Comment on theories of interfacial tension of polymeric interfaces

A simple scaling argument is shown to give the correction to the interfacial tension of two infinite-molecular-weight immiscible homopolymers arising from the translational entropy of finite-sized chains. It is suggested that the qualitative nature of the Flory-Huggins model, as shown by Binder and coworkers, precludes meaningful detailed comparison of experimental data with current theoretical expressions for the interfacial tension.     

Transient interfacial tension and dilatational rheology of diffuse polymer-polymer interfaces

We demonstrate the influence of molecular weight and molecular weight asymmetry across an interface on the transient behavior of the interfacial tension. The interfacial tension was measured as a function of time for a range of polymer combinations with a broad range of interfacial properties using a pendant/sessile drop apparatus. The results show that neglecting mutual solubility, assumed to be a reasonable approximation in many cases, very often does not sustain. Instead, a diffuse interface layer develops in time with a corresponding transient interfacial tension. Depending on the specific combination of polymers, the transient interfacial tension is found to increase or decrease with time. The results are interpreted in terms of a recently proposed model [Shi et al., Macromolecules 37, 1591 (2004)], giving relative characteristic diffusion time scales in terms of molecular weight, molecular weight distribution, and viscosities. However, the time scales obtained from this theoretical approach do not give a conclusive trend. Using oscillatory dilatational interfacial experiments the viscoelastic behavior of these diffusive interfaces is demonstrated. The time evolution of the interfacial tension and the dilatational elasticity show the same trend as predicted by the theory of diffuse interfaces, supporting the idea that the polymer combinations under consideration indeed form diffuse interfaces. The dilatational elasticity and the dilatational viscosity show a frequency dependency that is described qualitatively by a simple Fickian diffusion model and quantitatively by a Maxwell model. The characteristic diffusion times provided by the latter show that the systems with thick interfaces (tens of microseconds and more) can be considered as slower diffusive systems compared to the systems with thinner interfaces (a few micrometers in thickness and less) can be considered as fast diffusive systems.     

Method to evaluate the strength of interfaces in polymeric systems

It is shown that CT (compact tension) specimens known from fracture mechanics are adequate to study the strength of interfaces between polymers. The interface is coplanar with the initial notch. The capability of this method is demonstrated by quantifying: (a) the healing procedure of cracks in polystyrene; (b) by elaborating the influence of powders on the welding process in polystyrene; (c) by measuring the strength of interfaces between the different polymers SAN, PMMA, HIPS, PS, PC, and (d) by characterising the influence of a compatibilizer in the interface between PE and HIPS.     

Transient interfacial patterns and instabilities associated with liquid film adhesion and spreading

A surface force apparatus was used to study surface shape changes during the adhesion and spreading of a polymer melt on a bare mica surface. Transient fingers were observed during the initial, rapid spreading process, pointing radially out from the initial adhesive contact point. The fingers had microscopic widths and lengths but submicroscopic thicknesses. They eventually disappeared, leaving a more slowly growing circular neck with a smooth, featureless polymer-air surface. The mean radius of the spreading meniscus (neck) was found to follow a scaling relationship with time of the form (ri + ro)/2 proportional, variant tn, with n = 0.128, while the ends of the fingers grew according to ro proportional, variant tn, with n = 0.10. These rates agree with the values of n = 0.100-0.125 predicted by classical wetting theories for circular macroscopic droplets (i.e., radially symmetric, without fingers) spreading on a solid surface. The lifetime of the transient fingering patterns increases with the polymer viscosity as tau proportional, variant etan, with n = 2.1 +/- 0.2. A circular trough or depression in the film was observed just beyond where the fingers ended, which appears to be a source of the material for the advancing fingers. In addition, beyond the trough, circular ripples/waves were observed on the polymer melt film surface. Such patterns may arise quite generally whenever a perturbation occurs that changes the local forces, thereby inducing a bulge or depression in a liquid film or surface. Thus, we observe similar fingers and ripples/waves during the spreading of liquid polybutadiene on (the immiscible and more viscous) liquid poly(dimethylsiloxane), suggesting that the phenomenon may exist in various liquid adhesion and spreading situations. For low viscosity liquids such as water and low molecular weight oils, our scaling relations suggest that the transient patterns will exist for only a few microseconds; this is likely the reason for why they have not yet been observed.     

Particle laden fluid interfaces: Dynamics and interfacial rheology

We review the dynamics of particle laden interfaces, both particle monolayers and particle + surfactant monolayers. We also discuss the use of the Brownian motion of microparticles trapped at fluid interfaces for measuring the shear rheology of surfactant and polymer monolayers. We describe the basic concepts of interfacial rheology and the different experimental methods for measuring both dilational and shear surface complex moduli over a broad range of frequencies, with emphasis in the micro-rheology methods. In the case of particles trapped at interfaces the calculation of the diffusion coefficient from the Brownian trajectories of the particles is calculated as a function of particle surface concentration. We describe in detail the calculation in the case of subdiffusive particle dynamics. A comprehensive review of dilational and shear rheology of particle monolayers and particle + surfactant monolayers is presented. Finally the advantages and current open problems of the use of the Brownian motion of microparticles for calculating the shear complex modulus of monolayers are described in detail.     

Direct determination of interfacial energy between brittle and polymeric films

Very few analyses exist for determining the adhesion f thin brittle films in the micron range on low modulus polymeric substrates. A theoretical model and experimental means are developed to determine the adhesion for this situation using the concept of fracture mechanics and Griffith's energy criterion. An expression for the adhesion is derived in terms of the external loading, material constants, and thicknesses of the films. The model is compared with measurements of brittle films vacuum deposited onto polymeric substrates. Excellent agreement is obtained and interfacial energies are calculated. Since the moduli of thin films are very difficult to determine accurately, the effect of a variation in measurements of this material parameter on the adhesion is calculated. The effect of varying preparative conditions is also discussed.     

Synthesis of polymeric nanocapsules with a crosslinked shell through interfacial miniemulsion polymerization

A water‐soluble comonomer, N‐isopropylacrylamide (NIPAM), and an oil‐soluble crosslinker, divinylbenzene (DVB), have been combined in a system for the synthesis of nanocapsules with crosslinked shells through interfacial miniemulsion polymerization by encapsulating a liquid nonsolvating hydrocarbon. Oligomers of poly(N‐isopropylacrylamide) (PNIPAM) were dehydrated and separated from the aqueous phase and were adsorbed by the nanodroplets or latex particles and then anchored at their interfaces by means of a crosslinking reaction. Nanocapsules were then formed through encapsulation of the hydrocarbon by the newly produced polymers at the interfaces of the droplets. The crosslinked structure gradually grew to stabilize the shell morphology. The incorporation of NIPAM into the shell copolymers has been verified by FTIR and solid‐state ¹³C NMR data. The fact that the number of nanocapsules increases with increasing amounts of DVB and NIPAM supports the formation of nanocapsules following interfacial (co)polymerization. Therefore, a mechanism for the formation of nanocapsules through interfacial (co)polymerization induced by NIPAM and DVB is proposed. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1522–1534, 2009     

Encapsulation of living cells with polymeric systems

Microencapsulation of cells producing recombinant proteins or hormones leads to immunoprotection and immobilization in culture or in vivo. We are investigating three different strategies for the production of calcium cross-linked alginate beads of a small size with immobilized and immunoprotected mammalian cells: a) the AirJet technology (coaxial gas flow extrusion), b) the vibrating nozzle technology, and c) the JetCutter technology. A alginate/poly-L-lysine/alginate complexation was used as the polymeric system. All three methods may be used for production of homogeneous beads with a diameter of approximately 350 μm. While the vibrating nozzle technique was limited to an alginate viscosity of 0.2 Pa·s or less, the AirJet and JetCutter technology were less sensitive to higher viscosities. High frequency Scanning Acoustic Microscopy is used for mechanical characterization of the microspheres as well as for investigation of surface properties.     

Ab initio Hartree-Fock instabilities in closed-shell molecular systems

The Hartree-Fock instability of twelve polyatomic systems is studied at theab initio level. It is found that all the systems with at least one double bond, exhibit a non-singlet instability. On the other hand instabilities of singlet type as well as instabilities of non-real type appear only in a small number of cases. The existence of these instabilities is discussed with respect to the location of low-lying excited states and to the weight of ionic structure.     

Electrochemical heparin sensing at liquid/liquid interfaces and polymeric membranes

The monitoring of heparin and its derivatives in blood samples is important for the safe usage of these anticoagulants and antithrombotics in many medical procedures. Such an analytical task is, however, highly challenging due to their low therapeutic levels in the complex blood matrix, and it still relies on classical, indirect, clot-based assays. Here we review recent progress in the direct electrochemical sensing of heparin and its analogs at liquid/liquid interfaces and polymeric membranes. This progress has been made by utilizing the principle of electrochemical ion transfer at the interface between two immiscible electrolyte solutions (ITIES) to voltammetrically drive the interfacial transfer of polyanionic heparin and monitoring the resulting ionic current as a direct measure of heparin concentration. The sensitivity, selectivity, and reproducibility of the ion-transfer voltammetry of heparin are dramatically enhanced compared to those of traditional potentiometry. This voltammetric principle was successfully applied for the detection of heparin in undiluted blood samples, and was used to develop highly sensitive ion-selective electrodes based on thin polymeric membranes that are intended for analytical applications beyond heparin detection. The mechanism of heparin recognition and transfer at liquid/liquid interfaces was assessed quantitatively via sophisticated micropipet techniques, which aided the development of a powerful ionophore that can extract large heparin molecules into nonpolar organic media. Moreover, the reversible potentiometric detection of a lethal heparin-like contaminant in commercial heparin preparations was achieved through the use of a PVC membrane doped with methyltridodecylammonium chloride, which enables charge density dependent polyanion selectivity.     

Adsorption at liquid interfaces: the generalized Langmuir isotherm and interfacial structure

The thermodynamics of adsorption of amphiphilic surface-active compounds at the interface between two immiscible liquids is considered. At the interface, these molecules are supposed to replace a few of the adsorbed molecules of both solvents. Classical isotherms of adsorption (Frumkin, Langmuir, Henry) were based on the model of nonpenetrable interface, where an adsorbate can substitute only molecules of one solvent. However, at the interface between two immiscible electrolytes, like nonpolar oil-water interfaces, or liquid membrane amphiphilic molecules can substitute molecules of both solvents; therefore, classical isotherms are not applicable in these cases. The generalization of Langmuir and Frumkin isotherms for permeable and nonpermeable interfaces, known as the Markin-Volkov (MV) isotherm, gives the possibility to analyze adsorption and the interfacial structure in a general case. In the present paper, the adsorption isotherms of pentafluorobenzoic acid at the octane-water interface at various pH were measured by the drop-weight method. The thermodynamic parameters of pentafluorobenzoic acid (PFBA) adsorption at the octane-water interface were found. From the measurements of PFBA adsorption, the structure of the octane-water interface was determined. Substitution of one adsorbed octane molecule requires approximately three adsorbed PFBA molecules. This result shows that the orientation of solvent molecules at the interface is different from the bulk solution. Adsorbed octane molecules have a lateral orientation with respect to the interface. Gibbs free energy of adsorption equilibrium and thermodynamic parameters of PFBA adsorption show that the adsorption of PFBA at the octane-water interface is accompanied by a reduction in the attraction between adsorbed PFBA molecules as the pH decreases to the acidic region.     

Spatial displacement correlations in polymeric systems

The spatial correlations of the monomer displacements are studied via molecular-dynamics simulations of a melt of fully flexible, unentangled polymer chains with different length, interacting potential, density, and temperature. Both the scalar and the vector characters of the correlations are considered and their extension quantified in terms of suitable dynamical correlation lengths. Displacements performed at both short, i.e., vibrational, and long times, i.e., comparable to the structural relaxation time, are investigated. On both time scales the spatial correlations are modulated according to the radial distribution function g(r) to an extent which is determined by the character of the correlations, the time scale of the displacements and the structural slowing down. The spatial correlations of the short-time displacements have clear directional character. The modulus correlations of the long-time displacements are more marked, especially for sluggish states. Analogous findings are found by experiments on colloids. By inspecting the dynamical heterogeneities of states with slowed-down dynamics, it is observed that fast monomers exhibit correlations which are stronger and more differing from the bulk than the slow ones. It is shown that states with identical average vibrational monomer displacement exhibit identical spatial correlations of the monomer displacements pertaining to the subsets of the fast and the slow monomers characterizing both the short-time and the long-time dynamical heterogeneities.     

Adsorption at liquid interfaces: The generalized Frumkin isotherm and interfacial structure

The thermodynamics of adsorption of amphiphilic surface-active compounds at the interface between two immiscible liquids is considered. At the interface, these molecules are supposed to replace a few of the adsorbed molecules of both solvents. Classical isotherms of adsorption (Frumkin, Frumkin-Damaskin, Langmuir, Henry) were based on the model of non-penetrable interface, where an adsorbate can substitute only molecules of one solvent. At the interface between two immiscible electrolytes, nonpolar oil/water interfaces, and liquid membranes amphiphilic molecules can substitute molecules of both solvent and classic isotherms cannot be used. The generalization of Frumkin isotherm for permeable and non-permeable interfaces, known as the Markin-Volkov isotherm, gives the possibility to analyze adsorption in a general case. The adsorption isotherms of pentafluorobenzoic acid at the octane/water interface at different pHs were measured by the drop-weight method. The thermodynamic parameters of pentafluorobenzoic acid (PFBA) adsorption at octane/water interface were determined. From the measurements of PFBA adsorption, the structure of the octane/water interface was determined. Substitution of one adsorbed octane molecule requires approximately three adsorbed PFBA molecules. This result shows that the orientation of solvent molecules at the interface is different from the bulk. Adsorbed octane molecules have a lateral orientation with respect to the interface. Gibbs free energy of adsorption equilibrium and thermodynamic parameters of PFBA adsorption show that the adsorption of PFBA at the octane/water interface is accompanied by a reduction in the attraction between adsorbed PFBA molecules as the pH decreases to the acidic region. Published in Russian in Elektrokhimiya, 2006, Vol. 42, No. 10, pp. 1194–1200. The text was submitted by the authors in English.     

Characterization of polymeric surfaces and interfaces using time-of-flight secondary ion mass spectrometry

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is used for chemical analysis of surfaces. ToF-SIMS is a powerful tool for polymer science because it detects a broad mass range with good mass resolution, thereby distinguishing between polymers that have similar elemental compositions and/or the same types of functional groups. Chemical labeling techniques that enhance contrast, such as deuterating or staining one constituent, are generally unnecessary. ToF-SIMS can generate both two-dimensional images and three-dimensional depth profiles, where each pixel in an image is associated with a complete mass spectrum. This Review begins by introducing the principles of ToF-SIMS measurements, including instrumentation, modes of operation, strategies for data analysis, and strengths/limitations when characterizing polymer surfaces. The sections that follow describe applications in polymer science that benefit from characterization by ToF-SIMS, including thin films and coatings, polymer blends, composites, and electronic materials. The examples selected for discussion showcase the three standard modes of operation (spectral analysis, imaging, and depth profiling) and highlight practical considerations that relate to experimental design and data processing. We conclude with brief comments about broader opportunities for ToF-SIMS in polymer science.     

Lamination method for the study of interfaces in polymeric thin film transistors

A method for the fabrication of polymeric thin-film transistors (TFTs) by lamination is described. Poly(dimethylsiloxane) stamps were used to delaminate thin films of semiconducting polymers from silicon wafers coated with a self-assembled monolayer (SAM) formed from octyltrichlorosilane. These supported films were laminated onto electrode structures to form coplanar TFTs. The fabrication process was used to make TFTs with poly(3-hexylthiophene), P3HT, and poly[5,5'-bis(3-dodecyl-2-thienyl)-2,2'-bithiophene], PQT-12. TFTs, where these polymers were laminated onto gate dielectrics coated with SAMs from octyltrichlorosilane, had effective field-effect mobilities of 0.03 and 0.005 cm2/(V s), respectively. TFTs where PQT-12 was laminated onto gate dielectrics that were not coated with a SAM also had mobility of 0.03 cm2/(V s). In contrast, TFTs fabricated by spin-coating PQT-12 onto the same structure had mobilities ranging from 10-3 to 10-4 cm2/(V s). These results suggest that the lower mobilities of polymer TFTs made with hydrophilic gate dielectrics are caused by molecular ordering in the semiconducting film rather than electronic effects of dipolar groups at the interface.     

Healing of interfacial surfaces in polymer systems

Literature data on the problems related to the healing of interfacial surfaces in polymers are revisited. Specific features and behavior of the contacting surfaces of polymer films in the rubbery and glassy states, as well as in heterophase polymer systems, are analyzed. Particular details associated with the healing of interfacial surfaces in polymers, which are capable of chemical interactions with each other, are considered. Special attention is focused on the analysis of the phenomena taking place on the newborn interfaces formed owing to the deformation of polymers in different physical states. Processes providing healing of shear bands and crazes during annealing of deformed polymer glasses are discussed. The above phenomena are shown to present evident practical interest from the viewpoint of the development of advanced nanocomposites based on polymer matrices.     

Morphology control in polymeric multiphase systems with block copolymers and structured latexes

Some application possibilities of block copolymers for the morphology control in polymeric multiphase systems are reviewed. The microdomain formation of block copolymers in the solid state is illustrated in the case of functionalized block copolymers and for interpolymer complexes. The preparation of “hairy latex” by emulsion polymerization in the presence of hydrophilic-hydrophobic block copolymers is shown in connection with their applications in the controlled agglomeration process of latexes and for the preparation of polymer particles having microvoids. The surface activity of block copolymers in polymeric oil-in-oil systems is illustrated for silicone oil filled polymers. These materials have a low kinetic friction coefficient and act as reservoir systems with the lubricant incorporated in the polymer matrix.     

Polymer-enzyme conjugates can self-assemble at oil/water interfaces and effect interfacial biotransformations

Native water-soluble enzymes were transformed into interface-binding enzymes via conjugation with hydrophobic polymers, thus enabling interesting interfacial biocatalysis between immiscible chemicals at oil/water interfaces. Such interfacial biocatalysis demonstrated a significantly improved catalytic efficiency as compared to traditional biphasic reactions with enzymes contained in the bulk aqueous phase. Particularly, polystyrene-conjugated beta-galactosidase showed a catalytic efficiency that was more than 145 times higher than that of the native enzyme for a transgalactosylation reaction. It is believed that the improved accessibility of the biocatalysts to chemicals held in both phases across the interface is the key driver for the enhancement of enzyme activity.     

Studies on interfacial electrochemical phenomena in pigment-vehicle systems

Sedimentation potentials under a centrifugal field are studied with the disperse system titanium dioxide, alkyd resin and xylene. The relationships between the sedimentation potential and the rotation speed, the distance apart of the electrodes in the cell and the total weight of particles in the disperse system are examined. From the theoretical evaluation of the initial sedimentation potential, the zeta potential of the pigment in the disperse system is calculated. The calculated value was almost coincident with that obtained by the electrophoretic method. As the sedimentation potential is proportional to the total weight of pigment in the space between the electrodes in the cell, the particle-size distribution of pigment can be obtained from the sedimentation potential-time curve. The particle-size distribution in the above-mentioned disperse system was examined with respect to the effect of rotation speed and the pigment content. The modal diameter for the particle-size distribution obtained by this method had almost the same value as that obtained by the electron microscope method. By use of this method for particle-size analysis, the particle-size distribution for particles of diameter < 1 mum can be obtained in a short time (ca, 10-30 min) and the zeta potential of the particles in the disperse system can be calculated.     

Assessment of polymeric flocculants in oily water systems

Oil-in-water emulsions are usually formed during oil production and treatment. Before being discarded, such dispersions should be treated. In order to improve the oil–water separation process using physical processes (decantation, flotation, centrifugation, etc.) the particle size of the disperse phase should be increased. This may be achieved through flocculation, which consists in the agglomeration of various particles or drops, using, as flocculating agents, high molecular weight hydrophilic macromolecules. A few studies have been carried out on the flocculation of finely divided oil drops in brine with the aid of generally ionic polyelectrolytes. This work shows the results obtained using nonionic polymers as flocculants. Commercial samples of poly(ethylene oxide-b-propylene oxide) and poly(vinyl alcohol) were evaluated through flocculation–flotation tests as well as the drop size distribution. The performance of such additives as flocculants for oil–water dispersions is related to their structure, composition, molecular weight and hydrophilic–lipophilic balance. The composition of the produced water is also an important parameter when choosing the features of the flocculant additive.