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- Herausgeber: Wiley-VCH Verlag GmbH & Co. KGaA
- Kategorie: Fachliteratur
- Sprache: Englisch
The result of decades of research by a pioneer in the field, this is the first book to deal exclusively with achieving high-performance metal-polymer composites by chemical bonding. Covering both the academic and practical aspects, the author focuses on the chemistry of interfaces between metals and polymers with a particular emphasis on the chemical bonding between the different materials. He elucidates the various approaches to obtaining a stable interface, including, but not limited to, thermodynamically driven redox reactions, bond protection to prevent hydrolysis, the introduction of barrier layers, and stabilization by spacer molecules. Throughout, chemical bonding is promoted as a simple and economically viable alternative to adhesion based on reversible weak physical interaction. Consequently, the text equips readers with the practical tools necessary for designing high-strength metal-polymer composites with such desired properties as resilience, flexibility, rigidity or degradation resistance.
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Table of Contents
Cover
Title Page
Copyright
Preface
Chapter 1: High-Performance Metal–Polymer Composites
1.1 Introduction
References
Chapter 2: Interpretation of Adhesion Phenomena – Review of Theories
2.1 General
2.2 Mechanical Interlocking
2.3 Interdiffusion
2.4 Interphase Formation
2.5 Weak Molecular Interactions (Cohesive Forces)
2.6 Electrostatic Attraction
2.7 Contaminations, Role of Water, or Humidity
2.8 Coupling Agents
2.9 Use of Glues (Adhesives)
2.10 Hydrophobic Recovery
References
Chapter 3: Interactions at Interface
3.1 Composites and Laminates
3.2 Laminate Processing
3.3 Polymers as Substrate or as Coating
3.4 Chemical Reactions at Surfaces
3.5 Reactions of Metal Atoms with Polyolefins
3.6 Reaction of Metal Atoms with O-Functional Groups at Polymer Surfaces
3.7 Reactions of Metal Atoms with Amino Groups on Polymer Surfaces
3.8 Silane and Siloxane Adhesion-Promoting Agents
References
Chapter 4: Chemical Bonds
4.1 Bonds in Polymers
4.2 Reactions of Chemical Bonds during Pretreatment
4.3 Chemical Bonds at Interface
References
Chapter 5: Functional Groups at Polymer Surface and Their Reactions
5.1 OH Groups at Surface
5.2 Primary Amino Groups at Polymer Surfaces
5.3 Carboxylic Groups as Anchor Points for Grafted Molecules
5.4 Bromination
5.5 Silane Bonds
5.6 Click Chemistry
5.7 ATRP
5.8 Grafting
5.9 Polymers Deposited onto Silicon or Glass
5.10 Molecular Entanglement of Macromolecules of Coating and Substrate at Polymer Surfaces (Interpenetrating Network at Interface)
References
Chapter 6: Pretreatment of Polyolefin Surfaces for Introducing Functional Groups
6.1 Situation at Polyolefin Surfaces
6.2 Physical and Chemical Attacks of Polyolefin Surfaces
6.3 A Few General Remarks to the Pretreatment of Polyolefins
6.4 Introduction of Functional Groups to polyolefin Surfaces
6.5 Usual Pretreatment Processes and Their Advantages and Disadvantages
6.6 Surface Oxidation by Atmospheric-Pressure Plasmas (Dielectric Barrier Discharge-DBD, Atmospheric Pressure Glow Discharge-APGD or Corona Discharge, Spark Jet, etc.)
6.7 Flame Treatment
6.8 Silicoater Process (Pyrosil)
6.9 Laser Ablation
6.10 UV Irradiation with Excimer Lamps
6.11 Ozone
6.12 Mechanical Pretreatment
6.13 Cryogenic Blasting
6.14 Skeletonizing
6.15 Roughening for Mechanical Interlocking and Increasing of Surface Area by Plasma and Sputter Etching
6.16 Solvent Cleaning
6.17 Solvent Welding
6.18 Chemical Treatment by Chromic Acid and Chromo-Sulfuric Acid
6.19 Chemical Etching and Functionalizing of Fluorine-Containing Polymers
6.20 Oxyfluorination
6.21 Sulfonation
6.22 Sputtering for Film Deposition
6.23 Cross-linking as Adhesion Improving Pretreatment (CASING)
6.24 Monosort Functionalization and Selective Chemical Reactions
References
Chapter 7: Adhesion-Promoting Polymer Layers
7.1 General
7.2 Historical Development
7.3 Influence of Plasma Wattage on Chemical Structure of Plasma Polymers
7.4 Pulsed-Plasma Polymerization
7.5 Pressure-Pulsed Plasma
7.6 Copolymerization in Pulsed Plasmas
7.7 Some Additional Details to the Mechanisms of Plasma Polymerization
7.8 Often-Observed Abnormal Side Reactions Occurring in the Plasma Only
7.9 Structure of Plasma Polymers
7.10 Use of Plasma Polymers as Adhesion-Promoting Layers
7.11 Adhesion Promotion of Very Thick Layers
7.12 Summary
References
Chapter 8: Monosort Functional Groups at Polymer Surfaces
8.1 Introduction
8.2 Bromination of Polyolefin Surface by Exposure to the Br
2
Plasma
8.3 Bromoform as Precursor
8.4 Deposition of Plasma Polymers Carrying C−Br Groups
8.5 Loss in Bromine Groups by Wet-Chemical Processing
8.6 Other Halogenations
8.7 C−Br as Anchoring Point for Grafting
8.8 Underwater Capillary Discharge Plasma or Glow Discharge Electrolysis (GDE)
8.9 Conclusions
References
Chapter 9: Chemical Grafting onto Monosort Functionalized Polyolefin Surfaces
9.1 General Aspects
9.2 Grafting of Spacers onto Radicals
9.3 Grafting of Spacers and Oligomers by Reaction with C−OH Groups at the Polyolefin Surface
9.4 Grafting of Linear Spacers and Oligomers onto C−Br Groups
9.5 Introduction of Spacers with Siloxane Cages (POSS)
9.6 Grafting via Click Reaction
9.7 Influence of Spacers on the Metal–Polymer Adhesion
9.8 Summary
References
Chapter 10: Conclusions and Outlook to the New Interface Design
10.1 Introduction
10.2 Physical Effects Produced by Covalent Bonding of Metal to Polymer
10.3 Introduction of Functional Groups onto Polyolefin Surfaces Associated with Damaging of Polymer Structure Near Surface
10.4 Thermal Expansion Coefficients of Metals and Polymers
10.5 Differences between Al–Polyolefin and Polyolefin–Al Laminates
10.6 Protection of Covalent Metal–Polymer Bonds along the Interface
10.7 Reaction Pays for Grafting Spacer Molecules onto Polyolefin Surfaces
10.8 Special Requirements for Metal Deposition Especially Aluminum
10.9 Used Ways to Introduce Spacers for Maximum Adhesion
References
Chapter 11: Short Treatise on Analysis Chemical Features
11.1 General
11.2 Bulk Analysis
11.3 Surface Analysis
References
Index
End User License Agreement
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Guide
Cover
Table of Contents
Preface
Begin Reading
List of Illustrations
Chapter 1: High-Performance Metal–Polymer Composites
Figure 1.1 Model of a more than 10 000 years old spearhead made of flint stone and fixed by bitumen and bowstring.
Figure 1.2 Example for the principal structure of a polymer–metal laminate.
Figure 1.3 Examples of the schematic design of metal–polymer interfaces with interphases and the original bulk materials.
Figure 1.4 Problems with minimum contact area in case of laminating rough surfaces.
Figure 1.5 Schematic comparison of the strength of interactions (bond dissociation energy) and the measured total adhesion between a polymer and a coating, depending on the type of interaction and the density of these interactions along the polymer–coating interface.
Figure 1.6 Locus of failure in metal–polymer laminates.
Figure 1.7 Variants of covalent bonds across the interface between polymer and coating.
Figure 1.8 Continuous coupling of glass fiber and epoxy resin by covalent bonds.
Chapter 2: Interpretation of Adhesion Phenomena – Review of Theories
Figure 2.1 Example of interface and interphase structure of a metal–polymer laminate.
Figure 2.2 Overview on interactions along the interface and theories for their interpretation.
Figure 2.3 Overview on proposed adhesion mechanism.
Figure 2.4 Interatomic distances and bond energies in different types of physical and chemical bonds (A = reference atom, B = distances to the other atom depending on bond type).
Figure 2.5 Schematics of processes at surfaces and modes of layer growing if evaporating with metal.
Figure 2.6 Reactivity of different C−H bonds of a branched polyethylene toward attack of bromine.
Figure 2.7 Several types of surface roughness and different contact modes between rough surface and solid or viscous coatings.
Figure 2.8 Lap shear strength of polyurethane and steel in dependence of cleaning the steel by several methods and combinations expressed by the XPS C1s/Fe2p intensity coefficient.
Figure 2.9 Dependence of measured lap shear strength on C1s/Fe2p peak ratio produced by different pretreatments of steel [23].
Figure 2.10 Mechanical and chemical linking of two polymers A and B beginning with homopolymer A, graft copolymer AB, block copolymer AB, graft AB copolymer, interpenetrating network of AB, and interface-crossing interpenetrating network of AB.
Figure 2.11 Diblock copolymer composed of blocks A and B as coupling agent in blends of polymers A and B.
Figure 2.12 Incompatibility of polymers, segmental interdiffusion, and chain entanglement.
Figure 2.13 Entangled network of linear polymer chains, whereby the bold structure represents the minimum bridge structures to form a network.
Figure 2.14 Metallization of polymers by vacuum or liquid-phase processes.
Figure 2.15 Cross-sectional transmission electron micrographs showing the effect of deposition rate on the formation of the interface between gold and trimethylcyclohexane bisphenol polycarbonate (TMC-PC). A spread-out interface with extensive metal aggregation is obtained after 6 h of Au evaporation at 0.03 nm min
−1
at 235 °C.
Figure 2.16 Transcrystalline layer of polypropylene in the polarized light micrograph formed at the surface of poly(ethylene terephthalate) fibers.
Figure 2.17 Redox reaction of poly(tetrafluoroethylene) with evaporated potassium.
Figure 2.18 Destruction of aromatic rings in contact with transition metals, here chromium, as concluded from near-edge X-ray absorption fine spectroscopy (NEXAFS).
Figure 2.19 Assumed reactions between chromium and aromatic rings of polystyrene (PS) and poly(2,6-dimethyl-1,4-phenylene oxide) (PDMPO) [37].
Figure 2.20 XPS C1s and O1s signals of PDMPO before (virgin) and after evaporation of 4 Cr monolayers.
Figure 2.21 Loss in biaxial orientation of poly(ethylene terephthalate) by oxygen plasma exposure or chromium evaporation measured as NEXAFS determined order parameter (original orientation = 100%).
Figure 2.22 Etching of polymer upon exposure to oxygen plasma and establishing a modified surface layer with (constant) steady-state dimensions.
Figure 2.23 Failure in polymer laminates upon removing the coatings.
Figure 2.24 Failure propagation in a two-component laminate upon removing the coating (black) from the laminate (white). The middle row shows different propagation ways through the rough interface or near-interface layers. The top row presents the removed surface of coatings in a 2-D view, the bottom row those of the substrate surfaces.
Figure 2.25 Scheme of adsorption of macromolecules dissolved in a polymer solution onto the surface of a solid, thus forming an adsorption layer.
Figure 2.26 Vectorial model of sessile drop on the solid surface after Young's idea.
Figure 2.27 Common techniques for contact angle measurements.
Figure 2.28 Determination of contact angle hysteresis by measurement of advancing and receding angles.
Figure 2.29 Wilhelmy balance measurement of a submersion cycle: 1 – the sample approaches the test liquid and force/wetted length is zero; 2 – the sample is in contact with the liquid and forms a contact angle , the liquid rises up and forms a positive wetting force; 3 – the sample is more immersed, the buoyancy increases and the detected force decreases, and the force is measured for the advancing angle; 4 – the sample is partially removed from the liquid, and the force is measured for the receding angle.
Figure 2.30 Contact angles of a liquid (water) deposited onto surfaces with different surface energies and different roughness.
Figure 2.31 Calculation of the “critical surface tension” following Zisman's approach.
Figure 2.32 Schematics of surface dynamics caused by different environments.
Figure 2.33 Acid–base interactions of pyridine and alumina.
Figure 2.34 Acid–base contribution to the work of adhesion [192].
Figure 2.35 Schematic diagram of the electrical double layer along the polymer–metal interface and changes in the contact potential at the interface.
Figure 2.36 Failure at interface of composite systems caused by contaminations and overcoming this weak point by displacement with coupling agents or spacers as well as with adhesives or glues.
Figure 2.37 Principle of chemical bonding plasma-oxidized polypropylene and epoxy resins by use of the aminosilane adhesion promoter.
Figure 2.38 Principle of chemical bonding glass fibers and epoxy resins by use of aminosilane adhesion promoter.
Figure 2.39 A few coordinative bonds of chromium.
Figure 2.40 Synthesis of the diglycidylether of bisphenol A.
Figure 2.41 Curing of glycidylethers.
Figure 2.42 Bond strength of two-component epoxy resin adhesives in dependence of the mixing ratio of epoxy resin with curing agent.
Figure 2.43 Fracture energy in dependence of the temperature for rubber-toughened and unmodified epoxy resin adhesives.
Figure 2.44 Schematics of stress–strain behavior of adhesives layers produced different classes of adhesives.
Figure 2.45 Most important components of polyurethanes.
Figure 2.46 Urea and biuret groups.
Figure 2.47 Formation of allophanate groups.
Figure 2.48 Trimerization of isocyanates to polyisocyanurates.
Figure 2.49 Synthesis of polycyanurates from cyanoesters.
Figure 2.50 Formaldehyde–phenol reaction.
Figure 2.51 Reaction to novolacs.
Figure 2.52 Formation of a resol.
Figure 2.53 Methylol formation of melamine in the presence of formaldehyde and cut of a completely cured melamine–formaldehyde resin.
Figure 2.54 Urea–formaldehyde reaction to a dimethylol condensation product and cut of urea–formaldehyde resin with ether and methylene bridges.
Figure 2.55 Synthesis of aromatic polyesters.
Figure 2.56 Reactions to unsaturated polyesters and their curing.
Figure 2.57 Silicon resins.
Figure 2.58 High-temperature polyimide adhesive.
Figure 2.59 Thermostable polymer adhesives.
Figure 2.60 Schematic of the “hydrophobic recovery” and surface dynamics.
Figure 2.61 Decrease in surface energy during the storage of plasma-modified polypropylene under ambient air condition.
Figure 2.62 Loss in tensile shear strength of polyurethane–polypropylene composites by exposure of plasma treated polymers to air (storage) before gluing (forming the composite).
Chapter 3: Interactions at Interface
Figure 3.1 Survey on conditions for the formation of covalent bonds between polymer substrates and polymer coatings (metal deposits).
Figure 3.2 Differences in surface energies of polyethylene, plasma-oxidized polyethylene (PE-O
2
), polyethylene–maleic anhydride copolymer (PE-MAA), evaporated metallic aluminum, and native (oxy and hydroxy) aluminum substrate surfaces (=bayerite or boehmite structure).
Figure 3.3 Possible interactions between adsorbate and substrate.
Figure 3.4 Schematics of S
N
1and S
N
2 reactions at polymer surfaces exemplified by the substitution of Br moieties by amino groups containing grafting substances.
Figure 3.5 Examples for reduction reactions along the aluminum–polymer interface evaporating thin films of aluminum.
Figure 3.6 Proposed metal bonding to carboxylic groups of poly(acrylic acid) [38, 44].
Figure 3.7 Fourier transform infrared spectroscopy–attenuated total reflection (FTIR–ATR) spectra of stearic acid thermally evaporated with an ultrathin silver layer (a few monolayers).
Figure 3.8 Adhesive bond strength of evaporated metal films onto various polymer substrates [66].
Figure 3.9 Proposed subsequence of thermodynamically determined reactions along the interface between aluminum and OH-group containing polymer.
Figure 3.10 Comparison of surface energies on both sides of Al–polyethylene and polyethylene–Al laminates.
Figure 3.11 Role of thermodynamics and connection to real existing adhesion.
Figure 3.12 Indirect formation of OH groups at polyethylene surface upon exposure to the oxygen plasma by formation and attachment of OH species.
Figure 3.13 Octaamino-POSS. ( = polymer or solid surface)
Chapter 4: Chemical Bonds
Figure 4.1 Molecular orbitals of carbon atoms in single, double, and triple bonds.
Figure 4.2 CH, CH
2
, and CH
3
bonds in different types of polyethylene.
Figure 4.3 Concurrence of C−H and C−C bond scission in polymers during surface modification, formation of a thin layer of low-molecular-weight oxidized material (LMWOM) and its different behavior by deposition with a metal layer or with a viscous coating and mechanical separation, for example, by peel test.
Figure 4.4 Energy wins by delocalizing in aromatic rings.
Figure 4.5 Reactions of aromatics at substitution or upon exposure to UV irradiation or plasma [7].
Figure 4.6 Comparison of product spectra received after oxidative pretreatment of polyethylene with nontreated polymers containing OH or COOH groups in their original structure.
Figure 4.7 Loss in aromaticity in graphene by electrophilic or radical addtion of bromine connected with loss in electrical conductivity.
Figure 4.8 Thermodynamic and kinetic behavior of aliphatic polymers upon exposure to plasma-produced bromine atoms.
Figure 4.9 Examples of preferred and preformed degradation products in the polymer structure upon exposure to plasma, UV, or heat.
Figure 4.10 Important rearrangement processes of O-containing polymers upon exposure to plasma.
Figure 4.11 Reaction scheme of polymer–polymer (cross-)linking using dicumylperoxide.
Figure 4.12 Example of a synthesis route for the preparation of a comb-like graft copolymer.
Figure 4.13 Aminosilane as coupling agent in maleic-acid-grafted polypropylene.
Figure 4.14 Illustration of σ and π bonding involving s, p, and d orbitals.
Figure 4.15 Structure of model coatings for interface analysis.
Figure 4.16 Assumed succession of reactions between chromium and aromatic rings of polystyrene (PS) and poly(2,6-dimethyl-1,4-phenylene oxide) (PDMPO) [32].
Figure 4.17 XPS C1s und O1s peaks of poly(ethylene terephthalate) and poly(2,6-dimethy-1,4-phenylene oxide) before and after evaporation with 0.4 nm chromium.
Chapter 5: Functional Groups at Polymer Surface and Their Reactions
Figure 5.1 Significance of monosort functional groups at high density at polymer surfaces for chemical grafting.
Figure 5.2 Esterification and ester hydrolysis reactions at surfaces.
Figure 5.3 Esterification with acid chlorides and transesterification.
Figure 5.4 Reaction of OH groups at polymer surfaces with silanes.
Figure 5.5 Grafting route of silanes and alanine onto hydroxy-modified polypropylene surface and derivatization [2].
Figure 5.6 Addition reaction between a hydroxy group at a solid surface with an isocyanate.
Figure 5.7 Grafting route of toluenediisocyanate and different fluorophores onto hydroxy-modified polypropylene surfaces.
Figure 5.8 Possible linking of isocyanates onto SH groups at the substrate surface.
Figure 5.9 Found chemical reactions at polyolefin surfaces upon exposure to the ammonia plasma.
Figure 5.10 Possible wet chemical reaction of primary amino groups at polyolefin surfaces. ( = polymer or solid surface.)
Figure 5.11 Reactions of amino and nitro groups at polymer surfaces suited for anchoring of organic molecules.
Figure 5.12 Proposed mechanism of COOH formation on polyolefin surfaces [31].
Figure 5.13 Graft reactions of amino groups to polyolefin surfaces, transesterification, and reduction of COCl.
Figure 5.14 Changes in surface composition of polytetrafluoroethylene upon exposure to hydrogen or ammonia low-pressure plasma as measured by XPS.
Figure 5.15 Grafting onto C−Br groups at polyolefin surfaces. ( = polymer or solid surface.)
Figure 5.16 Reaction of amino and methacrylic silanes with fiber surfaces and polymer resin.
Figure 5.17 Principle of the formation of substituted triazine rings at polypropylene surfaces via “click” reaction onto azide groups introduced by plasma surface bromination with molecular bromine and their nucleophilic substitution by sodium azide.
Figure 5.18 Surface grafting of substituted acetylenes onto Br-containing plasma polymer surfaces using the click chemistry [71].
Figure 5.19 Anchoring of ethynyl-terminated poly(3-hexylthiophene) onto grafted 3-azidopropyltrimethoxysilane by click chemistry to a SAM (self-assembled monolayer) [72].
Figure 5.20 Polyolefin surface functionalization by use of azides/nitrenes.
Figure 5.21 ARGET–ATRP reaction (R−X = alkyl halide as dormant species; Cu
I
−X/ligand = activator that can be oxidized; Cu
II
−X
2
/ligand = deactivator that can be reduced; R* = monomer radical; M = monomer;
k
a
= activation rate coefficient;
k
p
= polymerization rate coefficient;
k
t
= termination rate coefficient, EH = ethyl hexanoate).
Figure 5.22 Principle of “grafting from” and “grafting to” for generating polymer brushes onto glass, metal, or polymer surfaces.
Figure 5.23 Schematic dependence of brush thickness, molecular orientation and grafting density (
R
g
– gyration radius,
D
– the distance between grafting sites, h is averaged length of the extended polymer chain, and σ is the grafting density).
Figure 5.24 Grafting of fluorescein onto polypropylene that was coated with plasma-polymerized allylamine.
Figure 5.25 Dye molecules covalently linked to polypropylene surface.
Figure 5.26 Amino acid sequences grafted to polypropylene (cf. Figure 5.3).
Figure 5.27 Covalently and spacer-bonded fluorescence sensors without support by cucurbituril jacket or without for elimination of undesired quenching by interaction with the substrate surface.
Figure 5.28 Erecting of aliphatic spacer molecules with dye sensors chemically linked to polypropylene by cucurbituril threadings.
Figure 5.29 Mechanism of dendrimer (sphere) immobilization modes.
Figure 5.30 Grafting of poly(ethylene glycol) onto polyolefin surfaces.
Figure 5.31 Molecular structure of the dendrimeric polyglycerol compared with a tree.
Figure 5.32 Reaction pathway to a partially aminated polyglycerol (Ms – methanesulfonyl chloride).
Figure 5.33 Simplified view of dendrimer linking to polypropylene surfaces via bromination.
Figure 5.34 Summary of complex molecules grafted onto polypropylene surfaces.
Figure 5.35 Silanization of glass surfaces and chain extension.
Figure 5.36 Adsorptive loop formation [101].
Figure 5.37 Aliphatic diamines A form molecular loops at brominated polypropylene surface and during a chain-growth polymerization to polymers B whereby the chain grows through the loops.
Figure 5.38 Comparison of polymer–polymer interdiffusion, welding, and sewing by entanglement of molecular loops.
Chapter 6: Pretreatment of Polyolefin Surfaces for Introducing Functional Groups
Figure 6.1 Comparison of different methods in maximum total functionalization (
X
) and in maximum desired monosort functionalization (
Y
).
Figure 6.2 Overview on variants of molecules and oligomer grafting onto modified polyolefin surfaces.
Figure 6.3 General behavior of metal–polymer adhesion depending on the duration/intensity of exposure to low-pressure glow discharge plasma.
Figure 6.4 Scheme of different molecular interactions along the interface of two materials and their assumed energetic contribution to the adhesion.
Figure 6.5 Self-assembled monolayer (SAM) of octadecyltrichlorosilane (OTS), polypropylene (PP), and poly(ethylene terephthalate) exposed to the oxygen low-pressure radio-frequency glow discharge plasma and their oxygen introduction (O/C by XPS), loss in molecular orientation (by near-edge X-ray absorption fine structure-NEXAFS) and 90° peel strength measurement with double-faced adhesion tape.
Figure 6.6 Establishment of a steady state of polymer etching and simultaneous advancing of a modified (oxidized, cross-linked) zone into the polymer bulk produced by plasma vacuum UV irradiation.
Figure 6.7 Possible surface structures of partially crystalline polyolefins after exposure to the oxygen low-pressure plasma.
Figure 6.8 Etching rate and fitting results of the XPS-C1s peak of polypropylene depending on time of exposure to the oxygen low-pressure plasma.
Figure 6.9 Superposing of surface layer modification, etching, and fixed XPS sampling depth at polymer exposure to low-pressure oxygen plasma.
Figure 6.10 Schematics of the grafting process postulated by Kato
et al
. [66].
Figure 6.11 Peel strength of aluminum from oxygen plasma-modified polymers (Al thickness = 150 nm, LP-GD = low-pressure glow discharge, DBD = dielectric barrier discharge).
Figure 6.12 Unspecific surface functionalization of polymers on exposure to oxygen or ammonia plasma and generating of a much more desired specific (monosort) functionalization by deposition of plasmapolymer layers with one type of functional groups or by exposure to bromine plasma.
Figure 6.14 Schematic view on a self-assembled monolayer (SAM) of octadecyltrichlorosilane (OTS) before and after exposure to the low-pressure oxygen plasma for 4 s.
Figure 6.14 Near edge X-ray absorption fine structure (NEXAFS) spectra of oxygen plasma-treated self-assembled monolayer of octadecyltrichlorosilane.
Figure 6.15 Langmuir–Blodgett layer made from stearic acid exposed to the oxygen low-pressure plasma.
Figure 6.16 MALDI spectra of hexatriacontane (HTC, C
36
H
74
) with silver adduct exposed to oxygen plasma.
Figure 6.17 Changes in structure (O/C, aromaticity, carbonyls) of polymers (PS, PC, PET, PP) on exposure to oxygen low-pressure plasma measured by means of NEXAFS (aromaticity, carbonyls) and XPS (O/C).
Figure 6.18 Gas-phase analysis on exposure of polyethylene to the oxygen plasma by means of mass spectrometry.
Figure 6.19 Principal reactions of polymers with release of hydrogen.
Figure 6.20 Gel-permeation chromatograms of oxygen plasma-modified polymers.
Figure 6.21 Classes of polymer degradation on exposure to plasma.
Figure 6.22 MALDI-ToF-MS spectrum of hexatriacontane after 30 s exposure to the oxygen plasma.
Figure 6.23 Polar contribution of surface energy versus oxygen concentration at polymer surface introduced by low-pressure oxygen plasma exposure.
Figure 6.24 ThFFF chromatograms of 230 nm thick polystyrene coatings on Si wafers exposed for different time duration to oxygen low-pressure plasma.
Figure 6.25 Merged chromatograms/fractograms of GPC and ThFFF of polystyrene exposed to the oxygen plasma.
Figure 6.26 FTIR–ATR (diamond) spectra of hexatriacontane (h-HTC) exposed either to NH
3
or to ND
3
plasma.
Figure 6.27 Comparison of oxygen introduction into polypropylene depending on exposure time to atmospheric or low-pressure plasma.
Figure 6.28 Glow zone of atmospheric dielectric barrier discharge.
Figure 6.29 Surface energy of polyethylene as a function of oxygen concentration for low and atmospheric pressure (DBD in air) plasmas.
Figure 6.30 Plasma jets for polymer surface treatment.
Figure 6.31 Tensile shear strengths of PU–PP specimen as a function of oxygen concentration at polypropylene surfaces introduced by atmospheric and low-pressure plasmas.
Figure 6.32 Flaming of sheet materials.
Figure 6.33 Absorbance of different polymers in the regions of UV-C and VUV.
Figure 6.34 Schematic presentation of different modifications of polystyrene spin-coating films using Ar plasma treatment and UV irradiation.
Figure 6.35 C1s peaks of spin-coated polystyrene without treatment, 2 and 600 s exposure to the argon plasma below the Faraday cage and pulsed plasma-polymerized styrene.
Figure 6.36 O/C ratio and surface energy of polystyrene exposed to UV, plasma, plasma + Faraday cage or produced by pulsed plasma polymerization.
Figure 6.37 Ozonolysis of olefinic double bonds [181].
Figure 6.38 Mechanical abrasion methods.
Figure 6.39 Micro and macro roughness of polymer surfaces after mechanical pretreatment.
Figure 6.40 Fingerprint at paper surface accentuated Si signals in mapping of secondary ion mass spectrometry.
Figure 6.41 Solvent cleaning procedures.
Figure 6.42 Principal scheme of interdiffusion during solvent-assisted “welding” of two partially compatible polymers.
Figure 6.43 Chromic acid-etched (96 h, 70 °C) polypropylene with spherulitic etch structure.
Figure 6.44 Equipment for magnetron sputtering of polytetrafluoroethylene (PTFE) developed by Biederman [275].
Figure 6.45 Schematics of CASING process, surface functionalization, and surface roughening.
Figure 6.46 Schematic comparison between plasma functionalization, plasma polymerization, and CASING.
Figure 6.48 Reaction scheme on reduction of O-functional groups to OH groups and their labeling with TFAA.
Figure 6.49 XPS survey scans of polypropylene surfaces, after exposure to oxygen plasma (cw-plasma, 6 Pa, 2 s), after labeling with TFAA and after reduction with diborane and TFAA derivatization.
Figure 6.49 Comparison of reduction efficiency of several reduction agents in terms of OH group formation for each polyethylene surface with 27% O/C.
Figure 6.50 NH
3
, N
2
, and NH
3
+ H
2
pulsed-RF plasma exposure of polypropylene surfaces and XPS-measured N, O, and NH
2
concentrations (X) (10 min, 6 Pa, 30 W, pulse frequency 1000, duty cycle 0.1).
Figure 6.51 C1s XPS peaks of NH
3
, N
2
, and NH
3
+ H
2
(1 : 3) plasma-treated polypropylene depending on treatment time (continuous wave radio-frequency plasma, 100 W, 6 Pa).
Figure 6.52 Time-dependence of oxygen and nitrogen incorporation into the polypropylene surface on exposure to the ammonia plasma (continuous wave radio-frequency plasma, 100 W, 6 Pa) as measured by XPS analysis after transport of samples from the plasma chamber to the spectrometer (exposure to air about 30 min).
Chapter 7: Adhesion-Promoting Polymer Layers
Figure 7.1 Principle of thin-film deposition of polymers by plasma polymerization of precursors/monomers or electrospray ionization deposition of high-molecular-weight classic polymers.
Figure 7.2 Principle of thin-film deposition of polymers by plasma polymerization of precursors/monomers carrying functional groups or electrospray ionization deposition of polymers with functional groups.
Figure 7.3 Labeling of radicals in freshly deposited plasma polymers.
Figure 7.4 Post-plasma oxygen incorporation of plasma polymers upon exposure to ambient air as well as after NO gassing.
Figure 7.5 Proposed model of plasma polymer produced from ethylene (PPE) with branches, cross-links, aromatic rings and double bonds, “atomic polymerization,” benzene ring cracking and acetylene–cyclobutadiene–benzene–styrene mechanism, and polystyrene formation.
Figure 7.6 Schematics of pulsed-plasma (pp) and continuous-wave (cw) polymerization and their products.
Figure 7.7 Schematic view of the correlation between high pressure, high sticking rate and chain propagation for chemical gas-phase polymerization. In the last line, the principal particle densities are depicted for the low-pressure plasma ignition and the high-pressure chemical chain propagation.
Figure 7.8 Principle of plasma and pressure-pulse synchronization and measured response of the plasma system.
Figure 7.9 Pressure- and power-pulsed plasma (10 W).
Figure 7.10 XPS-C1s-signals of pulsed-plasma-polymerized ethylene with additional pulsing of the pressure (a) and without (b) it.
Figure 7.11 Idealized structures of allyl alcohol homo- and copolymer ethylene or butadiene.
Figure 7.12 Yield in OH groups for plasma-initiated copolymerization of allyl alcohol and butadiene, ethylene, and styrene.
Figure 7.13 XPS C1s signals of ethylene, allyl alcohol, and allyl alcohol–ethylene copolymer (RF, 1000 Hz, 0.1 duty cycle, 100 W, 26 Pa).
Figure 7.14 Thermodynamics of plasma polymerization and conclusions to thermochemical reactions caused by the surplus in heat.
Figure 7.15 Theoretical and CHN analysis-measured hydrogen ratio in a plasma-deposited styrene–allyl alcohol (1 : 1) copolymer.
Figure 7.16 Fourier transform infrared spectroscopy–attenuated total reflection (FTIR-ATR) spectra of plasma-polymerized allylamine with NH-/NH
2
-specific bands (in gray), a significant CN band.
Figure 7.17 Energy distribution to collision and radiation processes during plasma polymerization.
Figure 7.18 Valence-band X-ray photoelectron spectroscopy spectra of continuous-wave plasma-polymerized ethylene before and after exposure to ambient air in comparison to spectra of commercial polyethylene and polypropylene.
Figure 7.19 Near-edge X-ray absorption fine structure (NEXAFS) spectra of pulsed-plasma-polymerized acetylene, ethylene, and butadiene in comparison to commercial polyethylene.
Figure 7.20 Thermal field-flow fractograms of continuous-wave (cw) and pulsed-plasma-polymerized styrene. The duty cycle for pulsing was 0.1, the repetition frequency was 1000, and the wattage was adjusted to 30 W only during pulses giving the same dose as with cw plasma = 3
W
eff
.
Figure 7.21 Comparison of IR spectra of pulsed-plasma PS (cw-RF plasma, 50 W; 0.015 ms plasma on/0.085 ms plasma off; 4.5 Pa; 80 sccm) and commercial PS standard (150 000 g mol
−1
) using the Grazing Incidence Reflectance–FTIR (α = 70°).
Figure 7.22 Valence band, survey scan, and C1s peak of X-ray photoelectron spectroscopy of pulsed-plasma-polymerized styrene.
Figure 7.23 Comparison of thermal weight loss of continuous-wave and pulsed-plasma-polymerized styrene by thermogravimetry (TG) and differential thermogravimetry (DTG).
Figure 7.24 Dielectric relaxation spectra (log ϵ″-dielectric loss) of pulsed-plasma-polymerized and chemically polymerized poly(1,2-butadiene) in dependence on temperature.
Figure 7.25 Retention of functional groups during polymerization.
Chapter 8: Monosort Functional Groups at Polymer Surfaces
Figure 8.1 Electron energy distribution in the low-pressure glow discharge and locus of dissociation and ionization energy.
Figure 8.2 Variants of polyolefin modification with monosort functional groups and chemical grafting via functional groups.
Figure 8.3 General plasma-initiated processes for covalent linking of molecules or monomers, oligomers, and polymer chains to plasma-functionalized or plasma-activated polyolefin surfaces.
Figure 8.4 Schematic sketch of principal was for introduction of bromine to polyolefin surfaces by bromine plasma, bromoform plasma, and deposition of allyl bromide plasma polymer layers.
Figure 8.5 Illustration of preferred radical substitution reaction on tertiary C−H moieties compared with that on secondary C−H groups.
Figure 8.6 Proposed reaction mechanisms with plasma-activated bromine and polyethylene chains.
Figure 8.7 Dependence of bromine percentage in the plasma polymer deposited from allyl bromide on wattage input into the bromine plasma.
Figure 8.8 Bromine introduction onto the surface of polyethylene on exposure to bromine, bromoform, or allyl bromide plasma.
Figure 8.9 Schematic view of loss of brominated material bromine concentration during contact with a solvent (tetrahydrofuran = THF) as occurring in wet-chemical synthesis.
Figure 8.10 Solvent resistance of the top-most surface layer of polyolefins during intense dipping for 10 min into ultrasonicated tetrahydrofurane (THF).
Figure 8.11 Br, Cl, and O introduction onto polyethylene and polypropylene surfaces in dependence on exposure time to the bromotrichloromethane plasma for washed and unwashed samples using the cw mode, 6 Pa and 100 W.
Figure 8.12 Br and O introduction onto polyethylene and polypropylene surfaces in dependence on exposure time to the trichloromethane plasma for washed and unwashed samples using the cw mode, 6 Pa and each 100 W.
Figure 8.13 F and O introduction onto polyethylene and polypropylene surfaces in dependence on exposure time to the trifluoromethane plasma for washed and unwashed samples using the cw mode, 6 Pa and each 100 W.
Figure 8.14 I and O introduction onto polyethylene and polypropylene surfaces in dependence on exposure time to the diiodomethane plasma for washed and unwashed samples using the cw mode, 6 Pa and each 100 W.
Figure 8.15 Ionization potentials and measured electron energies in rf plasma of haloforms and related halogen-containing plasma gases.
Figure 8.16 Survey on maximal halogenations introductions and co-introduction of oxygen for haloform (and related substances) plasmas.
Figure 8.17 Schematics for changing the C−Br functionality chemically to OH (SH) and NH
2
functional groups.
Figure 8.18 Chemical grafting onto C−Br groups at polymer surfaces.
Chapter 9: Chemical Grafting onto Monosort Functionalized Polyolefin Surfaces
Figure 9.1 Adhesion improvement of Al–polyethylene system in dependence on hydroxy and carboxylic group concentrations at polymer surface () as well as for aliphatic (CH
2
)
6
spacers with terminal OH or COOH groups grafted onto the polyolefin surface ().
Figure 9.2 Comparison of peel strength values of Al–PP systems modified at the interface with plasma-deposited poly(allylamine) and different grafted spacers R referenced to one spacer per 100 C atoms.
Figure 9.3 Influence of spacer structure on peel strength referenced to each one terminal group.
Figure 9.4 Dependence of peel Al–PP strength on chain length of spacer and terminal spacer group.
Figure 9.5 Comparison of functional groups carrying plasma polymers alone for adhesion promotion and in combination with additionally introduced spacer molecule (GAH=glutaraldehyde).
Figure 9.6 Peel strength of flame-resistant coatings from polyolefin laminates.
Figure 9.7 Chemical (or mechanical) fixation of molecules, oligomers, and polymers onto modified polyolefin surfaces.
Figure 9.8 Grafting routes via monosort functional groups or multifunctional groups and their chemical conversion to graft groups.
Figure 9.9 Hydrolysis of Al−O−C bonds.
Figure 9.10 Principal tasks of spacer molecules at the interface of polyethylene and aluminum.
Figure 9.11 Reactions on C-radical sites introduced by plasma exposure or irradiation with monomer, oxygen, and DPPH (DPPH = diphenylpicrylhydrazil).
Figure 9.12 Principal bonding between polyolefin and organic coating.
Figure 9.13 Nucleophilic substitution of C−Br moieties by diols, glycols, diamines, and dithiols.
Figure 9.14 Density of various spacer molecules grafted onto C−Br groups at polyethylene surface by the number of methylene units in the respective spacer molecule.
Figure 9.15 Graft density of diols, glycols, and diamines onto plasma brominated polypropylene surfaces with each 30% Br/C.
Figure 9.16 Calculated configuration of amino-POSS grafted onto brominated polyethylene surfaces by one bond (black = Si, white = O).
Figure 9.17 Grafting of spacer molecules by click reaction.
Figure 9.18 Peel strength results of Al–PP laminates with and without spacers. Additionally, the peel strength is given per one bond or spacer.
Figure 9.19 Scheme of goals to produce best adherent and long-living polyolefin-coating composite (OH and NCO groups serve as representative example).
Chapter 10: Conclusions and Outlook to the New Interface Design
Figure 10.1 Peel strengths (90°) of 150 nm thermally evaporated copper from plasma-brominated (bromoform plasma) and diol spacer-grafted polypropylene substrates referenced to the contribution of one spacer end group per 100 C atoms of polypropylene.
Figure 10.2 Dimensional behavior of laminates and composited by thermal exposure depending on direct or spacer bonding, presented in relative units.
Figure 10.3 Assumed reaction kinetics (energy profiles) for the two-step reaction of Al with hydroxyl groups at polyolefin surfaces (
G
0
= Gibbs reaction enthalpy).
Figure 10.4 Schematic of physical interactions (a), such as van der Waals forces, and chemical bonds (b).
Figure 10.5 Diffusion experiment with water or heavy water through polymer films (left) or into the interface measured by ATR (and Kelvin probe) [17].
Figure 10.6 Consequences of chemical bonding of onto functional groups onto the surface of polyolefins, formation of molecular debris (fragments) with low mechanical stability or removing of fragments by solvent extraction connected with strong loss in anchoring groups (X).
Figure 10.7 Surface chemistry and chemical interactions depending on the nature of substrate for a system consisting of polyethylene and aluminum combined either by thermal Al evaporation or thermal pressing of the polymer.
Figure 10.8 Characteristic examples of grafting via oxidation-diborane reduction, plasma bromination, or allylamine plasma polymerization to monosort functional groups on polyolefin surfaces.
Figure 10.9 Schematics of Al–polyolefin and polyolefin–Al systems.
Figure 10.10 XPS-measured elemental composition of plasma-deposited poly(allylamine) and concentration of primary amino groups on its surface.
Figure 10.11 C1s signal after reaction of glutaraldehyde with amino groups of plasma-polymerized allylamine deposited on polypropylene.
Figure 10.12 Schematics of the multifunctional (ideal) spacer rearrangements along the aluminum–polypropylene interface.
Figure 10.13 Aminophenyl-substituted POSS molecule with suggested chain-extensions on terminal amino groups.
Figure 10.14 Scheme of hydrolysis and condensation of trialkoxysilanes.
Figure 10.15 Absorbance of OH groups in the FTIR–ATR spectrum of polypropylene coated with allylamine plasma polymer, chain-extended with glutaraldehyde and aminosilane depending on time of hydrolysis of the silane ethoxy groups. The time of hydrolysis was varied from 10 to 30, 60, and 120 min.
Figure 10.16 Reaction of HDI with poly(allylamine) modified PP (step 1, structure A) and subsequent reaction with 3-aminopropyldimethylmethoxysilane, 3-aminopropyldimethoxymethylsilane, and 3-aminopropyltrimethoxysilane (step 2). The subsequent hydrolysis resulted in structures B, C, and D.
Figure 10.17 Variation of interface design of Al–PP laminates; peel strength referenced to one functional group or spacer.
Figure 10.18 Influence of the spacer introduction with OH endgroups in comparison with laminates without spacers but same concentration of OH groups.
Figure 10.19 “Ideal structure” of a metal–polymer interface with its different functions without and with linear mechanical loading.
Figure 10.20 Possible failure mechanism under mechanical loading, however, not observed because of mechanical inseparability by using peel technique.
Figure 10.21 Variation of spacer chain length using ethylene glycol species.
Figure 10.22 Ethylene glycol grafting (red-oxygen) by Williamson's ether synthesis onto plasma-brominated polyethylene surface (indicated by each seven CH
2
groups) with increasing coiling with growing chain length of ethylene glycol graft.
Chapter 11: Short Treatise on Analysis Chemical Features
Figure 11.1 Scheme of bulk analysis methods.
Figure 11.2 Overview of most often used analysis methods for information on the polymer surface.
Figure 11.3 Principle of MALDI-ToF-MS.
Figure 11.4 Scheme of field-flow fractionation of polymers.
Figure 11.5 Schematic overview of different sampling/information depths of surface-sensitive analysis methods.
Figure 11.6 Effects of irradiation with X-rays on solid surface layers.
Figure 11.7 XPS survey spectrum of polystyrene.
Figure 11.8 Fitting of the C1s peak of polyethylene exposed to the oxygen plasma into four components.
Figure 11.9 Scheme of secondary ion mass spectrometry (SIMS).
Figure 11.10 Damaged zone at polymer surface shown schematically with the probability of releasing small or large fragments.
Figure 11.11 Principle of SEIRA with evaporated silver ellipsoids and adsorbed fatty acid.
Figure 11.12 Scheme of IRRAS for analysis of thiol-terminated SAM layer.
Figure 11.13 Schematic view of an evanescent wave in an ATR crystal covered with polymer foils on both sides.
List of Tables
Chapter 1: High-Performance Metal–Polymer Composites
Table 1.1 Interatomic forces in physical and chemical interactions
Chapter 2: Interpretation of Adhesion Phenomena – Review of Theories
Table 2.1 Interaction forces and their ranges
Table 2.2 A few bond dissociation enthalpies of important bonds in polymers [12]
Table 2.3 A few multiple bond dissociation enthalpies of important bonds in polymers [12]
Table 2.4 Dissociation enthalpies of halogen bonds in polymers [14]
Table 2.5 Relative reactivity of C−H bonds in polymers toward halogen atoms [13]
Table 2.6 Overview on mathematical expressions of theories related to surface tension and contact angle measurements
Chapter 3: Interactions at Interface
Table 3.1 Polymers and their tendency to form interactions with other solids
Chapter 6: Pretreatment of Polyolefin Surfaces for Introducing Functional Groups
Table 6.1 Zip-lengths for polymer depolymerization [21, 22]
Table 6.2 Advantages and disadvantages of methods for modification of polyolefin surfaces with functional groups.
Table 6.3 Resulting functional groups at polymer surface on its exposure to plasma
Chapter 7: Adhesion-Promoting Polymer Layers
Table 7.1 Deposition rate of monomers and precursors as polymer layers upon exposure to plasma (duty cycle, dc = 0.1) [1, 56]
Chapter 8: Monosort Functional Groups at Polymer Surfaces
Table 8.1 Processes for production of monosort functionalization on polyolefin surfaces.
Chapter 9: Chemical Grafting onto Monosort Functionalized Polyolefin Surfaces
Table 9.1 Functional groups at polyolefin surface and at bifunctional spacer molecules
Chapter 10: Conclusions and Outlook to the New Interface Design
Table 10.1 Linear thermal expansion coefficients of different materials
Chapter 11: Short Treatise on Analysis Chemical Features
Table 11.1 Overview on most often used analysis methods for information on the whole solid (bulk)
Table 11.2 Surface imaging of topography
Table 11.3 Surface-sensitive methods
Table 11.4 Band positions of important groups in polymers
Table 11.5 Absorption wavelength of important groups in polymers
Table 11.6 Chemical shifts in organic compounds
Table 11.7 Binding energies of the C1s peak
Metal-Polymer Systems
Interface Design and Chemical Bonding
Jörg Friedrich
Author
Prof. Jörg Friedrich
Siedlerweg 23
15537 Erkner
Germany
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Preface
Recently, a great variety of books on adhesion, written by well-known and excellent researchers, is available for the readers in science and industry. Composite materials and adhesives, metals, and welding become important to discuss adhesion.
Focus of research on adhesion gives the scientific explanation of the adhesion phenomenon from physical or technical perspective. The industrial interests are more focused on adhesive, primer, or glues, their production, composition, and processing.
This work discusses the chemical processes that play a role in the adhesion phenomenon, however, now from the polymer point of view. It also considers chemical and structural changes in the involved materials of composites contacting at the interface. To join different materials well, adherent, the interface, and their structure become significant.
The book focuses and intends also the intensified consideration of chemical processes along and across interfaces. It emphasizes the role of permanent, durable, and strong chemical bonds at the interface. Ultimately, it recommends the systematic synthesis of covalent bonds across the interfaces, which are favored compared to weak and sensitive physical interactions.
By means of several examples, the general molecular structure of an efficient construction of interface is introduced. The aims of new different interface elements are explained. The final goal of interface structuring is the mechanically and hydrolytically non-separable composite. By means of aluminum–polyolefin composites with extraordinary adhesion and durability, such behavior is technically realized.
The author was engaged in this field from the beginning of his scientific career as PhD student in 1972. In particular, he thanks his former supervisor Dr. habil. Joachim Gähde, who was and is a great guide in science. Several former colleagues are involved in the author's work and have contributed to many published scientific papers, including Mrs Dr Ingrid Loeschcke, who had measured materials with X-ray photoelectron spectroscopy since 1973. To bear in mind that the author, who was incorrect in political affairs and did not follow the official directives in former German Democratic Republic, nevertheless has got much support from the institute's director Prof. Horst Frommelt and the chief of the Central Institute of Organic Chemistry in the former Academy of Science in Berlin, Prof. Hans Schick. Among the technical staff, Mrs Gundula Hidde has to be emphasized. She has measured and prepared nearly all samples with excellent correctness, contributing own ideas, and produced results with high standard. Without her assistance, this book could not been written.
I also have to thank my coworkers and colleagues. Dr Gerhard Kühn and Prof. Andreas Schönhals were my deputy chiefs in the Federal Institute of Materials Research and Testing in Berlin, since 1995, working on a high scientific level and were coauthors in many papers. The author of the book can only present some examples of excellent contributing coauthors, including Dr Harald Wittrich, Dr Wolfgang Unger, Prof. Heinz Sturm, Prof. Christian Jäger, Dr Asmus Meyer-Plath, Dr Sascha Wettmarshausen, Dr Rolf-Dieter Schulze, Dr Steffen Weidner, Dr Günter Schulz, Dr Jana Falkenhagen, Dr Ralph-Peter Krüger, Dr Simone Krüger, my daughter, Prof. Alaa Fahmy Mohamed, Dr Konstantyn Grytzenko, and so on.
There were also important input and support from anonymous scientists, such as from DuPont, Dow Chemical, BASF, Bayer, Ahlbrandt, Fluor Technik System and others. I give special thanks to Dr Pierre Lutgen for introducing me to the new world of science around DuPont in 1989. Much support was given by Dr Wolfgang Saur from Switzerland. Close fruitful and helpful contacts should also be mentioned to Prof. Christian Oehr, Dr Kashmiri Mittal, Prof. Jose Miguel Martin Martinez, Prof. Voytek Gutowski, Prof. Michel Wertheimer, Prof. Wulff Possart, Prof. Farzaneh Arefi-Khonsari, Prof. Gerhard Blasek, Prof. Claus-Peter Klages, Prof. Hideyuki Sotobayashi, Prof. Eldar Bahadur Zeynalov, Prof. Norihiro Inagaki, Prof. Jürgen Meichsner, Prof. Hans-Ulrich Poll, Dr sc. nat. Helmut Drost, Prof. Hans-Jürgen Tiller, president Prof. Manfred Hennecke, Prof. Andreas Hampe and others.
Without the contributions of coworkers, colleagues, and partners this book would not exist.
I thank also the publisher Wiley-VCH and its coworkers for having given me the opportunity to illuminate the “old” adhesion from the “chemical” point of view.
Last but not the least, I have to thank my wife, Dr Waltraud Friedrich, my daughters, and all my grandchildren for understanding that I have blocked little time for my great family. And, not to forget, I have to thank my parents, in particular my father, who introduced me to natural science, who was a generally educated scientist of chemistry, food chemistry, pharmacy, and medicine.
Berlin, August 2017
Chapter 1High-Performance Metal–Polymer Composites: Chemical Bonding, Adhesion, and Interface Design
1.1 Introduction
Most published books on adhesion are focused on the discussion of reversible physical interactions along the interface of polymers and coatings. Such adhesion can be described fairly well in terms of thermodynamics. In contrast, mechanical anchoring due to rough surfaces and mechanical interhooking is determined by mechanics. Chemical interactions or chemisorptions may be caused by hydrogen bonds produced by polar groups containing a covalently bonded H atom and an atom with a free pair of electrons. Oxygen and nitrogen groups are often involved in hydrogen bonds. Chemical bonds are often in focus of speculation but seldom clearly detected. Only in a few cases, chemical bonds between polymers and coatings were consciously prepared. This book will present some examples for systematic introduction of covalent bonds between polymers and coatings along the interface. The efficiency to form chemical bonds instead of physical interactions is high because of higher binding energies; thus, a strong adhesion promotion by dense chemical bonds is expected.
Sticking two solids together using vegetable resins is one of the oldest examples for adhesion in the history of mankind, at least in the period as Homo sapiens were arriving in Europe (about 40 000 years ago) [1]. It is also found that the foregoing species, the Homo neanderthalensis (180 000–30 000 years ago), may also be Homo erectus (1 000 000–180 000 years), invented glue as essential to produce their most formidable hunting weapon using bitumen or asphalt and heated it for better gluing. The finding in 1963 in Königsaue is at least 40 000 years old, that in Campitello is 200 000 years old, and that in Inden-Altdorf about 128 000–115 000 years old (Figure 1.1) [2–4].
Figure 1.1 Model of a more than 10 000 years old spearhead made of flint stone and fixed by bitumen and bowstring.
The base of this development of weapons was the found in the lances in Schöningen (Germany), more than 300 000 years old, hardened at the top by fire [5].
Now, let us consider the basics of adhesion in a composite or laminate. Two different solids with almost different chemical compositions, structures, reactivities, surface properties, and mechanical strengths collide in one atomic layer, and the transition from one to another solid takes place in one atomic layer. This transition from solid A to solid B is called interface (Figure 1.2).
Figure 1.2 Example for the principal structure of a polymer–metal laminate.
This atomic gap between solid A and solid B has to be bridged by physical, chemical, or mechanical forces to achieve proper adhesion. Often, a clear transition from solid A to solid B in one atomic layer is not found. Adjacent to the interface, polymers often show a new molecular orientation caused by the interaction with the coating material. Such an example is the “trans-crystalline” orientation of polymers in coatings caused by the texturing action of the metal substrate [6]. This behavior is similar to that of the well-known epitaxy. Thus, the interface region of a composite or laminate consists of the ultimate interface, transition zones in the two neighboring solids (interphases), and the intact original morphology of the two solids (bulk) (Figure 1.3).
Figure 1.3 Examples of the schematic design of metal–polymer interfaces with interphases and the original bulk materials.
Often, contaminations and additives accumulated at the polymer surface, metal oxide skin, and aged and/or oxidized polymer species at the surface/interface hinder the direct interaction of the two solids in a laminate.
Another problem is the contact area between two solids. The greater the contact area, the higher is the concentration of interactions and the stronger is the adhesion. Thus, roughness can increase the contact area, when one solid can wet and, therefore, adapt the rough surface topography of the other solid (Figure 1.4). Such adaptation occurs when the coating is evaporated, molded, or is a dip- or spin-coating film.
Figure 1.4 Problems with minimum contact area in case of laminating rough surfaces.
Now, let us have a look at the binding energies of interactions between two solid phases. The energy of interactions grows moderately from physical interactions to hydrogen bonds. Nevertheless, such van der Waals interactions and hydrogen bonds have low binding energies in comparison to those of chemical bonds. However, such low binding energies can be compensated partially by a high concentration of such interactions, that is, the addition of such many very weak interactions results in a great sum, also in strong adhesion in comparison to rare strong chemical bonds (Figure 1.5). The conclusion is that a great number of strong chemical bonds are needed to achieve a maximum in adhesion.
Figure 1.5 Schematic comparison of the strength of interactions (bond dissociation energy) and the measured total adhesion between a polymer and a coating, depending on the type of interaction and the density of these interactions along the polymer–coating interface.
It will be shown in the following chapters that a high density in chemical bonds across the interface can be realized. However, in such a case, two new difficulties appear. First, the chemical bonding across the interface is equal to or even stronger than the bonds in the polymer represented by the cohesive strength of the polymer in laminate materials; thus, the failure at mechanical loading shifts from the interface to the polymer bulk, termed as cohesive failure (Figure 1.5).
And, secondly, the chemical bonding makes the interface inflexible, and at mechanical loading, adjacent material layers fail (near-interface failing). To avoid such failing by stiffened near-interface layers, flexibilization of the interface is needed as realized by long-chain aliphatic spacers or viscoelastic polymer adhesion-promoting layers (Figure 1.6).
Figure 1.6 Locus of failure in metal–polymer laminates.
Chemical bonds across the interface between two polymers are most often covalent bonds, such as C−C, C−O−C, CO−O, CNH2−O, etc. bonds. Their formation is possible by chemical reactions of different functional groups of the two laminated polymers, by graft reactions or by use of peroxide for linking. The bond strengths of such covalent bonds are in the range of 350–400 kJ mol−1 or more, greater than the physical interactions by a factor of at least 100.
If the polymers are compatible in a thermodynamic sense, that is, have similar structure or equal chain segments, interdiffusion may also occur [7]. The compatible chain segments of polymer A and polymer B interpenetrate in a small interface layer. Solvent-induced swelling or heating supports interdiffusion. In such a case, the relating polymers A1 and A2 can coil in the interdiffusion zone as the macromolecules of a homopolymer. This molecular entanglement provides adhesion strength along the (former) interface similar to the cohesive strengths of polymers A1 and A2.
Functional groups on polymer surfaces or introduced on polyolefin surfaces can react with metal atoms or with its hydroxy groups at the surface of the oxide coating of the metal to chemical bonds (Figure 1.7).
Figure 1.7 Variants of covalent bonds across the interface between polymer and coating.
The aim of this book is to overcome simple physical interactions in composites and to establish, in the adhesion community, new polymer pretreatment processes, new interface design by more chemical processing.
The higher binding energy, at least one order of magnitude, achieved by chemical (covalent) bonds compared to physical interactions between polymer and coating molecules should increase the adhesion in laminates and composites considerably. Thus, if covalent bonds are more densely distributed across the interface, a significantly higher adhesion in laminates or composites should be achieved. It can be compared with the cross-linking of polyolefins by peroxides producing a harder but more brittle polymer bulk with all its advantages and disadvantages.
Now, two solids are strongly bonded together by covalent bonding; however, the interface is simultaneously made more stiff and inflexible. Thus, the mechanical loading is redistributed from the interface in the (often) weaker solid, and the failure is relocated to the vicinity of interface as determined by interfacial thermodynamics and formation of internal stress [8]. Strong interfacial covalent bonds weaken the adjacent covalent bonds in the solid. For example, in polymers, the failure propagation changes from the interface to such weaker near-interface layer, which is associated with a considerably lower adhesion. It was shown that peeling is always assisted by internal stress, here, caused by strong covalent bonds along the interface and by different thermal expansion, whether tensile or compressive, because the stored elastic energy released by mechanical separation of the joint can drive the crack through the weakened near-interface layer of the polymer [9]. Such simple dislocation of failure to near-interface weakened polymer layers is not the optimum solution of the adhesion problem, but it is a significant advantage compared to a poor interfacial failure.
