrogenation Effect of NH3 Plasma 385

8.1.9 Modification of Polyolefin Within a 2μm-Deep Surface Layer 386

8.1.10 Bulk Analysis by NMR 389

8.1.11 Summary of All Attempts to Increase the Yield in NH2 Groups 391

8.1.12 Ammonia Plasma – Undesired Side and Post-Plasma Reactions 392

8.1.13 Deposition of Plasma Polymers Carrying Amino Groups as an Alternative to Ammonia Plasma Treatment 393

8.1.14 Chemical Labeling and Protection of NH2 Groups 394

8.1.15 Post-Plasma Chemical Grafting Onto NH2-Groups 396

8.1.16 Amino Groups at Polymer Surfaces – A Summary 399

8.2 Bromine Plasma 399

8.2.1 Chemical Aspects 399

8.2.2 Theoretical Considerations of the Plasma Process Using Bromine 404

8.2.3 Comparison of Halogen Chemistry 406

8.2.4 Behavior of Plasma-Brominated Surface Layers in Solvents 408

8.2.5 Plasma Polymerization of Vinyl and Allyl Bromide 410

8.2.6 Attempts to Increase Br Concentration in the Plasma Polymer Layers by Admixture of Br2 to Allyl Bromide or Bromoform 412

8.2.7 Dependence of Bromine Introduction Onto Polyolefin Surfaces on Plasma Parameters 412

8.2.8 Electron Temperature in the Bromoform Plasma 415

8.2.9 Yields in Introduction of Other Halogens 415

8.2.10 Plasma Bromination of Other Polymers 417

8.2.11 Chemical Post-Plasma Synthesis of New Monosort Functional Groups by Conversion of Plasma-Introduced Bromine Groups 418

8.2.12 Grafting of Molecules onto Br Groups by Nucleophilic Substitution 419

8.2.13 Grafting Density at Polyolefin Surfaces 422

8.2.14 Comparison of Surface Bromination of Polyolefins with Other Processes 426

8.2.15 Plasma Bromination of Graphitic and Carbon Surfaces 427

8.2.16 Efficiency in Bromination and Grafting of Carbon in Comparison to Polyolefins 441

8.2.17 Conclusions to Plasma Bromination 445

References 446

9 Noble Gas Plasmas 457

9.1 Characterization of Noble Gas Plasmas 457

9.2 Polymer Crosslinking Caused by Noble Gas Plasmas 458

9.3 Vacuum-Ultra Violet Radiation Emitted by Noble Gas Plasmas 460

References 464

10 Plasma Polymerization 467

10.1 Introduction 467

10.2 Milestones in History 470

10.3 General Features of Plasma Polymers 473

10.4 Mechanisms of Plasma Polymerization 475

10.4.1 Absence of Often Proposed Plasma-Induced Radical Chain-Growth Polymerization to Linear Macromolecules? 477

10.4.2 Radical Polymerization of Allyl Monomers 480

10.4.3 Ion-Molecule Reactions 482

10.4.4 Role of Polymerizing Intermediates 483

10.4.5 Crosslinking 483

10.4.6 Polymerization in Continuous-Wave Plasma 486

10.4.7 Pulsed Plasma Polymerization 491

10.4.8 Pressure- and Plasma-Pulsed Discharge 500

10.5 Special Aspects of Plasma Polymerization 506

10.5.1 Fragmentation-(poly)Recombination 506

10.5.2 Atomic Polymerization 506

10.5.3 Rearrangement and Crosslinking of the Already Deposited Plasma Polymer Layer by Plasma Particle Bombardment and Vacuum-UV Irradiation 507

10.5.4 Formation of Unsaturations 508

10.5.5 Formation of CH3 Groups 510

10.5.6 H/C Ratio in Plasma Polymers and “Quasi-Hydrogen-Plasma” 511

10.5.7 Hydrogen Exchange Between Plasma and Polymer Deposit 516

10.5.8 Existence of Crystalline and Supermolecular Structures in Plasma Polymers 517

10.5.9 Influence of Monomer or Precursor Type 518

10.5.10 Role of Pressure and Flow Rate 518

10.5.11 Role of Energy Dose 520

10.5.12 Plasma Polymerization of n-Hexane and Other Hydrocarbons 520

10.5.13 Dependence of Deposition Rate on Position of Sample in the Plasma Zone 524

10.5.14 Retention of Monomer Structure in Plasma Polymer –Changes in Aromaticity and Substitution 525

10.5.15 Molecular Weight Distribution 527

10.5.16 Energetic Balancing 529

10.6 Locus of Plasma Polymerization 530

10.6.1 Adsorption or Gas Phase? 530

10.6.2 Powder Formation 531

10.6.3 Redeposition of Etched Products as Layer 532

10.6.4 Special Effects of Irradiation of Growing Polymer Layer by Vacuum-UV Radiation from Plasma 533

10.6.5 Formation of a “Polymer Skin” 535

10.6.6 Graft Polymerization 535

10.7 Plasma Polymers with Monosort Functional Groups 537

10.7.1 OH Groups 540

10.7.2 COOH Groups 544

10.7.3 NH2 Groups 548

10.8 Attempts to Increase the Yield of Functional Group 556

10.8.1 Optimization of Plasma Conditions for Generation of NH2 Groups 556

10.8.2 Attempts to Increase the Concentration of NH2 Groups by Addition of Ammonia to Allylamine Plasma Polymerization 556

10.8.3 Alternative Methods 564

10.8.4 Plasma-Produced Amino Groups for Promotion of Adhesion 564

10.9 Plasma Copolymerization 566

10.9.1 General Remarks on the Background of Copolymerization and Its Definition 566

10.9.2 Copolymers with Allyl Alcohol 569

10.9.3 Copolymers with Acrylic Acid 575

10.9.4 Allylamine Copolymers 576

10.10 Grafting Onto Plasma Polymers as Special Case of ‘Graft-Copolymerization’ 580

10.10.1 General Aspects 580

10.10.2 Direct Grafting Onto Radical Sites 582

10.10.3 Grafting Onto Peroxy Radicals/Hydroperoxides 582

10.10.4 Reactions with OH Groups 583

10.10.5 Reactions with COOH Groups 584

10.10.6 Reactions with NH2 Groups 584

10.10.7 Reactions with Br Groups 585

10.10.8 Other Methods 585

10.11 Significant Side Reactions 585

10.11.1 Details of the IR Bands at 2200 cm-1 588

10.11.2 DSC Results 590

10.11.3 Post-Plasma Oxidation 591

10.11.4 Attempts to Eliminate Post-Plasma Oxidations 596

10.12 Plasma Polymers Deposited by Atmospheric-Pressure Plasmas 597

References 598

11 Technical Applications 621

11.1 Introduction 621

11.2 Adhesion Promotion 622

11.2.1 Polymer Surface Modification 624

11.2.2 Combination of Plasma Pretreatment and Wet-Chemical Post-Plasma Treatment 628

11.2.3 Deposition of Adhesion-Promoting Polymer Films 629

11.2.3.1 Direct Grafting 629

11.2.3.2 Grafting via Peroxy Route 630

11.2.3.3 Co-Evaporation or Sputtering of Metals During Plasma Polymerization 630

11.2.3.4 Plasma Polymer Coating 631

11.3 Cleaning 633

11.4 Wettability 635

11.5 Etching of Polymers 637

11.5.1 Preparation and Excavation of Supermolecular Structures of Polymers for Their Characterization by Electron Microscopy 637

11.5.2 Ashing 638

11.6 Barrier Layers or Barrier Formation 638

11.6.1 Organic and Inorganic Barrier Layer for Limiting Diffusion 638

11.6.2 Fluorination of Polymers 639

11.7 Anti-Fouling Layers 641

11.8 Sterilization 642

11.9 Water Purification and Desalination 643

11.10 Flame Protection 643

11.11 Textile Modification 644

11.12 Modification of Carbon Fibers and Nanotubes 644

11.13 Silent Discharge and Excimer Radiation 645

11.14 Conducting Films 646

11.15 Scratch-Resistant Coatings 646

11.16 Underwater Plasma 647

References 650

Index 671

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