Thermodynamic Work Of Adhesion Research Articles

Role of Chain Architecture in the Adhesion of Block Copolymers

We have used Johnson-Kendall-Roberts contact mechanics methodology to study the self-adhesion of block copolymers. The block copolymers studied contain building blocks that have different physical states as well as different dynamic characteristics. The polymers chosen are poly(styrene)-poly(isoprene) (PS-PI), poly(styrene)-poly(ethylethylene) (PS-PEE) diblocks and PS-PI-PS, PS-PEE-PS triblocks that have the tail and looped chain surface architectures, respectively. Our results for unloading of the diblock/triblock mixtures, where two surfaces are separated by decreasing the amount of compression, show a maximum in adhesion vs mixture composition when the contact time is short. For contact times longer than 30 min, only unloading of triblock-dominated mixtures shows considerable buildup of adhesion with time. The surface energies obtained from loading experiments do not show any detectable influence of chain architecture on the thermodynamic work of adhesion.

Thermodynamic work of adhesion and peel adhesion energy of dimethoxysilyl-terminated polypropylene oxide/epoxy resin system jointed with polymeric substrates

The characterization of the interfacial surface of a dimethoxysilyl-terminated polypropylene oxide (DMSi–PPO)/diglycidylether of bisphenol A (DGEBA) system, which has the phase structure of DGEBA particles in a DMSi–PPO matrix, was investigated by using model joints with polymeric substrates. The surface free energy (γ) of the DMSi–PPO/DGEBA system had varied with the γ of each substrate. When the system contacted to low surface free energy materials such as Teflon, polypropylene, and polyethylene, the γ of the system showed about 14.3–31.6 mJ/m2; on the other hand, when the system contacted to high surface free energy substrates such as polyethylene–telephthalate and polyimide, the γ of the system showed 50.4 and 64.6 mJ/m2, respectively, because the concentration of the DGEBA as a polar component in the system changed around these interfaces. In the low surface energy substrates used, the actual peel adhesion energy value was in good agreement with the thermodynamic work of adhesion (Wa) determined independently. However, in the high surface energy materials used, the peel adhesion energies were 103–104 times larger than Wa because the energy was dissipated viscoelastically at the jointed points. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1920–1930, 2001

Equilibrium and adhesion of Nb/sapphire: The effect of oxygen partial pressure

We derive a formula, useful for first-principles calculations, which relates the free energy of an oxide/metal interface to the free energies of surfaces and the work of separation of the interface. We distinguish the latter mechanical quantity from the thermodynamic work of adhesion, and we describe explicitly how both may be calculated. Our formulas for interfacial and surface energies are cast in terms of quantities which can be calculated or looked up in tables, and include as additional parameters the ambient temperature and partial pressure of oxygen ${P}_{{\mathrm{O}}_{2}}.$ From total-energy calculations for the $\mathrm{Nb}(111)/\ensuremath{\alpha}\ensuremath{-}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ (0001) interface, free Nb and ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ surfaces, we obtain firstly numerical estimates of the works of separation, which are independent of ${P}_{{\mathrm{O}}_{2}}.$ We then obtain surface energies, interfacial energies, and the equilibrium work of adhesion as a function of ${P}_{{\mathrm{O}}_{2}}.$

Rupture of Hydrophobic Microcontacts in Water: Correlation of Pull-Off Force with AFM Tip Radius

The purpose of this work was to investigate the functional dependence of the adhesion (pull-off) force on probe radius in AFM force measurements. Pull-off forces for hydrophobic AFM probes in contact with hydrophobic substrates were measured in water. Chemical functionalization of probes and substrates was accomplished by self-assembly of octadecanethiol on Au. Average pull-off forces for 10 SAM/Au-coated probes were measured and found to depend linearly on the probe tip radii, which were determined by scanning electron microscopy (SEM). Using the JKR contact mechanics model, the thermodynamic work of adhesion (Wad) determined from the pull-off forces was 110 ± 10 mJ/m2 which compares favorably with estimates of Wad based on reported interfacial energies. These results verify the linear dependence of the adhesive force on probe radius, as expected from contact mechanics models. They also verify that SEM can provide self-consistent and reproducible estimates of the probe radius.

Adhesion: Comparison Between Physico-chemical Expected and Measured Adhesion of Oxygen-plasma-treated Carbon Fibers and Polycarbonate

The adhesive interaction between oxygen-plasma-treated, polyacrylonitrile-based, high-tensile-strength carbon fibers and a polycarbonate matrix has been studied. Several models have been used to predict the impact of the plasma treatment process on the strength of adhesion between both jointing partners. These approaches have been the thermodynamic work of adhesion which was calculated from the solid surface tensions, based on the results of contact angle measurements versus test liquids, the contact angle which was directly obtained via polycarbonate melt droplets on single carbon fibers and the zeta (ς)-potential data provided by streaming potential measurements. The results have been compared with the interfacial shear strength determined from the single-fiber fragmentation test. Additionally, the single-fiber tensile strength of the oxygen-plasma-treated carbon fibers was determined. We confirmed that any physico-chemical method on its own fails to describe exactly the measured adhesion. However, for the investigated system, the conscientious interpretation of the data obtained from wetting measurements, in conjunction with the thermodynamic approach, is sufficient to predict the success of a modification technique which has been applied to one component in order to improve adhesion.

Adhesion of Triblock Copolymer-Based Thermoreversible Gels and Pressure Sensitive Adhesives

ABSTRACTTriblock copolymers with poly (methyl methacrylate) (PMMA) end blocks and a poly (n-butyl acrylate) (PnBA) midblock have been synthesized as model pressure sensitive adhesives and thermoreversible gels. These materials dissolve in a variety of alcohols at temperatures above 60 °C to form freely flowing liquids. At lower temperatures the PMMA end-blocks associate so that the solutions form ideally elastic solids. In our case the solvent is 2-ethylhexanol, polymer volume fractions vary from 0.05 to 0.3, and the elastic moduli are close to 10,000 Pa. We have conducted three types of experiments to elucidate the origins of adhesion and bulk mechanical properties of these materials: 1) Weakly adhering gels: The adhesive properties of the gels are dominated by the solvent. Very little adhesion hysteresis is observed in this case, although we do observe hysteresis associated with the frictional response of the layers. 2) Strongly adhering gels. By heating the gels in contact with a PMMA surface, it is possible to bond the gels to the surface. Development of adhesion as the PMMA blocks penetrate into the PMMA substrate can be probed in this case. The cohesive strengths of the gels are found to be substantially greater than their elastic moduli, so that these materials can be reversibly extended to very high strains. These properties have enabled us to probe the origins of elastic shape instabilities that play a very important role in the behavior of thin adhesive layers. 3) Dried gels – model pressure sensitive adhesives. By removing the solvent at low temperatures, the underlying structure of the gel is preserved, giving a thin elastic layer with excellent performance as a pressure sensitive adhesive. Resistance to adhesive failure, expressed as a velocity-dependent fracture energy, greatly exceeds the thermodynamic work of adhesion. This energy is further magnified by ‘bulk’ energy dissipation when the stress applied to the adhesive layer exceeds its yield stress.

Study on surface and mechanical fiber characteristics and their effect on the adhesion properties to a polycarbonate matrix tuned by anodic carbon fiber oxidation

Polyacrylonitrile (PAN) based high strength carbon fibers were anodically oxidized using the galvanostatic mode in alkaline electrolyte solutions to influence the chemical surface composition. The change of chemical and physical properties was investigated using scanning electron microscopy (SEM), photoelectron spectroscopy (XPS), energy dispersive X-ray analysis (EDX) and contact angle as well as zeta (ζ)-potential measurements. An initially improved wettability for polar liquids, particularly water, was observed for oxidized carbon fibers. This result was confirmed by ζ-potential measurements. The chemical state of the oxygen containing surface groups changes during anodic oxidation in K2CO3/KOH due to further oxidation of C–OH and CO groups to COOH groups. Therefore the surface acidity increases, which leads to a shift of the isoelectric point to lower pH values and increases the negative ζplateau value. The ζ–pH as well as the ζ–concentration dependence show the same tendency. During anodic oxidation of carbon fibers in KNO3/KOH electrolyte solution beside ‘normal’ (like C–OH, CO and COOH) surface oxides also carboxylate groups (COO−K+) were formed at the fiber surface in contrast to an oxidation in K2CO3/KOH which introduces ‘normal' surface oxides. No influence could be observed of such an anodic oxidation on the single fiber tensile strength. Contact angle measurements of polycarbonate melt droplets onto single carbon fibers show no dependence of the surface composition. The interfacial shear strength, measured using the microdroplet pull-off test were compared with the thermodynamic work of adhesion. The calculated as well as the measured adhesion show the same absence of any influence of fiber treatment.

Quantitative adhesion measures of multilayer films: Part II. Indentation of W/Cu, W/W, Cr/W

Sputtered copper and tungsten thin films both with and without tungsten and chromium superlayers were tested by using nanoindentation probing to initiate and drive delamination. The adhesion energies of the films were calculated from the induced delaminations using the analysis presented in “Quantitative adhesion measures of multilayer films: Part I. Indentation mechanics.” Copper films ranging in thickness from 150 to 1500 nm in the as-sputtered condition had measured adhesion energies ranging from 0.2 to 2 J/m2, commensurate with the thermodynamic work of adhesion. Tungsten films ranging in thickness from 500 to 1000 nm in the as-sputtered condition had measured adhesion energies ranging from 5 to 15 J/m2. The superlayer was shown to induce radial cracking when under residual tension, resulting in underestimation of the adhesion energy when the film was well adhered. Under conditions of weak adherence or residual compression, the superlayer provided an excellent means to induce a delamination and allowed an accurate and reasonably precise quantitative measure of thin film adhesion.

Fracture behavior of isotropically conductive adhesives

The fracture behavior of several commercial, silver-filled epoxies was studied using a combination of fracture mechanics, surface science, and microscopy. Three-point bend tests revealed that the bulk fracture toughness of the silver-filled epoxies fell within a narrow range 1.1-1.3 MPa-m/sup 0.5/. Both electron and optical microscopy studies indicated that crack path deflection due to the silver-particles was the primary micromechanical deformation mechanism. Surprisingly, the interfacial fracture energies between the epoxies and a copper surface ranged from 50 to 900 J/m/sup 2/. Contact angle measurements on the cured epoxies indicated that some epoxy surfaces are more active than others. However, the correlation between thermodynamic work of adhesion and fracture energy is rather weak and suggests only a modest trend. In summary, although the use of contact angles/surface energies to predict adhesion is promising, much more effort is required to make it a reliable screening tool. Fortunately, the use of interfacial fracture mechanics can detect differences in adhesive strength, and should allow packaging engineers to select die attach adhesives with improved adhesion.

Influence of a monofunctional acid-base environment on peel mechanics

There is a strong practical interest in being able to predict the strength of an adhesive bond with a non-destructive technique. The general fracture mechanics (GFM) theory, which assumes a linear relationship between the work of separation and the thermodynamic work of adhesion, allows for the possibility of such a correlation. In this study, a plasticized poly(vinyl butyral) (PVB) and quartz adhesive-adherend system is used to investigate the influence of the work of adhesion on the work of separation in a modified 180° peel test. The work of adhesion may be varied by separately introducing various Lewis acids and bases as functional groups in the adhesive or adherend surfaces, or into the medium in which fracture is carried out. Wetting and surface tension measurements, obtained via the Wilhelmy technique, are used to determine surface energies and acid-base properties. The strong acid functionality of quartz suggests that a basic environment should yield a lower work of adhesion value than an acidic environment. Applying GFM theory allows this prediction to be extended to the work of separation. The measured adhesion strength results obtained agreed qualitatively with this prediction.

Electron beam-initiated surface modification of elastomers

Ethylene-propylene diene monomer (EPDM) containing dicyclopentadiene (DCPD) and ethylidene norbornene (ENB) as the termonomers, styrene-butadiene rubber (SBR), and acrylonitrile-butadiene rubber (NBR) have been surface-modified by 10% methyl ethyl ketone (MEK) solutions of trimethylol propane triacrylate (TMPTA) at an irradiation dose of 100 kGy. The irradiation dose and TMPTA concentration were optimized using samples treated with 2, 5, 10, 20, and 50% TMPTA and 50, 100, 200, and 500 kGy doses. Two per cent solutions of acrylate rubber having diene, chloro, and epoxy groups at the reactive sites and tripropyleneglycol diacrylate (TPGDA) and tetramethylol methane tetracrylate (TMMT) were also employed as the surface modifiers. The level and nature of the vulcanization system were varied. The modified rubbers were characterized by attenuated total reflection infrared (ATR-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and contact angle measurements. IR and XPS studies confirmed the generation of polar groups such as C=O and -C-O-C on the surfaces. The contact angles and the surface energy change with the nature of the modifiers, rubbers, diene monomers, the crosslinking system and the level of the curing agent. The total surface energy and the thermodynamic work of adhesion of the different systems have been correlated with the amount and the nature of the polar groups generated.

An Investigation of the Cling of Thin Polymeric Films

The results of an investigation into the mechanisms and debonding energy associated with the cling between thin polymeric films and various substrates are presented in this paper. The thermodynamic work of adhesion as well as electrostatic attraction apparently play significant roles in the cling of a film to a substrate. Peel tests are conducted and strain energy release rates are determined which show different debonding energies for the various film-substrate combinations.

Adhesion issued in epoxy-based chip attach adhesives

This paper focuses on the effects of surface chemistry and processing conditions on the adhesive strength of epoxy-based chip attach adhesives on typical leadframe substrates. The surface chemistry is characterized using a three liquid probe method, which quantifies the polar and apolar contributions to the thermodynamic work of adhesion. Practical adhesion is quantified using interfacial fracture mechanics. The effect of processing conditions on adhesion is summarized using "bondability diagrams", which are modeled after moldability diagrams used in reaction injection molding processes. Preliminary bondability diagrams are given for several silver-filled, epoxy-based chip attach adhesives. These diagrams are constructed from a fundamental understanding of cure kinetics, degradation mechanisms, void phenomena, and contraction during cure. Verification of such diagrams is in progress and the agreement thus far is very satisfactory.

The role of adhesion in the mechanical properties of filled polymer composites

Filled polymer composites have been prepared in which the energetics of the filler surfaces was systematically varied in order to investigate the dependence of the mechanical properties of the composite on the interfacial strength as predicted by the thermodynamic work of adhesion at the filler-matrix interface. A high-purity silica filler was used, treated with three different organofunctional silane coupling agents (two alkylsilanes and an aminosilane) to varying degrees from zero to complete coverage. The surface energetics of the modified fillers was characterized using both inverse gas chromatography (IGC) and dynamic contact angle analysis (DCA). While the surface energy assessments from IGC were higher than those obtained with wetting measurements, as expected, the trends with fractional coverage of silane were the same for each method, and were used to evaluate the thermodynamic work of adhesion. Highly filled polymer composites were prepared by dispersing the variously treated silica fillers into the amorphous thermoplastic matrix polymers: poly(methyl methacrylate) and poly(vinyl butyral). Specimens of the composites were tested mechanically to give the yield stress. The poly(methyl methacrylate) composites all failed cohesively in the matrix, unaffected by any of the filler surface treatments. The poly(vinyl butyral) composites, however, all displayed purely interfacial failure, with the yield stress strongly dependent on the type and extent of the filler surface treatment. While all three silanes were found to decrease the filler surface energy, and consequently the thermodynamic work of adhesion, with higher surface coverage, corresponding decreases in the yield stress were found only for the alkylsilanes. For the aminosilane, the measured yield stress was found to increase with surface coverage and therefore to decrease with the work of adhesion. The difference in behavior between the two types of coupling agent is explained in terms of acid-base effects.

Influence of the concentration of trimethylol propane triacrylate on the electron beam-induced surface modification of EPDM rubber

EPDM rubber has been surface- and bulk-modified with varying concentrations of trimethylol propane triacrylate (TMPTA) in the presence of an electron beam at a constant irradiation dose of 100 kGy. The surface properties of the modified rubber have been characterized by contact angle measurements, surface energy and thermodynamic work of adhesion, infrared spectroscopy (IR), and X-ray photoelectron spectroscopy. The surface energy of EPDM increased from 44.8 to 53.8 mJ/m2 on modification with 10% TMPTA, due to the increased contribution of γs (-) and γs AB. With increasing TMPTA concentration, γs and γs AB showed an optimum value at 10% concentration. The IR studies indicated increased peak absorbances at 1730, 1260, 1120 and 1019 cm-1 and hence increased -C(=O)- and C-O-C concentrations. These results are further supported by XPS data.

The Rule of Interfacial Equilibrium

The detailed analysis of the well-known relation γ12 = γ1 −γ2 has been done (γ12, γ1 and γ2 are interfacial and surface tensions of phases 1 and 2). The analysis of equilibrium of a liquid drop at the interface with another liquid allowed us to prove that this relation should be modified by including in it the value γ2* which is considered as the result of all interactions at the interface and is accepted depending on γ1. The value γ2* represents the surface tension of a phase in the ternary point solid-liquid-gas, i.e. in the zone of interfacial nonuniformity. The modified form of the relation, called the rule of interfacial equilibrium, allows us to show that thermodynamic work of adhesion is equal to the cohesion energy of the interphase formed by phase 2.

Adhesion of ceramic coatings sprayed onto graphite

In order to expand the fields of application and to improve the performance of graphite (Cg), it is necessary to reduce its permeability towards oxygen and to limit its reactivity and especially its oxidation. It is therefore essential to protect it from the environment through the use of ceramic coatings. Adhesion between ceramic coatings and graphite is controlled by the mechanical stresses in the coatings and the thermodynamic work of adhesion. Different metal-graphite systems were examined and it was shown that the adhesion depended particularly on the thermal expansion coefficient mismatch between the two materials and on the stability of the metal carbide. Thus, the role of the addition on the graphite surface of elements such as Cr, Mo, Al, Si, and O in the adhesion of metals or ceramics to graphite has been identified.

Field Investigation of Corrosion-Protection Performance of Bridge Decks Constructed with Epoxy-Coated Reinforcing Steel in Virginia

The corrosion-protection performance of epoxy-coated reinforcing steel (ECR) in three 17-year-old bridge decks in Virginia was assessed. The decks had an upper mat of ECR and a lower mat of bare steel. Surface cracking in the right traffic lane was visually surveyed and 12 cores randomly located in the lowest 12th percentile cover depth in each deck were drilled. The concrete core and the extract ECR were visually inspected. In the concrete moisture content, absorption, percent saturation, carbonation depth, and the effective chloride diffusion constant were measured. In the ECR physical damage, coating thickness, adhesion loss and corrosion at damaged sites, and undercoating corrosion at adhesion test sites were measured. The chloride content of the concrete and carbonation of the ECR trace were determined. Significant coating adhesion loss occurred before the chloride arrived at the bar depth. The debonding of the epoxy is wet debonding, which is predicted by the negative thermodynamic work of adhesion. ECR will extend the service life of only 5 percent of the bridge decks in Virginia. Thus, its use is not cost-effective. Additional decks should be evaluated to confirm the results of this and other studies.

Quantitative Measurement of the Effect of Annealing on the Adhesion of Thin Copper Films Using Superlayers

ABSTRACTNanoindentation at loads from 100 to 750 mN was used to measure the interfacial adhesion energy of sputtered copper thin-films, for which the release of elastic strain energy drives delamination. The as-sputtered films were of four thicknesses, nominally 225 nm, 400 rn, 600 nm and 1000 nm, and were deposited onto Si/SiO2 wafers; one wafer of each two-wafer run was annealed at 600°C for 2 hrs in a nitrogen atmosphere. Subsequently, a nominally 700 nm thick sputtered superlayer of tungsten was deposited over all wafers. The tungsten overlayer was used to promote delamination at the copper-SiO2 interface. The energies for fracture ranged from 1 to 80 J/m2, increasing with increased indentation depth, and a trend of higher adhesion energy for annealed films. The large values of adhesion energy with respect to the thermodynamic work of adhesion are attributed primarily to plasticity and/or void nucleation within the copper film, which appears to be strongly influenced by the constraint of an overlayer.

Analytical Currents: Adhesive forces and the thermodynamic work of adhesion

Analytical Currents: Adhesive forces and the thermodynamic work of adhesion