Group contribution method for the estimation of contact angle trends on modified polymer surfaces

Group contribution methods can be used as an aid in evaluating contact angle data on modified polymer surfaces. Good correlation was seen between experimental contact angle titration data on a hydrolyzed polyimide surface and equations involving water-octanol partition coefficients using parameters derived from group contribution methods. The methods used captured features of the experimental contact angle titration data observed during ionization of surface functional groups.     

Assessing the accuracy of contact angle measurements for sessile drops on liquid-repellent surfaces

Gravity-induced sagging can amplify variations in goniometric measurements of the contact angles of sessile drops on super-liquid-repellent surfaces. The very large value of the effective contact angle leads to increased optical noise in the drop profile near the solid-liquid free surface and the progressive failure of simple geometric approximations. We demonstrate a systematic approach to determining the effective contact angle of drops on super-repellent surfaces. We use a perturbation solution of the Bashforth-Adams equation to estimate the contact angles of sessile drops of water, ethylene glycol, and diiodomethane on an omniphobic surface using direct measurements of the maximum drop width and height. The results and analysis can be represented in terms of a dimensionless Bond number that depends on the maximum drop width and the capillary length of the liquid to quantify the extent of gravity-induced sagging. Finally, we illustrate the inherent sensitivity of goniometric contact angle measurement techniques to drop dimensions as the apparent contact angle approaches 180°.     

Factors influencing the contact angle value. The contact angle,as a characteristic of the properties of solid surfaces

A survey of publications concerning the properties of solids in relation to wetting phenomena is presented. Factors influencing the contact angle value as well as problems of objective approach to research into wetting phenomena are discussed. Peculiarities of the direct and reverse processes during the formation of the solid—liquid—vapor three-phase contact and the inevitability of contact angle hysteresis for polar solids and liquids are analyzed. It is suggested that contact angle hysteresis is due to high energy of the interaction between the liquid and the solid and hence a long relaxation time of the three-phase contact system. Specific features of the response of a solid surface to all surface processes (“chemomechanics”) is discussed. Cleaning of solid surfaces as well as surface preparation for repeated measurements is considered. It is shown that good reproducibility of results is possible if conditions for sample preparation are met. The results of determination of the activation energy for wetting of glass surface with water are presented. The influence of the structure of solids (their hardness) on the contact angle values is demonstrated. Inevitability of the presence of different-type active sites characterized by different dissociation constants (pK_a) on the surface of solids is discussed. The pK_a values and content of these surface sites obtained from potentiometric titration and wetting data are estimated. The estimates thus obtained are in reasonable agreement with each other and can thus be used in practical applications. However, potentiometric titration is currently inappropriate for evaluating the content of individual surface sites as well as the surface charge.     

Why surfaces modified by flexible polymers often have a finite contact angle for good solvents

Polymers adsorbing from a dilute solution onto the solvent-vapor interface generate a nonzero surface pressure. When the same polymers are end-grafted onto a surface such that a so-called polymer brush is formed, one will find that the solvent wets this compound interface partially. The partial wetting and the finite surface pressure are intimately linked properties of the polymer-solvent-vapor combination. It is shown that the spreading parameter in the wetting problem is proportional to the surface pressure in the adsorption case. Complete wetting is only possible when this surface pressure is nonpositive. The wetting characteristics are hardly influenced by the grafting density and chain length characterizing the brush. We argue that the grafted polymer chains can bridge to the solvent-vapor interface, thereby preventing the wetting film to become macroscopically thick. We present experimental data underpinning our self-consistent field analysis. Indeed, finite contact angles should be expected in various systems in which bridging attraction contributes to the disjoining pressure in wetting films.     

Raman microspectroscopic mapping: a tool for the characterisation of polymer surfaces

Raman mapping by point illumination of polymer surfaces is discussed with examples taken from the plasma treatment of polypropylene (PP) and subsequent grafting of polystyrene (PS). Maps can be constructed for surface properties such as crystallinity, blend components and distribution of grafted PS. The Raman sampling volume was estimated for confocal operation using a 50x objective lens.     

A thermodynamic approach for determining the contact angle hysteresis for superhydrophobic surfaces

Contact angle hysteresis (CAH) is critical to superhydrophobicity of a surface. This study proposes a free energy thermodynamic analysis (of a 2-D model surface) that significantly simplifies calculations of free energy barrier associated with CAH phenomena. A microtextured surface with pillar structure, typical of one used in experimental studies, is used as an example. We demonstrate that the predicted CAH and equilibrium contact angles are consistent with experimental observations and predictions of Wenzel's and Cassie's equations, respectively. We also establish a criterion for transition between noncomposite and composite wetting states. The results and methodology presented can potentially be used for designing superhydrophobic surfaces.     

Effect of orientation and mobility of polymer molecules at surfaces on contact angle and its hysteresis

The contact angle of a water droplet on the surface of a solid polymer or hydrogel (water-swollen three-dimensional network) depends on whether a hydrophilic moiety of the polymer molecule is oriented towards the air interface or towards the bulk of the solid, but not on the hydrophilicity of the molecule. Therefore, the short-range rotational mobility of a polymer molecule has a major influence on the apparent hydrophilicity of a polymer surface as measured by the contact angle of water. By the came principle, the abnormally large hysteresis effect observed in advancing and receding contact angles of water on some polymer surfaces can be attributed to the reorientation of hydrophilic moieties of polymer molecules at the surface. These factors are demonstrated by selected polymer surfaces with different degrees of mobility at the polymer-air interface.     

等离子体改性聚合物表面动力学的动态接触角法研究

不同聚合物经CF~4/CH~4等离子体处理后,在浸水过程中表面动力学衰减常数对温度通过Arrhenius关系作图,对于所研究的聚合物都有一个明显的转折点。转折点处的温度称作表面构型转变温度(T~s),大约为15℃,与表面邻近水的Drost-Hansen温度一致。T~s以上及以下的活化能数值较小,说明表面构型变化的本质可看作是由于基团的翻转运动,而不需要整个大分子或链段的迁移运动。在浸水过程中,接触角滞后Δθ在表面构型转变温度T~s附近有转变,并有极小值,此后随着温度的升高出现极大值,继续升高温度接触角滞后Δθ又反而下降。     

Determining the contact angle between liquids and cylindrical surfaces

One of the simplest methods of measuring the quantities for estimating the adhesion properties of materials (i.e., the adhesion work, the surface energy, and the interfacial tension between certain liquids and a surface) requires the determination of the contact angle between the liquid and the surface. In the case of plane surfaces the determination of the drop dimensions makes it possible to calculate the contact angle by the sessile drop method, but in the case of cylindrical surfaces (such as the monofilaments), several methods were developed to improve the accuracy of the contact angle measurements. This paper presents a comprehensive method for precise evaluation of the contact angle between liquid drops and monofilaments by establishing a differential equation describing the drop contour. This equation makes it possible to accurately compute the contact angle using the dimensions of the drop. A comparison of the values of the contact angle calculated by our method and those obtained by other approaches is made. We applied our method in the case of polyamide-6 monofilaments treated using dielectric barrier discharge, knowing their medical applications in surgical sutures.     

New advances on the characterisation of polymer surfaces by HREELS

Self assembled monolayers (SAMs) of thiols functionalised with oligothiophene functions were used as models for polymer surfaces. Thienyl functionalisation simulates the surface region and demonstrates that the analysed depth is of the order of 10 Å. Spectra of polystyrene (PS) films were recorded and show that impact mechanism dominates, which corroborates the high sensitivity of HREELS applied to polymer surfaces. Different orientation of phenyl groups is unravel for PS films deposited on platinum and silicon substrates, which is here related to a different conformation of the PS chains in the surface region of the film. To illustrate the state of the art of HREELS applied to organic films, spectra were recorded with old and new generation spectrometers.     

Spectral quality enhancement as an aid to polymer characterisation

The limit to the information on the microstructure of polymers that may be obtained from their vibrational spectra is often set by overlapping bands, markedly broader than the spectrometer resolution. Following the initial pioneering work of Pohl and Hummel, using a simple curve fitting technique, mathematical methods of spectral quality enhancement have progressed very considerably. These methods are reviewed critically and new results are presented. The desirability of imposing some parameter constraints in curve fitting is demonstrated, and this has led to the development of methods for peak narrowing, as an aid to peak finding. The two important approaches, derivative spectroscopy and Fourier selfdeconvolution (FSD), are discussed in terms of their application to the complex of overlapping bands in the v(C-Cl) region of the infrared spectrum of poly(vinyl chloride). A development of the FSD method termed Fourier band isolation, is described and preliminary results indicate that it has considerable promise. The maximum entropy method is discussed briefly.     

Isoelectric points of plasma‐modified and aged silicon oxynitride surfaces measured using contact angle titrations

Acid‐base properties of metal oxides and polymers can control adhesion properties between materials, electrical properties, the physical structure of the material and gas adsorption behavior. To determine the relationships between surface isoelectric point, chemical composition and aging effects, plasma‐surface treatment of amorphous silicon oxynitride (SiO_xN_y) substrates was explored using Ar, H₂O vapor, and NH₃ inductively coupled rf plasmas. Overall, the Ar plasma treatment resulted in nonpermanent changes to the surface properties, whereas the H₂O and NH₃ plasmas introduced permanent chemical changes to the SiO_xN_y surfaces. In particular, the H₂O plasma treatments resulted in formation of a more ordered SiO₂ surface, whereas the NH₃ plasma created a nitrogen‐rich surface. The trends in isoelectric point and chemical changes upon aging for one month suggest that contact angle and composition are closely related, whereas the relationship between IEP and composition is not as directly correlated. Copyright © 2010 John Wiley & Sons, Ltd.     

Magnetically induced decrease in droplet contact angle on nanostructured surfaces

We report a magnetic technique for altering the apparent contact angle of aqueous droplets deposited on a nanostructured surface. Polymeric tubes with embedded superparamagnetic magnetite (Fe(3)O(4)) nanoparticles were prepared via layer-by-layer deposition in the 800 nm diameter pores of polycarbonate track-etched (PCTE) membranes. Etching away the original membrane yields a superparamagnetic film composed of mostly vertical tubes attached to a rigid substrate. We demonstrate that the apparent contact angle of pure water droplets deposited on the nanostructured film is highly sensitive to the ante situm strength of an applied magnetic field, decreasing linearly from 117 ± 1.3° at no applied field to 105 ± 0.4° at an applied field of approximately 500 G. Importantly, this decrease in contact angle did not require an inordinately strong magnetic field: a 15° decrease in contact angle was observed even with a standard alnico bar magnet. We interpret the observed contact angle behavior in terms of magnetically induced conformation changes in the film nanostructure, and we discuss the implications for reversibly switching substrates from hydrophilic to hydrophobic via externally tunable magnetic fields.     

Determination of pK a of benzoic acid- and p-aminobenzoic acid-modified platinum surfaces by electrochemical and contact angle measurements

Acidity constant values of benzoic acid (BA)-modified platinum electrode (Pt-BA) and p-aminobenzoic acid (pABA)-modified platinum electrode (Pt-NHBA) surfaces were determined using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and contact angle measurements (CAM). Diazonium tetrafluoroborate salt reduction and pABA oxidation reactions were used to prepare (Pt-BA) and (Pt-NHBA) surfaces, respectively. Both surfaces exhibited pH dependence with [Fe(CN)₆]^3?/4? redox probe solutions at different pH; this allowed us to estimate the surface pK _a values. Acidity constants for Pt-BA surface were found to be pK _a (3.09 ± 0.25), (4.89 ± 0.11), and (3.91 ± 0.54) by CV, EIS, and CAM techniques, respectively, while the values for Pt-NHBA surface were pK _a (3.16 ± 0.45), (4.24 ± 0.40), and (5.64 ± 0.12). The Pt-BA surface pK _a values were lower in CV and CAM measurements relative to the bulk solution of BA, while a higher value was observed in EIS for Pt-BA surface. The pK _a values determined for Pt-NHBA surface via both CV and EIS were lower than the bulk value; however, the result obtained from CAM was one unit higher than pK _a of bulk pABA.     

Electrokinetic and contact angle measurements of grafted carbon fibers

 The grafting method that has successfully been applied to methacrylic acid and liquid crystalline monomers was expanded to prepare amphoteric carbon fiber surfaces using 2-(N,N-dimethylamino)ethyl methacrylate as monomer. The obtained carbon fiber surfaces were characterized by contact angle and ζ-potential measurements. The expected basic behavior was not observed, instead an amphoteric character of the modified carbon fiber surface was found. The fiber surfaces display a basic character in the acidic pH-range, while they are acidic in the alkaline part of the pH-scale. An important influence is derived from the amount of initiator used to graft the monomers onto the fibers. The smaller the initiator concen-tration used during polymerization, the larger the amount of amino functionalities introduced to the carbon fiber surface. The wetting behavior versus water depends on the overall conformation of the immobilized polymer. During immersion into water the polymer acts hydrophobic, while during emersion, a hydrophilic character is observed, probably derived from conformational changes and swelling during the contact angle measure-ments in water. Received: 9 June 1998 Accepted: 13 August 1998     

A modified contact thermal heater for short-time polymer destruction studies

A new design of rotation heater has been suggested for the short-time polymer thermal stability registration.     

Stabilizing contact angle hysteresis of paraffin wax surfaces with nanoclay

The addition of montmorillonite clay modified with an alkylammonium salt surfactant (i.e., organoclay) to paraffin wax is found to reduce the decay in wetting properties associated with its heating in the melt. It was previously shown that holding wax in its molten form prior to characterization reduces crystallinity when the solid forms. This results in the development of microscale amorphous regions at wax surfaces, which appear to be more polar given the abundance of methylene linkages versus methyl groups. These regions are believed to impact the receding angles for more polar liquids almost exclusively. It is known that the introduction and exfoliation of a small amount of the organoclay greatly enhances the stiffness, strength, and toughness of paraffin wax. Here, it is shown that the organoclay also promotes the formation of coatings possessing fewer thermal cracks and helps maintain higher crystallinity levels. Fresh wax surfaces containing the clay are slightly rougher than those without, which produces a slight increase in hysteresis. However, the significant drops in receding angles found for paraffin wax samples cast from the melt subsequent to heating are absent.     

FTIR-ATR-spectroscopic analysis of the protein adsorption on polymer blood contact surfaces

Summary Model experiments have been carried out in a flow-through attenuated total reflectance (ATR-) cell mounted in a Fourier Transform Infrared- (FTIR-) spectrometer to study in situ the adsorption of some important blood proteins on several organic and inorganic surfaces from D₂O-solutions. The model compounds used were fibrinogen, albumine and ₁-acid glycoprotein as well as polystyrene, polyetherurethane and germanium (the material of the ATR-prism).The intensities of the amide I bands of the respective adsorbed proteins were in the 10 m AU-range and have been plotted as a function of contact time between solution and surface for different reaction conditions. (Isolated, ground and negatively charged surfaces, before and after rinsing with D₂O, different pD-values.) From the data it was concluded, that the proteins adsorb in two layers, the first, irreversibly bound within a few seconds to minutes and the second build up slowly. This second layer could be removed by application of negative charge (–9 V) in the case of fibrinogen on polystyrene, and to a lesser extend for albumine and ₁-acid glycoprotein but not by rinsing with D₂O. From these IR spectra of fibrinogen, structural changes upon buildings up and desorption of the second layer as compared to the first could be derived.

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FTIR-ATR spektroskopische Analyse der Proteinadsorption an polymeren Blutkontaktoberflächen

Zusammenfassung In einem mit einer ATR-Durchflußzelle ausgestatteten Fourier-Transformations-Infrarotspektrometer wurden Modellexperimente zum In-situ-Studium der Adsorption einiger wichtiger Blutproteine an verschiedenen organischen und anorganischen Oberflächen aus D₂O-Lösung durchgeführt. Als Modellsubstanzen wurden Fibrinogen, Albumin und ₁-Glycoprotein sowie Polystyrol, Polyetherurethan und die Oberfläche des Ge-Reflexionselementes eingesetzt. Die Intensitäten der Amid-I-Banden der jeweiligen adsorbierten Proteine lagen im Bereich vonE=0,01 und wurden als Funktion der Kontaktzeit für verschiedene Reaktionsbedingungen dargestellt (isolierte, geerdete bzw. negativ geladene Oberflächen, vor und nach D₂O-Spülung, verschiedene pD-Werte). Es konnte gezeigt werden, daß die Proteine zunächst rasch eine irreversible Schicht bilden (Sekunden bis Minuten), worauf sich langsam eine zweite Schicht aufbaut. Diese konnte durch Aufladung des Reflexionselementes auf –9 V im Fall von Fibrinogen auf Polystyrol signifikant, für Albumin und ₁-Glycoprotein nur in geringerem Ausmaß wieder entfernt werden. D₂O-Spülung allein greift diese zweite Schicht nicht an. Aus den IR-Spektren dieses Fibrinogenexperiments konnten weiters Strukturänderungen während des Auf- und Abbaues der zweiten Schicht im Vergleich zur ersten abgeleitet werden.

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This work was supported by the Austrian Science Foundation (Fonds zur Förderung der wissenschaftlichen Forschung in Österreich); Proj. no. 3427.     

A modified Cassie–Baxter relationship to explain contact angle hysteresis and anisotropy on non-wetting textured surfaces

The Cassie–Baxter model is widely used to predict the apparent contact angles obtained on composite (solid–liquid–air) superhydrophobic interfaces. However, the validity of this model has been repeatedly challenged by various research groups because of its inherent inability to predict contact angle hysteresis. In our recent work, we have developed robust omniphobic surfaces which repel a wide range of liquids. An interesting corollary of constructing such surfaces is that it becomes possible to directly image the solid–liquid–air triple-phase contact line on a composite interface, using an electron microscope with non-volatile organic liquids or curable polymers. Here, we fabricate a range of model superoleophobic surfaces with controlled surface topography in order to correlate the details of the local texture with the experimentally observed apparent contact angles. Based on these experiments, in conjunction with numerical simulations, we modify the classical Cassie–Baxter relation to include a local differential texture parameter which enables us to quantitatively predict the apparent advancing and receding contact angles, as well as contact angle hysteresis. This quantitative prediction also allows us to provide an a priori estimation of roll-off angles for a given textured substrate. Using this understanding we design model substrates that display extremely small or extremely large roll-off angles, as well as surfaces that demonstrate direction-dependent wettability, through a systematic control of surface topography and connectivity.