On the interpretation of contact angle hysteresis

The determination of solid surface free energy is still an open problem. The method proposed by van Oss and coworkers gives scattered values for apolar Lifshitz-van der Waals and polar (Lewis acid-base) electron-donor and electron-acceptor components for the investigated solid. The values of the components depend on the kind of three probe liquids used for their determination. In this paper a new alternative approach employing contact angle hysteresis is offered. It is based on three measurable parameters: advancing and receding contact angles (hysteresis of the contact angle) and the liquid surface tension. The equation obtained allows calculation of total surface free energy for the investigated solid. The equation is tested using some literature values, as well as advancing and receding contact angles measured for six probe liquids on microscope glass slides and poly(methyl methacrylate) PMMA, plates. It was found that for the tested solids thus calculated total surface free energy depended, to some extent, on the liquid used. Also, the surface free energy components of these solids determined by van Oss and coworkers' method and then the total surface free energy calculated from them varied depending on for which liquid-set the advancing contact angles were used for the calculations. However, the average values of the surface free energy, both for glass and PMMA, determined from these two approaches were in an excellent agreement. Therefore, it was concluded that using other condensed phase (liquid), thus determined value of solid surface free energy is an apparent one, because it seemingly depends not only on the kind but also on the strength of interactions operating across the solid/liquid interface, which are different for different liquids.     

Contact angle measurements and interpretation: wetting behavior and solid surface tensions for poly(alkyl methacrylate) polymers

Low-rate dynamic contact angles of a large number of liquids were measured on a poly(ethyl methacrylate) (PEMA) polymer using an automated axisymmetric drop shape analysis profile (ADSA-P). The results suggested that not all experimental contact angles can be used for the interpretation in terms of solid surface tensions: eight liquids yielded non-constant contact angles and/or dissolved the polymer on contact. From the experimental contact angles of the remaining four liquids, we found that the liquid-vapor surface tension times the cosine of the contact angle changes smoothly with the liquid-vapor surface tension, i.e. γlv cos ζ depends only on γlv for a given solid surface (or solid surface tension). This contact angle pattern is again in harmony with those from other methacrylate polymer surfaces of different compositions and side-chains. The solid-vapor surface tension of PEMA calculated from the equation-of-state approach for solid-liquid interfacial tensions was found to be 33.6 ± 0.5 mJ/m2 from the experimental contact angles of the four liquids. The experimental results also suggested that surface tension component approaches do not reflect physical reality. In particular, experimental contact angles of polar and nonpolar liquids on polar methacrylate polymers were employed to determine solid surface tension and solid surface tension components. Contrary to the results obtained from the equation-of-state approach, we obtained inconsistent values from the Lifshitz-van der Waals/acid-base (van Oss and Good) approach using the same sets of experimental contact angles.     

Hydrophilic/electron-acceptor surface properties of metallic biomaterials and their effect on osteoblast cell activity

Mice osteoblast cells were cultured on samples of 316L stainless steel and titanium alloy (Ti-6Al-4V) with different surface treatments. The resulting cell differentiation was correlated with the solid surface tension and electron-acceptor surface tension parameter of the biomaterial samples. Both of these characteristics were determined through contact angle measurements, using the Lifshitz–van der Waals/Lewis acid–base interaction model. Before calculating the surface tension of the biomaterials, the experimental contact angles were corrected for the effect of roughness using the Wenzel equation. For this calculation, the roughness characteristics of the solid surfaces were determined using atomic force microscopy and interferometric profilometry at the nano-scale and micro-scale, respectively. It was found that osteoblast cell differentiation directly related to the implant surface tension and electron-acceptor properties. The alkaline phosphatase activity increased both with increasing surface tension and increasing electron-acceptor surface tension parameter of the implant materials. These results suggest that the formation of surface hydroxyl groups with acidic character gives rise to enhanced attachment of osteoblast cells.     

Contact angle and IGC measurements for probing surface-chemical changes in the recycling of wood pulp fibers

The objective of this study was to use dynamic contact angle (DCA) analysis and inverse gas chromatography (IGC) to probe the surface-chemical changes in wood pulp fibers during recycling. A simplified wet-dry-rewet cycle was performed on hardwood bleached kraft fibers to represent the recycling process. The DCA measurements revealed that the overall effect of recycling was an increase in the non-polar (dispersive) component and a corresponding decrease in the polar component of the surface free energy, hence resulting in a total surface free energy that remained essentially unaltered. The DCA experiment also showed that virgin fibers lost both their electronaccepting (γ_S ⁺) and their electron-donating (γ_S ^-) characteristics when converted to paper. Upon rehydration, the fibers recovered some surface acidity (γ_S ⁺) but surface basicity (γ_S ^-) continued to decrease. The changes in polar surface free energy correlate well with the changes in hydroxyl number determined independently using the acetylation method. IGC could not detect changes in the dispersive component of the surface free energy induced by recycling. The acid-base (KA and KB) changes in the IGC measurements were also indistinguishable between virgin fibers and recycled fibers. This research concludes that DCA analysis is preferable to IGC in better reflecting the surface changes in fiber recycling, and γS-can at least be treated as an empirical parameter to complement the hydroxyl availability data in distinguishing among virgin, paper, and recycled fibers.     

Effects of chemical structure changes on thermal,mechanical, and crystalline properties of rigid rod epoxy resins

Effects of chemical structure changes on the thermal, mechanical, and crystalline properties of rigid rod epoxy resins have been studied for azomethine epoxy, biphenol epoxy, and tetramethyl biphenol epoxy. Rigid rod epoxies have exhibited better properties than those of the flexible bisphenol A epoxy. The chemical structures of both rigid rod epoxy and curing agent control the properties of cured rigid rod epoxies. When a flexible curing agent (methyl cyclohexane 1,2‐dicarboxylic anhydride) was used, the chemical structure of rigid rod epoxy has dominated effects on the properties. Thus, the azomethine epoxy has shown the best thermal and mechanical properties among three rigid rod epoxies. While a rigid curing agent (sulfanilamide) was used, the physical properties of cured epoxies are not only dependent on the chemical structures of epoxies but also on the ease of formation of ordered network. Among the cured rigid rod epoxies, only the biphenol epoxy cured by sulfanilamide exhibits a liquid crystalline network. It has the highest glass transition temperature (219°C) and the lowest coefficient of thermal expansion (20.8 μm/m°C). However, the most thermal stable system is azomethine epoxy cured with sulfanilamide. It has a weight loss (39%) at 450°C. Their excellent thermal and mechanical properties of rigid rod epoxies are useful in composites, printed wiring boards, integrated circuit encapsulations, etc. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 446–451, 2000     

Epoxy‐amine networks with varying epoxy polydispersity

Solid, high molecular weight DGEBA‐based epoxies were blended with high purity liquid DGEBA to create several resins with equivalent epoxy equivalent weights, but with polydispersity indices (PDIs) ranging from 3 to over 10. The resins were cured with a stoichiometric amount of polyetheramine and compared to a nonblended epoxy with PDI of 1.8. Modulus, glass transition temperatures, and molecular weight between cross‐links were measured using dynamic mechanical analysis. Coefficients of thermal expansion (CTE) were measured and used to extend room temperature density measurements as a function of temperature. Fracture properties were also measured. Overall, the increased polydispersity has almost negligible effect, with the main difference occurring in the slope of the glassy CTE, with more polydisperse epoxies having a slower increase in CTE. In comparison to previous work where bimodal amines were blended with DGEBA, we conclude that epoxy resins are far more sensitive to distributions in the flexible portion, rather than the more rigid one. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41503.     

Temperature influence on the physicochemical surface properties and adhesion behaviour of Enterococcus faecalis to glass and silicone

The interest in studies on the physicochemical surface properties of bacteria has increased because they are related to the causes of the initial adhesion of microorganisms to biomaterials and the subsequent biofilm formation on indwelling medical devices. The determination of physicochemical parameters such as hydrophobicity or surface tension is usually done at room temperature, not taking into account the real temperature at which bacteria cause infection inside the human body. In this work, the influence of the experimental temperature on the surface physicochemical characteristics and adhesion behaviour of Enterococcus faecalis ATCC29212 to glass and silicone has been studied. Water, formamide and diiodomethane contact angles on bacterial lawns changed when the experimental temperature was increased from 22°C to 37°C. Moreover, hydrophobicity, as determined by water contact angle, increases with temperature, in agreement with the higher and lower adhesion to silicone and glass, respectively, observed at 37°C, with respect to the results at 22°C. Also, when the formamide and diiodomethane contact angles are considered, the changes in the adhesion behaviour to glass and silicone are predicted by the sum of Lifshitz-van der Waals and acid-base interaction free energies if the measurement temperature is the same as the bacterial growth temperature, i.e. 37°C.     

Influence of the fused silica surface dehydroxylation on the adhesion of epoxyacrylate protective coatings used for optical fibers

Measurements have been made of the adhesion of liquid and UV-cured epoxyacrylates to a fused silica surface. The fused silica surface was dehydroxylated in the 200-900°C temperature range. Also, the contact angles of water, diiodomethane and formamide on the fused silica surface were measured. Using the contact angle results, the 'harmonic mean' method and the acid-base interactions approach, the dispersion (Lifshitz-van der Waals) and electron donor and electron acceptor components of the fused silica surface as well as epoxyacrylate polymer surface free energy were calculated. It was found that, probably because of the physically adsorbed water, the hydroxylated surface of the fused silica is basic and that the adhesion of the epoxyacrylate polymer to this surface depends on its basicity.     

Evolution of surface chemistry and physical properties during thin film polymerization of thermotropic liquid crystalline polymers

By applying a novel thin film polymerization technique, X-ray photoelectron spectroscopy (XPS), and the Lifshitz-van der Waals acid-base (LWAB) theory, we have determined the time evolution of surface chemistry and surface free energy during the polymerization of liquid crystalline poly(p-oxybenzoate/2,6-oxynaphthoate) at a molar ratio of 50/50. The surface free energy components of these main-chain liquid crystalline copolyesters were calculated from contact angle measurements using a Ramé-Hart goniometer and a three-liquid procedure (water, glycerol, and diiodomethane). The experimental data suggest that the Lewis base parameter (y-) during thin film polymerization decreases rapidly with the progress of polymerization, while the Lewis acid parameter (γ⁺) and the Lifshitz-van der Waals parameter (γ^LW) are almost invariant. The surface roughness data measured by atomic force microscopy (AFM) suggested that the increase in water contact angle (or the decrease in y-) was not caused by the change in surface roughness, but by the change in surface chemistry, i.e. due to the reaction of acetoxy and carboxy groups to release acetic acid during the polymerization reaction. In addition, the XPS results coincide with our previous Fourier transform infrared spectroscopy results showing that the condensation polymerization is much faster in the beginning than in the later stages. Consequently, the decrease in y- in the early stages of the polymerization is well explained.     

Acid–base surface free energies of solids and the definition of scales in the Good–van Oss–Chaudhury theory

The overwhelming basicity of all analysed surfaces strongly dependent on the choice of liquid triplet used for contact angle measurements and the negative values sometimes obtained for the square roots of the acid-base parameters can be summarized as the main problems arising from the application of the Good-van Oss-Chaudhury (GvOC) theory to the calculation of Lewis acid-base properties of polymer surfaces from contact angle data. This paper tries to account for these problems, namely: (1) the Lewis base, or electron donor component, is much greater than the Lewis acid or electron-acceptor component because of the reference values for water chosen in the original GvOC theory. A direct comparison of the acidic component with the basic one of the same materials has no meaning. A new reference scale for water which is able to overcome this problem is suggested. For the calculation of acid-base components, a best-fit approach is proposed which does not require any starting information about the liquids or polymers and can yield estimates of the acid-base parameters for both the liquids and the polymers involved; (2) the strong dependence of the value of the acid-base components on the three liquids employed is due to ill-conditioning of the related set of equations, an intrinsic and purely mathematical feature which cannot be completely cured by any realistic improvement in experimental accuracy. To reduce or eliminate the effect, one only needs a proper set of liquids, representative of all kinds of different solvents; (3) the negative coefficients appear as a simple consequence of measurement uncertainty, combined with the possible ill-conditioning of the equation set. We cannot exclude, however, that in some cases they could have a different origin.     

Polar Interfacial Interactions in RPLC

The three principal forces that govern the interaction between dissolved proteins and solid (flat or particulate) surfaces immersed in a polar liquid are: A. Lifshitz-van der Waals forces (LW interactions) B. Polar (hydrogen bond, or Lewis acid-base) forces (AB interactions, which include “hydrophobic” interactions) C. Electrostatic forces (EL interactions) EL interaction energies can be determined via electrokinetic measurements. LW and AB interaction energies are derived from the LW component and the electron-acceptor (γ⊕) and electron-donor (γ⊖) parameters of the AB component of the surface tension, all three of which can be determined by contact angle (Θ) measurements on (a) hydrated layers of protein and (b) flat layers of particulate surface, using at least 3 polar liquids in each case. By this approach, the optimal conditions for attachment as well as for detachment of a given protein to a well-characterized solid surface or particle can be determined quantitatively. Reversed phase liquid chromatography of immunoglobulin G (1) on phenyl Sepharose (2) shows that the elution of the protein, by an increasing concentration of ethylene glycol (EG) in the aqueous medium (3), occurs at the EG concentration where the value of ΔG₁₃₂^TOT changes from negative to positive.     

A Probe on the Failure Mechanism in Rubber-Modified Epoxy Blends: Morphological and Acoustic Emission Analysis

In this work, diglycidyl ether of bisphenol A based epoxy resin (DGEBA) was modified with varying amounts of two liquid rubbers: carboxyl terminated copolymer of butadiene and acrylonitrile (CTBN); and a hydroxyl terminated polybutadiene (HTPB), using an anhydride hardener. The ultimate aim of this study was to investigate the failure mechanism operating in the rubber-modified epoxies and to evaluate this by correlating these results with the miscibility and interfacial adhesion between the components and the morphology of the cured network. Some of the mechanical and fracture properties, which are associated with the two-phase particulate morphology, were investigated. The visoelastic behavior of modified epoxies was also analyzed and variations in the shift of T _g values in toughened epoxies were explained. The samples were carefully analyzed by an acoustic emission technique to investigate the failure mechanism operating in them. From the response of force and number of acoustic events as well as from the amplitude of acoustic events, we were able to explain the failure mechanisms in the elastomer incorporated epoxy resins supplemented by morphological evidence.     

The Effect of Surface Activation in Polymer Matrix-Carbon Fiber Interactions

Various carbon fibers (CF) were surface modified by chemical and electrochemical treatments for the purpose of establishing organic functional groups on the fiber surface. A series of fibers, surface oxidized for various periods of time, was prepared. The amounts of surface functionalities formed were assessed by means of contact angle measurements on single fibers. A suitable set of probe liquids was used to determine the LW (Lifshitz-van der Waals) and acid-base components of carbon fiber surfaces. Similar tests were made on commercial, sized carbon fibers, polystyrene (PS) and polymethyl methacrylate (PMMA), and their surface energies determined in terms of LW, surface acidity and surface basicity components. Work of adhesion values were calculated of all combinations of CF and polymer matrix couples by using these surface energies of both constituents. The calculated work of adhesion values were correlated to the ILSS values obtained from single fiber pull out tests with PS and PMMA as matrices.     

A comparative study of accelerated weathering of epoxy resins based on DGEBA and HDGEBA

The durability of epoxy resins is an important factor in applications of composites to timber engineering. We carried out a comparative study of the effect of accelerated weathering on epoxy resins based on the glycidyl ether of bisphenol A (DGEBA) or hydrogenated diglycidyl ether of bisphenol A (HDGEBA) cured with 2,2,4-trimethylene-1,6-hexadiamine, using various characterization tools, including mechanical and thermal testing and SEM to evaluate the effect of exposure to UV light, moisture and elevated temperature cycles on the properties of the resins. Mechanical testing showed that after 6 months accelerated weathering reduced by 50% the tensile strength of DGEBA-TMDA while HDGEBA-TMDA epoxies lost slightly less strength over the same exposure period. Elongation at break increased after 6 months of accelerated weathering from 5.1?±?0.1% to 7.5?±?0.3% for DGEBA-TMDA epoxy specimens, and from 0.9?±?0.3% to 2.0?±?0.1% for HDGEBA-TMDA epoxy specimens. FTIR of the epoxies showed increases in the absorbance of hydroxyl and carbonyl groups for both epoxies on accelerated weathering, but the hydroxyl and carbonyl indexes reached values 90 and 40% higher respectively for DGEBA than HDGEBA-based epoxy resins. While susceptibility to accelerated weathering on the molecular scale was clearly greater for the DGEBA-based epoxy on the molecular scale, this did not translate into a significant deterioration in mechanical properties relative to HDGEBA over the time scale of the accelerated weathering experiments.     

Synthesis of solid epoxy resins base on BPC II and bisphenol A using conventional heating equipment and microwave

New solid state epoxy resins based on diglycidyl ether of 1,1‐dichloro‐2,2 bis (4‐hydroxyphenyl), ethylene (BPC) and BPA epoxy resins in the reaction with BPC and BPA were developed. Solid epoxy resins were synthesised by the use of two different heating methods: conventional and microwave reactor. The use of microwave radiation as novel heating medium as an alternative to the conventional methods, which provide a quicker and more effective synthesis. The solid epoxies have high melting points higher than 100°C and the polycondensation degree remains between n = 4–12. Epoxy value stays low and occurs around 0,02–0,1. BPA epoxy resins and diglycidylether of BPC II were compared in terms of reaction speed. It can be seen that the reaction of BPC diglycidylether occurs approximately 20% quicker given the same reaction conditions of temperature, and balance of catalyst. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 3850–3854, 2006     

不同固化剂对环氧树脂玻璃化温度影响的分子动态模拟研究

提出了一个分子动态模拟方法研究环氧树脂玻璃化转变温度(Tg)随固化剂结构变化。首先,建立了固化环氧树脂的一些简单分子模型;然后是一个分子动态模拟(MD)被重复执行。用模拟得到的数据作V-T关系曲线,用曲线上的转折点确定Tg的值。该模拟值与计算值有很好的吻合,尽管与实验值有较大的偏差,固化剂对Tg的影响可以用MD模拟定性得到。所提出的方法对于开发具有提高固化效果的固化剂有潜在的意义。     

Controlling the release of active iron and manganese ions from styrene‐butadiene rubber‐binding matrix containing chloride salts of them

A series of epoxy resin composites containing different contents of alkoxysilane functionalized polycaprolactone/polydimethylsiloxane (PCS‐2Si) were prepared after curing with polyamidoamine curing agent at different temperatures. The effects of PCS‐2Si content and curing temperature on morphologies, solvent resistance, and surface properties of the composites were studied. The scanning electron microscope results showed that increasing the PCS‐2Si content and curing temperature caused the changes of miscibility between epoxy and modifier, leading to different morphologies. Other data from solvent swelling and surface tension of composites cured at the same temperature illustrated that the modified epoxy resins with higher content of PCS‐2Si had less crosslinked networks, but lower surface tension. At the same time, the composites cured at higher curing temperature generally had more resistance to chemicals and higher surface tension due to the formation of highly crosslinked networks. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Anodic Oxidation of Carbon Fibres in Diammonium Hydrogen Phosphate Solution

Anodic surface treatment of high tensile-carbon fibres under galvanostatic conditions has been performed in diammonium hydrogen phosphate solution, containing an addition of ammonium rhodanide.

The oxidized fibres have been characterized by monofilament tensile strength, XPS measurements and surface energetic analysis. Additionally, the acid-base interactions have been evaluated by wetting with aqueous solutions of different pH values.

An addition of ammonium rhodanide to the diammonium hydrogen phosphate anodization bath affects the oxidation of carbon fibres in terms of decreasing both the amounts of the surface oxides as well as that of degradation by-products. At the optimal treatment conditions (I = 100 mA) no changes in the tensile strength or BET-surface area of the fibre have been observed. The rise in ILSS values of amine cured epoxy composites is not dependent on O_ls/C_ls ratio or surface free energy of the reinforcing fibres, but on the acidic as well as nitrogen functional groups on their surface.     

Understanding the Affinity between Components of Wood–Plastic Composites from a Surface Energy Perspective

To evaluate surface compatibility in wood-plastic composites (WPCs), the dispersion and acid–base components of surface energy of various thermoplastic resins (matrices) and several wood-based reinforcing materials were determined using inverse gas chromatography (IGC). Polypropylene (PP), nylon 6, poly(ethylene terephthalate) (PET), poly(trimethyl terephthalate) (PTT), high impact polystyrene (HIPS), and styrene maleic anhydride (SMA) were used as thermoplastic resins, while wood flour (hot water extracted and un-extracted), microcrystalline cellulose (MCC) (50 μm and 90 μm), α-cellulose (60 μm), and silicified microcrystalline cellulose (SMCC) (60 μm) were used as reinforcing materials. All matrices and reinforcing components were exposed to low vapor concentrations of apolar (decane, heptane, nonane, octane) and polar (chloroform, ethyl acetate, dichloromethane, acetone, and tetrahydrofuran) probes. Methane and helium were employed as reference and carrier gases, respectively. IGC retention times were used to determine the acid–base component of surface energy of the analyzed materials. The corresponding surface energy, work of adhesion, and work of cohesionwere calculated based on the van Oss–Chaudhury–Good approach (acid–base and Lifshitz–van der Waals interactions). Composite performance was analyzed by measuring tensile and flexural strengths according to ASTM standards. The results indicated that for the same type of filler (assuming similar shape and dimensions), the mechanical properties of the composites increased when the ratio of the work of adhesion to the work of cohesion increased. A similar trend was observed when the thermoplastic resin employed to create the composite possessed an acid–base component of surface energy greater than zero.     

Enhanced thermal conductivity of boron nitride epoxy‐matrix composite through multi‐modal particle size mixing

Castable particulate‐filled epoxy resins exhibiting excellent thermal conductivity have been prepared using hexagonal boron nitride (hBN) and cubic boron nitride (cBN) as fillers. The thermal conductivity of boron nitride filled epoxy matrix composites was enhanced up to 217% through silane surface treatment of fillers and multi‐modal particle size mixing (two different hBN particle sizes and one cBN particle size) prior to fabricating the composite. The measurements and interpretation of the curing kinetics of anhydride cured epoxies as continuous matrix, loaded with BN having multi‐modal particle size distribution, as heat conductive fillers, are highlighted. This study evidences the importance of surface engineering and multi‐modal mixing distribution applied in inorganic fillered epoxy‐matrix composite. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007