Thermodynamic Work Of Adhesion Research Articles

The science and mechanics of adhesion: An industrial view.

Undoubtedly, adhesion is one of the broadest terms in science and technology used to describe several bulk and interface related phenomena. While the thermodynamic work of adhesion is determined by contacting surfaces and their intrinsic surface energetics, it is important to understand how adhesive properties of materials are additionally governed and amplified by their dissipative rate processes in the bulk or near the interface as they go through large strains and deformation. Systematic review of the literature showed that the involved interfacial mechanisms were grouped into several categories ranging from micromechanical interlocking to molecular interdiffusion of surface constituents, a characteristic of most polymeric systems. This paper addressed the static and dynamic contributions to the adhesion energy and discussed its relation to microstructure and surface architecture in pressure sensitive and fracture in structural adhesives. While the focus was on industrial view of adhesion, parallels in adhesive dentistry were given where connections between adhesion, boundary geometry/compliance, shrinkage stress, material model, joint design, retention, and interfacial curing were made. Adhesion science and mechanics are complex multi-disciplinary fields involving surfaces, substrates, and loading system involving a broad range of mechanisms applicable to dentistry.

Computationally developed acrylated epoxidized methyl ester based pressure-sensitive adhesives

Healthcare sector is increasingly demanding pressure-sensitive adhesives (PSAs) to be bio-sourced and biodegradable to replace petro-based PSAs, which are neither renewable nor sustainable. This investigation employs MD simulations to develop bio-based PSAs by replacing low Tg monomers such as 2-ethylhexyl acrylate partially and fully with acrylated epoxidized methyl ester (AEME). The simulation procedures used in this study were verified by comparing specific properties with published literature. The glass transition temperatures of all developed systems show insignificant differences and are well below application as desirable for developing PSA products. Also, these values closely match Fox equation estimates and are in good accord with experimental values. Surface characteristics like surface energy and interfacial adhesion with Al substrate have been predicted. A ∼79 % increment in surface energy with AEME content ensures good substrate wettability and thermodynamic work of adhesion. The interaction energy with Al substrate has been increased by ∼365 % with addition of AEME reflecting improved adhesion strength and enhanced tackiness of the PSA copolymer. AEME provides polar groups like acrylate and hydroxyl groups, which helps improve adhesion strength. The polar groups associated with AEME and increased crosslinking density enhanced mechanical properties of developed PSA systems. This computational research lays the path for developing innovative bio-based PSAs used in adhesive bandages and transdermal patches.

Nanocracks in nature and industry.

Nanocracks in nature and industry.

Adhesion-cohesion balance of prepreg tack in thermoset automated fiber placement. Part 1: Adhesion and surface wetting

The constitution of prepreg tack in automated fiber placement (AFP) is affected by a sensitive balance between adhesive interfacial bond strength and cohesive strength of the prepreg resin. In an effort to explore the role of interfacial liquid-solid interaction on the tack of commercial aerospace-grade epoxy prepreg, a surface wetting analysis was performed on AFP-related substrates. The standard test liquid combination water/diiodmethane and extracted neat epoxy resin were used for contact angle measurement employing the sessile drop method and the Owens-Wendt-Rabel-Kaelble (OWRK) model. Additional rheological and topographical analyses were carried out to account for viscous resin flow on surfaces of different roughness. The results from the material characterization are discussed against the background of tack measurement by probe tack testing utilizing a rheometer. Significant differences between the investigated surfaces in terms of both the maximum tack level and the onset temperatures of adhesion were found as a function of test parameters relevant for contact formation. General agreement with the experimental tack results was observed employing a topographically extended version of the Dahlquist criterion. For each substrate, a temperature-dependent critical storage modulus could be determined that conforms to the onset temperature of tackiness. Contact angle measurements revealed a correlation between the thermodynamic work of adhesion and maximum tack and, moreover, the tack onset in the adhesive regime when additionally incorporating surface topography. Matching ratios of polar and dispersive surface free energy and surface tension components were found to favor the molecular interaction at the interface between prepreg resin and substrate.

Investigations on the Adhesive Contact Behaviors between a Viscoelastic Stamp and a Transferred Element in Microtransfer Printing

Microtransfer printing is a sophisticated technique for the heterogeneous integration of separately fabricated micro/nano-elements into functional systems by virtue of an elastomeric stamp. One important factor influencing the capability of this technique depends on the adhesion between the viscoelastic stamp and the transferred element. To provide theoretical guidance for the control of adhesion in the transfer printing process, a finite element model for the viscoelastic adhesive contact between a polydimethylsiloxane (PDMS) stamp and a spherical transferred element was established, in which the adhesive interaction was modeled by the Lennard-Jones surface force law. Effects of the unloading velocity, preload, and thermodynamic work of adhesion on the adhesion strength, characterized by the pull-off force, were examined for a loading-dwelling-unloading history. Simulation results showed that the unloading path deviated from the loading path due to the viscoelastic property of the PDMS stamp. The pull-off force increased with the unloading velocity, and the increasing ratio was large at first and then became low. Furthermore, the influence of the preload on increasing the pull-off force was more significant under larger unloading velocity than that under smaller unloading velocity. In addition, the pull-off force increased remarkably with the thermodynamic work of adhesion at a fixed maximum approach.

Epoxidised natural rubber as adhesion promoter in natural rubber based compounds

Natural rubber (NR) is the most preferred raw material for both tyre and non-tyre applications, which require good adhesion performance. It has a naturally sticky character to provide green tack. However, it has limitations to improve adhesion further due to its relatively low reactivity. In this study, epoxidised natural rubber (ENR) has been evaluated as an adhesion promoter for NR based model tyre compounds. ENR based adhesive compounds were prepared by blending NR with ENR in various ratios. Rheological, physical, and mechanical properties of NR/ENR compounds were studied. Sufficient cure rate and cure extent were achieved for elevated curing temperatures. The adhesion performance was measured with the T-peel adhesion test. In addition, thermodynamic work of adhesion (TWA) was calculated by using the surface free energy components of the compounds. The correlation between adhesion force and TWA was evaluated together with vulcanisation kinetic parameters in order to understand the adhesion mechanism. ENR incorporation was found to improve adhesion between selected tyre compounds, remarkably.

Transition in the Acid–Base Component of Surface Free Energy of Ice upon the Premelting of Its Second Molecular Bilayer

Molecular disordering of the ice surface occurs below the bulk melting temperature of 273 K, termed surface premelting. The top-most molecular layer begins gradually premelting at 200 K, and has been linked to its low coefficient of friction through an increase in molecular mobility. The second molecular bilayer premelts around 257 K, but no study has linked this transition to a change in any macroscopic phenomena. Here, we show that the thermodynamic work of adhesion between polydimethylsiloxane (PDMS) and ice changes abruptly at 257.0 $\pm$ 0.1 K. Surface-sensitive sum frequency generation spectroscopy shows that there are no molecular level changes at the PDMS surface or the ice-PDMS interface near the transition in adhesion, indicating that the transition arises from changes of the ice surface. Using existing contact angle data in the literature, we show that this transition is due to a decrease in the acid-base component of the surface free energy of ice by 17 $\pm$ 2 mJ/m$^2$ at 257.0 $\pm$ 0.1 K. The change in surface energy provides a possible explanation for a variety of unexplained phenomena seen across the literature including ice adhesion, friction, and the morphology of snowflakes.

Movement initiation of millimeter particles on a rotating rough surface: The role of adhesion

We performed experiments to determine the critical moment for movement initiation of a millimeter bead on a rotating rough surface. The corresponding critical angular velocities were measured for glass and stainless-steel ball bearings over two different rough surfaces with glued glass beads. A basic theoretical analysis was developed to explain the observed results. Although the expectation of a simple approach with the presence of the obstacles offered by a rough surface could be sufficient to describe the problem, we prove here that the sole consideration of these obstacles, and even friction, are insufficient to explain the results in the range of a few-millimeter glass particles. Where the thermodynamic work of adhesion between surfaces is significant, the adhesion forces must be considered in the force balance for particle detachment. This effect is a determinant for describing theoretically and numerically the dynamics of millimeter particles.

The Effect of Solvent Nature on the Adhesive Properties of Binary Styrene–n-Butyl Acrylate Copolymer Films Formed by Casting on Solid Substrates

A methodological approach has been proposed for studying the effect of solvent nature on the adhesive properties of films of binary copolymers with different chain microstructures. The approach is based on a complex analysis of the thermodynamic work of adhesion in polymer–liquid model systems and the results of mechanical tests in combination with quantum-chemical calculations. The approach has been used for the first time for copolymers of styrene and n-butyl acrylate having gradient and random distributions of comonomer units over the chains, with the copolymers being synthesized by radical polymerization with reversible chain transfer. The results obtained have shown that the adhesive properties of the binary copolymer films may be regulated by varying the chemical nature of solvents from which they are formed.

Interface Characterization Between Polyethylene/ Silica in Engineered Cementitious Composites by Molecular Dynamics Simulation.

Polyethylene is widely adopted in engineered cementitious composites to control the crack width. A clearer knowledge of the PE/concrete interfacial properties is important in developing engineered cementitious composites, which can lead to a limited crack width. Tensile failure and adhesion properties of the amorphous polyethylene/silica (PE/S) interface are investigated by molecular dynamics to interpret the PE/concrete interface. The influence of the PE chain length, the PE chain number and coupling agents applied on silica surface on the interfacial adhesion is studied. An increase of the adhesion strength of the modified silica surface by coupling agents compared with the unmodified silica is found. The failure process, density profile and potential energy evolutions of the PE/S interface are studied. The thermodynamic work of adhesion that quantifies the interfacial adhesion of the PE/S interface is evaluated. The present study helps to understand the interfacial adhesion behavior between ECC and PE, and is expected to contribute to restricting the crack width.

Improved Performance of LiFePO4 Cathode through Use of Single Lithium Ion Conducting Polymer Binders Based on Sp 3 Boron Atoms

Active materials and electrolytes are widely explored to improve the energy and power density of the batteries. However, polymer binder is the key component for the control of the electrode (composite) interfaces, which include the conductive agent/active materials interface, the conductive agent/current collector interface, and the initial active materials/electrolyte interface, facilitating electron and ion conductivity, retaining the physical structure of electrode and stable cycling improving the performance of the LIB. According to Sugita et al. [1] and the approach based on the thermodynamic work of adhesion, polar binders improve the composite electrode wettability, increasing the ionic conductivity in conventional composites; these showed highest capacity, suggesting that Li+ conduction inside the polymeric binder contributes to the enhancement of electrochemical performance. In this work, we report a serie of binders based on sp3 boron atoms and poly(ethylene glycol) PEG400 bridges and other alcohols. These binders were obtained by polymerization in-situ with lithium tetrametoxyborate (LTMB), on the LiFePO4 and SP-carbon surfaces (120°C, 12h), after homogenized in methanol by magnetic stirring. The electrodes based on polyborates as binders were processed and compared with electrodes with PVDF as binder, according to the methodology used by G. Guzman et al. [2] In addition, the effect of charge delocalization present in BO4 - groups, associated to strong electron-withdrawing substituents and their effect on the ionic conductivity in polyborates binders, was studied by electrochemical impedance spectroscopy EIS. The results suggest that increased in ionic conductivity of the SLICB relative to the PVDF, decreases the charge transfer resistance in LiFePO4 cathodes. During the process of stabilization of the cell at C-rate of C/10, the cathodes based on PVDF as binder have better capacity, however, it is easily degraded with the increase of the C-rate. In general, the use of SLICB allows improving the load capacity of LiFePO4 cathodes, mainly at high C-rate values. Besides, they allow for a lower degree of degradation, which is possible to observe in the reversibility tests at low C-rate values, see figure 1. Figure 1. Rate capabilities collected during 40 cycles for a Li-ion cell at subsequent C-rates of 0.1, 1, 10 and 0.1C. Cell contains a cathode composite comprised: LiFePO4: SP-Carbon and different polymer binders 86-07-07 %w) and electrolyte of 1 M LiPF6 in a mixture of PC:EC:DMC (1:1:1 V%). Acknowledgements This work was supported, GR thanks (project seciti/080/2017) IG thanks CONACYT (proyect 237343) for financial support. G. Guzman is grateful to CONACYT for the scholarship granted to pursue his doctoral studies. Reference s : [1] M. Sugita, H. Nakamura, M. Yoshio, IMLB-2006, No. 486. [2] G. Guzmán, J. Vazquez, G. Ramos, M. Bautista, I. González, Electrochimica Acta 247 (2017) 451–459 Figure 1

A quantitative correlation between polyethylene/graphene interfacial viscoelastic dissipation and deformation parameters: A molecular simulation study

In this study, the influence of applied tensile stretching rate and temperature on amorphous polyethylene/graphene (PE/G) and polyethylene/amino functionalized graphene (PE/aFG) interfaces were studied via molecular dynamics (MD) simulations. The simulations results indicated that the interfacial adhesion during stretching process was increased with increasing the stretching rate (V) while decreased with increasing the temperature (T). This behavior can be connected to changing the polymer chains friction arisen from inadequate reorientation time, which especially happened at the higher stretching rates and lower temperatures. Furthermore, the amino functionalization of G strengthened both the thermodynamic work of adhesion and interfacial viscoelastic energy dissipation function, φ (V, T), in the interface deformation process. In fact, relying the chains on aFG surface with stronger interfacial interactions also increased the required energy for the chains to disentangle and reorient on the surface. Moreover, the stretching rate dependency of the φ value well follows a power law at all the stretching temperatures. The effect of stretching rate and temperature on φ value was simultaneously investigated utilizing the time-temperature superposition principle. Master curves of the simulation data, φ (aTV), was obtained for PE/G and PE/aFG interfaces on which the horizontal (aT) and vertical shift factor (bT) were calculated using Williams-Landel-Ferry (WLF) equation and Bueche-Rouse theory, respectively.

Composite material based on the polyethylene terephthalate polymer and modified fly ash filler

The possibility of filling the recycled polyethylene terephthalate (rPET) with fly ash was studied to make a polymer composite material (PCM). It is shown that high adhesion between polymeric matrix and mineral filler is the key parameter to produce high performance PCM. For this purpose the acid-basic interaction as well as the thermodynamic work of adhesion between components of PCM were calculated. The technique of modifying fly ash filler with 5% concentration solution of sulfuric acid to increase acid-basic interaction has been elaborated. The resulting behavioral patterns are listed and compared to those of composites containing untreated fly ash particles.

Pull-off strength of thermoplastic fiber-reinforced composite coatings

The paper aims at presenting the results of pull-off strength tests of fiber-reinforced polymer composite coatings laminated on steel substrates. It contains the measured data on the thickness of manufactured coatings and the substrate surface’s roughness, according to the various methods of surface’s preparation with the means of abrasive blasting. The microstructure analysis of material’s cross-sections and damage analysis of samples after failure were also performed. The highest pull-off strength’s values for composite coatings were obtained for joints with the substrate modified by abrasive blasting with corundum F60 or simply degreased. To establish the compatibility of substrates with coatings the wettability of the chosen materials was tested and work of adhesion was calculated on its base. Concerning the wettability, it was found that the most preferable joints were characterized by the similar thermodynamic work of adhesion and consisted of the coating’s matrix (SBS) and the steel substrate degreased with acetone or modified with corrundum abrasive blasting and then degreased.

Strength and bonding characteristics of adhesive joints with surface-treated titanium-alloy substrates

This paper presents a study on the effect of surface treatments on the mechanical behavior of adhesively bonded titanium alloy joints. Several different treatments were selected for the preparation of Ti-6Al-4V alloy faying surfaces, and bonded joints were fabricated using surface-treated titanium alloy substrates and a film adhesive. Tensile tests were performed on single-lap specimens to evaluate the joint strength and to assess the failure mode, i.e. cohesive failure, adhesive (interfacial) failure or a mix of both. Contact angle measurements were also carried out, and the surface free energies of titanium alloys and the thermodynamic works of adhesion for the adhesive/titanium alloy interfaces were obtained. A three-dimensional finite element analysis was used to predict the strength of the specimens exhibiting cohesive failure. In addition, an expression of the relationship between the joint strength corresponding to interfacial failure and the thermodynamic work of adhesion was introduced based on the cohesive zone model (CZM) concept. It is shown that two surface treatments, Itro treatment and Laseridge, lead to cohesive failure and a significant increase in the joint strength, and the numerically predicted strength values are fairly close to the experimental values. These surface treatments are possible replacements for the traditional surface treatment processes. For degreasing, emery paper abrasion, atmospheric plasma treatment, sulfuric acid anodizing, nano adhesion technology and high-power lasershot, the specimens fail at the adhesive/substrate interface and the joint strength increases linearly with the thermodynamic work of adhesion as expected from our CZM-based expression.

Estimating work of adhesion using spherical contact between a glass lens and a PDMS block

This study proposes a method of estimating the thermodynamic work of adhesion using the spherical contact between a glass lens and a polydimethylsiloxane (PDMS) block. An equivalent stiffness of the measurement system is determined prior to the experimental measurements. Parameters such as force, contact radius, and displacement are measured during the contact processes that consist of a loading process and an unloading process. The elastic modulus of the PDMS is determined from the measured parameters. Hysteresis is observed in the contact process, showing that the process is in a non-equilibrium state. However, an equilibrium instant exists when the contact area attained a maximum value. The work of adhesion can be determined from the strain energy release rate using the parameters of the instant. The estimated work of adhesion is in the range estimated by another method. The limitation of the conventional method is also presented. The proposed method is suggested to be applicable to interface characterization in a practical adhesive contact with soft materials.

Prospects of the evaluation of adhesive interaction via chemical force microscopy

The adhesive interaction of some polymer surfaces with neutral, acidic, and basic samples formed on a cantilever specimen has been evaluated via atomic-force microscopy. The thermodynamic work of adhesion has been calculated using the Johnson–Kendall–Roberts model. The differences in the work of adhesion values obtained in the interaction of acidic, basic, and neutral samples with the same polymer surface have been found. The experimentally obtained values of work of adhesion have been carefully compared with theoretically calculated ones, which allowed the conclusion to be drawn that the area of research under consideration is promising.

Topochemistry of cellulose nanofibers resulting from molecular and polymer grafting

The aim of this study was to synthesize hydrophobic cellulose nanofibers (CNFs) using different chemical treatments including polymer and molecular grafting. For polymer grafting, immobilizing poly (butyl acrylate) (PBA) and poly (methyl methacrylate) (PMMA) on CNFs were implemented by the free radical method. Also, acetyl groups were introduced directly onto the CNFs surface by acetic anhydride for molecular grafting. The gravimetric and X-ray photoelectron spectroscopy analysis showed the high grafting density of PMMA on the surface of CNFs. AFM results revealed that molecular grafting created non-uniformity on the CNFs surface, as compared to polymer brushes. In addition, thermodynamic work of adhesion and work of cohesion for the modified CNFs were reduced in water and diiodomethane solvents. Dispersion factor was studied to indicate the dispersibility of CNFs in polar and non-polar media. Dispersion energy was reduced after modification as a result of decreasing interfacial tension and the dispersibility of modified CNFs was improved in diiodomethane.

Analysis of strength and response of polymer nano thin film interfaces applying nanoindentation and nanoscratch techniques

Polymer and hydrogel thin films on a compliant substrate are gaining significant attention these days in the fields of soft electronics, digital display, biomedical devices and sensors to name a few. The functional characteristics of these thin films depend on the performance of its interface with the substrate. The interface response is a non-linear function of surface energy and stiffness of a substrate beside other factors. The deviation of thin film properties under confinement from its bulk counterparts and its ultra-low physical dimension makes characterization of an interface extremely challenging. Probing and shearing of these interfaces with appropriate load value and up to the critical load rate can provide insight into the integrity of these interfaces under different service conditions. We have studied the response of PMMA thin film (150 nm) on a softer epoxy substrate and a stiffer glass substrate of increasing interfacial strength applying nanoindentation (NI) and nanoscratching. Pile up behaviour of the film material at confined state, the critical load at the interface proximity and the plastic work done during indentation as determined in this work can be successfully applied to characterize interface behaviours. These parameters also help in developing interface models and build qualitative correlations between thermodynamic work of adhesion and true mechanical work of adhesion at the interface. We hypothesize that the elasto-plastic region around load tip plays a significant role in determining pile up above film surface. We have discussed the possible molecular mechanisms in this region under stress leading to pile up.

Mathematical models of polymer-dentin physicochemical interactions and their biological effects

Dental adhesion is the result of a physicochemical interaction between tooth structure and the adhesive polymeric restorative material. Adhesion involves molecular interactions at the interface between these constituents. Furthermore, mechanical interlocking is a common type of adhesion important in dental materials. This type of bonding involves the penetration of the adhesive into the dental surface and requires different energetic considerations for an optimal interface. An adequate infiltration of adhesive monomers into demineralized dentin depends on several factors that are determined by the atoms on the surface of the structures and the effects of surface energy on the thermodynamic work of adhesion. The polarity, solubility and viscosity of the adhesive system and the surface energy and moisture of dentin tissue are key factors that contribute to adhesion energy. The main goal of dental material adhesion is to produce an interface that is strong and durable. Thus, it is important to optimize the infiltration of adhesive monomers into exposed collagen fiber networks and dentinal tubules in order to increase the strength of the Resin-dentin bonds and produce adequate dentin sealing.