Thermodynamic Work Of Adhesion Research Articles

Surface characterization of structural and stereo isomeric amino acids using sorption techniques and Raman spectroscopy

Event Abstract Back to Event Surface characterization of structural and stereo isomeric amino acids using sorption techniques and Raman spectroscopy Daniel Burnett1, Majid Naderi1, Anett Kondor1 and Manaswini Acharya1 1 Surface Measurement Systems, Ltd., United States Introduction: Amino acids are found in proteins and enzymes, which perform essential roles in nature. Different forms of compounds are known to have different physiological and therapeutic effects and therefore various effective methods are required to provide insight in their physical properties.[1] Considering that hydration is a major factor in determining the structural and isomeric forms of amino acids, the interaction with water vapor would provide fundamental information. Materials and Methods: Alanine samples were obtained from Across Organics. A unique combination of a fiber optic Raman probe with Dynamic gravimetric Vapor Sorption (DVS) was used to monitor the real-time transformation of different forms of Alanine. The sample was exposed to a particular RH while monitoring the change in mass. In-situ Raman spectra were collected during sorption experiments. Total surface free energies of different forms of Alanine were determined using IGC-SEA (Surface Measurement Systems Ltd.). IGC-SEA provides surface energy profiles of particulate/fibrous samples unlike conventional techniques which only yield a single average valued property. Results and Discussion: The combined water sorption/Raman spectroscopy results for ß-Alanine and DL-Alanine show significant characteristics in the region 800 – 1600 cm-1 for both materials. There are no changes in the Raman bands of ß-Alanine during the sorption process up to 90% RH. However, after the 90% RH stage and during the desorption stage the Raman Spectra show irreversible changes in ß-Alanine structure. The water sorption data for DL-Alanine indicates a relatively low uptake and confirms a stable material in agreement with the Raman spectrum. The difference of 125× in moisture uptake levels is attributed to water absorption into the bulk of ß-Alanine, which causes a phase change in ß-Alanine, whereas in the case of DL-Alanine there is no phase or structural changes. The dispersive and specific surface energy profiles at 0% RH indicate ß-Alanine has the higher total surface energy, thus is the most active and have higher affinity for water. At 90% RH, the acid-base surface energy of both samples is increased while the dispersive surface energy did not change significantly. Thus, the polarity, for both alanine samples increases with the relative humidity due to an increase in the acid-base surface energy of both samples. The wettability as the measure of the intermolecular interaction between the solid sample and water can is determined by the thermodynamic work of adhesion with water molecules (Figure 1). The higher work of adhesion shows higher the wettability (more hydrophilic). It is clearly observed that the wettability of both alanine samples increases with the RH, but the increase is more significant in the case of the ß-Alanine. Figure 1. Wettability of (a) DL-Alanine and (b) -Alanine at 0% RH (blue) and 90% RH (red). The work of adhesion values were calculated at 0.005 surface coverage (n/nm). Conclusions: IGC-SEA was able to measure the significant differences in surface chemistry for the ß-Alanine and DL-Alanine at 0 and 90% RH. These results correlate directly with water sorption behavior. The combination of gravimetric vapor sorption with Raman spectroscopy and IGC-SEA allows for better understanding of vapor-induced structural changes of biologically active ingredients.

Evaluation of Adhesion Forces for the Manipulation of Micro-Objects in Submerged Environment through Deposition of pH Responsive Polyelectrolyte Layers.

Optimization of surface treatment for reversible adhesion of micro-objects in liquid environment for the need in microassembly processes is presented. A spherical borosilicate probe and planar oxidized silicon wafer substrates were modified by deposition of pH sensitive polyelectrolyte films through layer-by-layer technique. Branched polyethylenimine (b-PEI) and poly(sodium styrenesulfonate) (PSS) were deposited in alternating manner on surfaces, and the influence of polyelectrolyte concentration, pH of deposition, and number of layers on the adhesion were successively examined. The multilayer buildup was followed by optical reflectometry (OR) and dissipative quartz crystal microbalance (QCM-D). The adhesion forces were monitored in aqueous environment at variable pH values by colloidal probe AFM microscopy. The thermodynamic work of adhesion was derived from the pull-off forces by using the Johnson-Kendall-Roberts (JKR) model and compared to the work of adhesion determined from contact angle measurements. It was found out that they correlate well, however, the values accessed from JKR model were underestimated, which was attributed mainly to the effect of surface roughness. Obtained results have demonstrated that it is possible to achieve repeatable reversible adhesion with the change of pH of submerged environment by appropriately tailoring the surface properties and therefore the prevailing surface forces.

Interfacial bonding of fine aggregate concrete to low modulus fibers

The present work describes the feasibility of using cost effective and low modulus polymeric fibers including Nylon 6,6 and Acrylic fibers as reinforcement of fine aggregate cementitious composites. For this purpose, the pullout energy and interfacial bonding strength of fibers to the different matrix compositions where evaluated using a pullout test. Surface free energy of the fibers and different fine aggregate composites were also evaluated using the sessile drop method. Fiber to cement bonding strength, thermodynamic work of adhesion and surface free energy (wetting affinity) were assessed by changing maximum aggregate size, water to cement (binder) ratio, using fly ash as supplementary cementing materials (SCMs) and fiber type. Both chemical and mechanical interfacial interactions were measured and it was found that the Nylon 6,6 fiber showed highest pullout energy and flexural toughness due to more mechanical interlocking to the matrix.

Investigation of the interface between polyethylene and functionalized graphene: A computer simulation study

The effect of surface chemical functionalization of a single graphene layer on its thermodynamic work of adhesion (WA) with polyethylene (PE) chains has been investigated using molecular dynamics (MD) simulation. For this purpose, amine (NH2), carboxyl (COOH), hydroxyl (OH), and methyl (CH3) functional groups were distributed randomly throughout the graphene surface using a Monte Carlo (MC) algorithm to achieve graphene functionalized structures with minimized potential energies. The MD simulation results showed that the thermodynamic WA between the PE and the functionalized graphene was larger than that between the PE and the pristine graphene. In fact, the electronegativity of functional groups and Van der Waals forces play influential roles in the thermodynamic WA between the PE and the functionalized graphene. In addition, the amount of thermodynamic WA was increased with increasing the functional group surface density, except for the graphene functionalized with the methyl groups. The segmental density of the PE chains near the single sheet surface was determined based on the density profile calculation. The polymer segments exhibited strong ordering and sharp density variations near the PE/graphene interface. The dynamic of chains was quantitatively characterized by calculating mean square displacement (MSD). Furthermore, the influence of functionality on the glass transition temperature (Tg) of the PE at the PE/graphene interface region was investigated. The results showed that the Tg at the PE/graphene interface was much higher than that of the bulk polymer. In fact, the functionalization of the graphene surface seems to considerably enhance the Tg of the polymer due to lowering the chains mobility.

Estimation of polyurethane-carbon black interactions by means of inverse gas chromatography

The properties of composites depend mainly on the interfacial interactions between filler and matrix that can be related to the adhesion between filler and polymer matrix. In this study the work of cohesion between the carbon black particles – Wcoh – and the thermodynamic work of adhesion – Wa – between four carbon blacks of different specific surface area and surface chemistry (nature and content of carbon–oxygen functional groups) and thermoplastic polyurethane were calculated by means of inverse gas chromatography (IGC) at infinite dilution. IGC derived data indicated that the work of adhesion increased by increasing the surface area of the carbon black, but the opposite trend was found in Wa/Wcoh and work of cohesion. According to the Wa/Wcoh values the filler particles should be well dispersed into the polyurethane matrix giving homogenous composites. The carbon black-thermoplastic polyurethane interactions determined by plate–plate rheology showed the same trend than that for the Wa/Wcoh values. However, the thermodynamic work of adhesion values derived from IGC were not in agreement with the carbon black-polyurethane interfacial interactions, likely due to the dominant effect of the carbon black in reducing the crystallinity and increasing the degree of phase separation of the thermoplastic polyurethane.

Adhesion of Polypropylene Fiber to Cement Matrix

In this research, the adhesion of polypropylene (PP) fibers to cementitious matrix has been investigated and the chemical bonding and mechanical interlocking between PP fiber and hardened cement paste has been studied. Furthermore, thermodynamic work of adhesion and loss-function (dissipation energy) has been calculated in the PP-cement matrix model system. To investigate the work of adhesion, the pull-out test has been used. Also, the surface free energy and contact angle of the PP monofilaments and cement matrix have been measured using a tensiometer and the fiber–cement interfacial interactions and thermodynamic work of adhesion and loss-function were calculated. Scanning electron microscopy (SEM) analysis was used to study the fiber–cement matrix interfacial transition zone (ITZ). The results showed that the application of theories of polymer–polymer adhesion in fiber–cement matrix systems was feasible. To verify the accuracy of the method, the adhesion of two other fibers (nylon 6,6 and acrylic polymer) was studied.

Adhesion phenomena in bonded joints

Adhesive bonding is a key joining technology in many industrial sectors including the automotive and aerospace industries, biomedical applications, and microelectronics. Adhesive bonding is gaining more and more interest due to the increasing demand for joining similar or dissimilar structural components, mostly within the framework of designing lightweight structures. When two materials are brought in contact, the proper or adequate adhesion between them is of great importance, so it is necessary to device ways to attain the requisite adhesion strength between similar or dissimilar materials including the different combinations of metallic materials, polymers, composite materials and ceramics. To make adhesion possible, it is necessary to generate intrinsic adhesion forces across the interface. The magnitude and the nature of those forces are very important. From a thermodynamic standpoint the true work of adhesion (or intrinsic property) of the interface create free surfaces from the bonded materials. Adhesion mechanisms have been known to be dependant on the surface characteristics of the materials in question. The intrinsic adhesion between the adhesive and substrates arises from the fact that all materials have forces of attraction acting between their atoms and molecules, and a direct measure of these interatomic and intermolecular forces is surface tension. Atomic/molecular understanding of adhesion should be extremely beneficial in selecting or creating the appropriate materials to attain the desired adhesion strength. In the present paper, the following topics are reviewed in detail: (a) the surfaces or interfaces of similar and dissimilar materials, (b) adhesion or bonding mechanisms in the adhesive joints (c) thermodynamic theory of adhesion: surface tension or surface free energy concepts including the wetting, wetting criteria, wettability, and thermodynamic work of adhesion, (d) dispersion and polar components of surface free energies, and finally (e) effect of surface roughness on wettability or adhesion.

Polymeric fibre adhesion to the cementitious matrix related to the fibres type, water to cement ratio and curing time

In this study adhesion of some polymeric fibres to a cement matrix was evaluated by both theoretical and experimental approach. The adhesion results are interpreted in terms of fibre nature, water to cement ratio and curing. It was found that practical work of adhesion (G) is ten times larger than thermodynamic work of adhesion (W) between fibre and cement matrix. By employing adhesion theory to the composite system, fibre/cement interactions are better described. The results indicated that chemical adhesion between cement matrix and fibres decreased with decrease in water to cement ratio while mechanical bonding increased. The application of adhesion theory to cement based composite leads to calculation of a new parameter which is called energy dissipation (loss function). It was found that the loss function of the fibre–cement system increases more than 21% by increase in the curing time of the matrix from 14 days to 28 days.

Characterization of nano scale adhesion at solid surface of oxidized PP wax/PP blends

The incorporation of oxidized PP wax OPPW additive into polypropylene resulted to the improved surface adhesion. Atomic force microscopy (AFM) was employed to show either the surface topography or the changes in adhesion force measurements between the probe tip and sample surface. The results of surface characterization tests revealed lower surface roughness and higher adhesion force for higher OPPW content in the blends. The lower surface roughness may be attributed to higher melt flow ability of OPPW and hence better mold filling during sample fabrication. The improvement of surface adhesion was in favor for wetting behavior of solid surface using three test liquids of distilled water, diiodomethane and formamide. The thermodynamic work of adhesion W a was calculated from surface free energy components using geometric mean model of Owens–Wendt–Rabel and Kaelble (OWRK approach) and also from the theory of van Oss–Good–Chaudhary (VOGC approach). The adhesion force measurements in AFM were also employed to derive the W a using Johnson–Kendal–Roberts (JKR approximation). The results indicate that the increase in W a was mainly due to polar part in OWRK approach and the acid–base part in VOGC approach. However, the results of work of adhesion in JKR approximation does not in general scale linearly with the respective OPPW content for the sample series, may be due to varying in near-surface modulus. The adhesion promoting effect of OPPW in the blends seems to work mainly via increasing surface acid–base interactions, better surface wettability and to some extent by plastic deformation mechanisms during debonding.

Measurement and analysis of micro-scale adhesion for efficient transfer printing

The adhesion-based transfer printing process allows the devices to be fabricated through low temperature process, which is important for realizing flexible electronics with high mobility. Therefore, comprehensive understanding about adhesion between the solid object and the surface of elastomeric stamp (generally, polydimethylsiloxane is used) and the optimal strategy to control adhesion can support the effective and high yield transfer. In this paper, a novel adhesive force measurement system is developed to measure dynamic adhesive forces from the surface on the flexible stamp. The thermodynamic work of adhesion at the moment of debonding of a tip of sensor from the flexible stamp is modeled and used for developing the adhesion control strategy. The measurement results show that the work of adhesion is strongly dependent on the peel-off velocity of tip, while the indentation force has only minor effects on it. The effect of Young’s modulus of elastomeric stamp is also investigated using the model of work of adhesion based on the relation between the adhesive force and peel-off velocity. The elastomeric stamp with low Young’s modulus is preferred for efficient transfer printing.

Experimental analysis of the strength of silver–alumina junction elaborated at solid state bonding

The mechanisms of ceramics–metal assemblies, particularly silver and alumina, can be better understood by studying the strength of their adhesion. These two materials are a priori non-reactive, their thermodynamic work of adhesion is low and the difference between their thermal coefficients of expansion in very considerable. In this study, the strength of silver–alumina junctions elaborated at solid state by thermo-compression is tested by an indirect tensile test and shearing one. The effects of several parameters such as: the pressure of bonding, the time of bonding, the temperature, and the oxygen dissolve in metal solid solution on the strength of the junction are analyzed. The obtained results show that the resistance of the junction is affected by all this parameters and it is essential to optimize these different parameters in order to increase the durability of the junction. It was also shown that the diffusion of the silver in alumina could be the cause of the damage of alumina near the interface.

Use of Surface Energy to evaluate Adhesion of Bituminous Crack Sealants to Aggregates

Problem statement: The performance of bituminous sealants in the field is partly controlled by properties and strength of the sealant-aggregate interface. The thermodynamic work of adhesion represents energy required for reversible separation of the two materials at the interface. The study of fracture includes the energy required for both reversible and irreversible processes during separation. Excess of work of facture over work of adhesion represents the energy consumed by irreversible processes in the specimen during loading and fracture. In addition, adhesion strength is related to the properties of constitutive components. Approach: The objective of this study was to measure or predict adhesion of hot-poured bituminous-based sealant to aggregates of different chemical composition. In order to accomplish, the study of fracture and the thermodynamic work of adhesion were estimated for 14 hot-poured bituminous-based sealants with two types of aggregate: Limestone and quartzite. The work of adhesion for each sealant-aggregate system was calculated from the contact angle measurements of system components at corresponding sealant installation temperature. Results: In general, limestone showed better adhesion to hot-poured crack sealant than quartzite. Interfacial parameters such as contact angles and surface tensions were successfully used to differentiate between sealants. Conclusion/Recommendation: The findings were in agreement with preliminary testing results of 14 sealants using a newly developed fixture in the direct-tension testing machine. Provided the sealant has an appropriate viscosity to fill the crack, as the sealant surface tension decreases, its adhesion strength increases.

Characterization of adhesion phenomena and contact of surfaces by soft colloidal probe AFM

We present a method based on colloidal probe atomic force microscopy (AFM) to measure adhesion energies and to study other contact phenomena of surfaces. The method employs an elastomeric colloidal probe, rendering the contact area between probe and sample much larger as compared to standard atomic force microscopy techniques. The technique allows us to determine the contact area via microinterferometry and measure the applied forces at the same time. The adhesion properties can then be accessed by using the Johnson–Kendall–Roberts (JKR) approach, i.e. measuring (a) the contact area as a function of applied load, and (b) the elastic parameters and the thermodynamic work of adhesion. We test this method in ambient conditions as well as in aqueous media on well-known surface chemistries, and can clearly characterize the contributions of capillary in air, hydration forces and hydrophobic interactions in water. This novel method provides a means to study the contact behavior of soft colloids and enhanced sensitivity for adhesion measurements.

Optoelectronic and Interfacial Properties of CNT and ITO on Borosilicate Glass and PET Substrates with Nano- and Hetero-structural Aspects

AbstractNano- and hetero-structures of carbon nanotube (CNT) and indium tin oxide (ITO) can control significantly piezoelectric and optoelectronic properties in Microelectromechanical Systems (MEMS) as sensing and actuators under cyclic loading. Optimized preparation conditions were obtained for multi-functional purpose of the specimens by obtaining the best dispersion and turbidity in the solution. Optical transmittance and electrical properties were investigated for CNT and ITO dipping and spraying coating on boro-silicate glass and polyethylene terephthalate (PET) substrates by electrical resistance measurement under cyclic loading and wettability test. Uniform dipping coating was performed using Wilhelmy plate method due to it simple and convenience. Specimen was applied with spraying coating additionally. The change in the electrical and optical properties of coated layer is mainly dependent upon the number of dip-coating, the concentration of CNT and ITO solutions, and the surface treatment condition. Electric properties of coating layers were measured using four-point probe method, and surface resistance was calculated using a dual configuration method. Optical transmittance of CNT and ITO coated PET film was also evaluated using UV spectrum. Surface energy and their hydrophilic and hydrophobic properties of CNT and ITO coated substrates were investigated by wettability test via static and dynamic contact angle measurements. As the elapsing time of cyclic loading passed, the stability of surface resistance and thus comparative interfacial adhesion between coated layer and substrates was evaluated to compare the thermodynamic work of adhesion, Wa. As dip-coating number increased, surface resistance of CNT coating decreased, whereas the transmittance decreased step-by-step due to the thicker CNT and ITO networking layer. Nano- and hetero-structural effects of CNT and ITO solution on the optical and electrical effects have been studied continuously.

Bacterial attachment and removal properties of silicon- and nitrogen-doped diamond-like carbon coatings

Si- and N-doped diamond-like carbon (DLC) coatings with various Si and N contents were deposited on glass slides using magnetron sputter ion-plating and plasma-enhanced chemical vapour deposition. Surface energy analysis of the DLC coatings revealed that with increasing Si content, the electron acceptor value decreased while the electron donor value increased. The antifouling property of DLC coatings was evaluated with the bacterium, Pseudomonas fluorescens, which is one of the most common microorganisms forming biofilms on the surface of heat exchangers in cooling water systems. P. fluorescens had a high value of the component (69.78 mN m−1) and a low value of the component (5.97 mN m−1), and would be negatively charged with the zeta potential of −16.1 mV. The experimental results showed that bacterial removal by a standardised washing procedure increased significantly with increasing electron donor values and with decreasing electron acceptor values of DLC coatings. The incorporation of 2%N into the Si-doped DLC coatings further significantly reduced bacterial attachment and significantly increased ease of removal. The best Si–N-doped DLC coatings reduced bacterial attachment by 58% and increased removal by 41%, compared with a silicone coating, Silastic® T2. Bacterial adhesion strength on the DLC coatings is explained in terms of thermodynamic work of adhesion.

Surface Energy of Modified Nanoclays and Its Effect on Polymer/Clay Nanocomposites

Surface properties of thermally stable phosphonium-modified montmorillonite were investigated at both room temperature and 220°C. These properties were compared with those of pristine and ammonium-modified montmorillonite. Surface properties at room temperature were calculated from contact angles measured using sessile drops. Several liquids with known polar and dispersion components of surface tension were used. Surface energy of nanofillers at 220°C was calculated from contact angles, using sessile drops of polymer melts. Two commercial polystyrene (PS) resins, with different melt flow characteristics, and high-density polyethylene (HDPE) were used. Isothermal TGA experiments were used to determine the thermal stability of the resins and nanofillers. The dispersion behavior and mechanical properties of the nanocomposites are correlated with the values of the Hamaker constant and thermodynamic work of adhesion for these polymer–nanoclay systems.

Mucoadhesion of pectin as evidence by wettability and chain interpenetration

Different types of pectin were characterized for gastrointestinal (GI) mucoadhesion. The mechanisms of mucoadhesion of these pectins were investigated by measurements of surface tension, contact angle, and FTIR studies. The surface tension of different GI fluids was relatively the same and found to decrease after addition of mucin. The contact angle of tested fluids on pectin surfaces behaved as time-dependent and the values at 30 s were used for comparison. The type of pectin, addition of mucin and pectin surfaces influenced the contact angle of GI fluids. The thermodynamic work of adhesion ( W ad/therm) and spreading coefficient were calculated. The positive values of W ad/therm indicated that the pectin surfaces could be wet spontaneously by adhesional process. The pectin with higher molecular weight and higher degree of substitution showed a lower W ad/therm, resulting from the low wettability. The spreading coefficient of tested fluids containing mucin on pectin surface was negative, suggesting that no spreading could be occurred spontaneously. ATR-FTIR studies revealed the changes resulting from interpenetration of pectin–mucin chains at film interface and the formation of hydrogen bonds. The results obtained from this study demonstrated that the wetting behavior and work of adhesion could be used to explain the mechanism of mucoadhesion of various pectins in GI conditions, and that a chain interdiffusion occurred at the interface of pectin film and mucin solution which supported the diffusion theory of mucoadhesion.

Effects of Flame Treatment on the Interfacial Energy of Polyethylene Assessed by Contact Mechanics

The effects of flame treatment of low-density polyethylene (LDPE) on the work of adhesion (W) and energy release rate (G) were assessed by a custom-built adhesion testing device (ATD). The contact area and the vertical displacement between planar LDPE films and PDMS lenses (untreated and UV/ozone treated) were varied as a function of the applied load, P. The adhesion and pull-off forces between the extracted PDMS lenses and the LDPE films were studied as a function of the duration of the flame treatment expressed in treatment numbers. A fracture mechanics approach was used to relate the applied load and the radius of the contact area to the energy release rate (G). The difference between the energy release rate and the thermodynamic work of adhesion, defined as the adhesion hysteresis (G - W), increased systematically with increasing treatment numbers for both lens types. In addition, the adhesion hysteresis appeared to be dependent on the contact time. Finally, the origin of the adhesion hysteresis was discussed. It was concluded that bonding by surface functional groups may be the dominating mechanisms for the changes in the observed hysteresis.

Measurement of time-dependent adhesion between a polymer film and a flat indenter tip

We revisited an elasticity problem of flat indentation on an elastic film bonded to a rigid substrate and obtained the force–depth relation in a simple form. With the obtained force–depth relation, Kendall's elastic equilibrium theory of adhesion was extended to the adhesion between a flat tip and a compressible elastic film. Thus, the thermodynamic work of adhesion at the moment of debonding of a flat tip from an elastic film was expressed in terms of pull-off force, elastic constants and geometric parameters. It is worth noting that the obtained relation for elastic films is still valid for viscoelastic films if viscoelastic losses are limited to the process zone of debonding. This makes it possible to study the time-dependent adhesion of viscoelastic polymer films. Indentation experiments with a flat diamond tip were performed on SU-8 films, and the results verified that the extended form of Kendall's theory correctly compensates the effect of the finite thickness of the films on the work of adhesion. The indentation results also showed that the work of adhesion is strongly dependent on the unloading velocity of the tip, while indentation depth and dwell time have only minor effects on the work of adhesion.

Dispersive Evaluation and Self-Sensing of Single Carbon Fiber/CNT-Epoxy Composites using Electro-Micromechanical Techniques

ABSTRACTSelf-sensing and interfacial evaluation were investigated with different dispersion solvents for single carbon fiber/carbon nanotube (CNT)-epoxy composites by electro-micromechanical technique and acoustic emission (AE) under loading/subsequent unloading. Optimized dispersion procedure was set up to obtain improved mechanical and electrical properties. Apparent modulus and electrical contact resistivity for CNT-epoxy composites were correlated with different dispersion solvents for CNT. CNT-epoxy composites using good dispersion solvent showed higher apparent modulus because of better stress transferring effect due to relatively uniform dispersion of CNT in epoxy and enhanced interfacial adhesion between CNT and epoxy matrix. However, good solvent showed high apparent modulus but low thermodynamic work of adhesion, Wa for single carbon microfiber/CNT-epoxy composite. It is because hydrophobic high advanced contact angle was shown in good solvent, which can not be compatible with carbon microfiber well. Damage sensing was also detected simultaneously by AE combined with electrical resistance measurement. Electrical resistivity increased stepwise with progressing fiber fracture due to the maintaining numerous electrical contact by CNT.