Latex film morphology and electrical potential pattern dependence on serum components: a scanning probe microscopy study

Dry films formed by surfactant-stabilized, peroxide-initiated styrene-butyl acrylate latex were examined by atomic and electric force microscopy (AFM and EFM). The effects of latex serum components on the films were observed by subjecting the latex to extensive dialysis prior to film formation, and comparing the results to as-prepared latex. The films formed with the dialyzed latex are smoother (as evidenced by roughness and fractal dimension measurements) than the films from the as-prepared latex, but they display large electric force gradients between neighboring domains. The films made with the as-prepared latex have the highest electric uniformity, with a maximum potential variation lower than 80 mV, while this reaches 200 mV in the dialyzed latex film.     

Brownian Dynamics Simulation of Film Formation of Mixed Polymer Latex in the Water Evaporation Stage

Brownian dynamics simulations of the filming process of a mixed polymer latex in the water evaporation stage were performed in order to explore the effect of surface potential on latex particle packing and distribution at a temperature far below the glass transitions of polymers in bulk. Polymer latex particles are modeled as spheres that interact via DLVO potential with various surface charge densities for emulsifier-free emulsion polymerized particles and dispersion polymerized particles. It is found that the distribution of modeled poly(methyl methacrylate) and polystyrene latex particles in the finally formed film exhibits a noticeable dependence of surface potentials of latex particles. When the difference of the surface potentials between binary mixed latex particles is small, the particles distribute randomly. In contrast, when the difference of the surface potentials between binary mixed latex particles is large, heterocoagulation occurs and the polymer latex in which the repulsive electrostatic potential is weak will form clusters in the film. The results are in agreement with laser confocal fluorescence microscopy observations of fluorescent dye labeled poly(methyl methacrylate) and polystyrene mixed latex films. The correlation between latex particles increases with increasing repulsive electrostatic potential, and the spatial order can be obtained at the end of the water evaporation stage. Copyright 2000 Academic Press.     

Thinning of drying latex films due to surfactant

Lateral non-uniformities in surfactant distribution in drying latex films induce surface tension gradients at the film surface and lead to film thinning through surfactant spreading. Here we investigate the influence of the surfactant driven to the air-water interface, during the early stages of latex film drying, on the film thinning process which could possibly lead to film rupture. A film height evolution equation is coupled with conservation equations for particles and surfactant, within the lubrication approximation, and solved numerically, to obtain the film height, particle volume fraction, and surfactant concentration profiles. Parametric analysis identifies the effect of drying rate, dispersion viscosity and initial particle volume fraction on film thinning and reveals the conditions under which films could rupture. The results from surface profilometry conform qualitatively to the model predictions.     

Change of Surface Structure upon Foam Film Formation

The charge distribution and coverage with surfactant molecules at foam film surfaces plays an important role in determining foam film structure and stability. This work uses the concentration depth profiling technique neutral impact collision ion scattering spectroscopy to experimentally observe the charge distribution in a foam film for the first time. The charge distribution at the surface of a foam film and the surface of the corresponding bulk liquid were measured for a cationic surfactant solution and the surface excess as well as the electric potential were determined. Describing the internal pressure of foam films by using the electrochemical potential is introduced as a new concept. The foam film can be seen to have a more negative surface charge compared to the bulk liquid surface due to re‐arranging of the surfactant molecules. It is discussed how the change in surface excess and electric potential change the electrochemical potential and the stability of the foam film.     

Kinetics of latex formation of PBMA latex in the presence of alkali soluble resin using atomic force microscopy

 The effect of alkali-soluble resin (ASR), poly(ethylene-co-acrylic acid), EAA, postadded to emulsifier-free monodisperse poly(butyl methacrylate) (PBMA) latexes on the kinetics of film formation was investigated using atomic force microscopy (AFM). Corrugation height of latex particles in films was monitored at various annealing temperatures as a function of annealing time. Enhanced polymer diffusion was found in a latex film containing ASR regardless of anneal-ing temperature. With increasing annealing temperature, a much higher rate of polymer diffusion was found in latex films containing ASR. These results can be interpreted that the low molecular weight and low Tg EAA resin adsorbed at the particle surface is more susceptible to diffusion than that of the PBMA in the film formation stage, thus it enhances the mobility of PBMA polymer. Received: 30 October 1997 Accepted: 20 March 1998     

Film formation from monodisperse acrylic latices 5. Drying and ageing in coalescing agent containing latex films

The influences of drying temperature, ageing time and ageing temperature on the film formation in coalescing agent containing latex films were investigated via turbidity measurements and atomic force microscopy. Coalescing agents used are 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate (TEX) and ethylene glycol monobutyl ether (EB). Latex films were dried at different temperatures. At a drying temperature of 40 degrees C, the differences in immersion patterns of neat and 1% coalescing agent containing latex films are too small to show significant improvement in film formation during 1 h of drying. Increase of the temperature from 40 to 50 degrees C, however, causes an improvement in film formation. At higher temperatures (95 degrees C), fast removal of the liquid phase cannot be prevented by the addition of 1% coalescing agent. Although addition of higher amounts of coalescing agent results in an increase of particle deformation and film surface smoothness, it intensifies also the water absorption of dried films. Ageing below the MFFT does not result in a significant improvement of film formation in the presence of 1% coalescing agent. Above the MFFT, coalescing agents act more efficiently and cause an improvement in film formation. By addition of higher amounts of coalescing agent the completion of the film formation process is delayed: for example, by the addition of 15% EB, nearly 1 year of ageing at room temperature is required for an efficient result.     

Study on the interaction between anionic and cationic latex particles and Portland cement

The interaction between organic latex polymers and the surface of hydrating cement was investigated by measuring the zeta potential and adsorbed amount of polymer on cement. First, differently charged model latex particles were synthesized in aqueous media by well-known emulsion polymerization technique. The latex polymers were characterized by differential scanning calorimetry (DSC), dynamic light scattering (DLS) and environmental scanning electron microscopy (ESEM). Electrokinetic latex surface properties were investigated by means of streaming potential measurements using a particle charge detector (PCD). It is shown that the anionic latexes adsorb a considerable amount of Ca²⁺ from the cement pore solution. Next, adsorption of the latex polymers on the surface of hydrating cement was confirmed by zeta potential measurements using the electroacoustic method. A water to cement ratio in the cement paste as low as 0.5 was studied, representing actual conditions in mortar and concrete. Additionally, adsorption isotherms were determined in a sedimentation test using the depletion method. For all latex polymers, Langmuir type adsorption isotherms were found. The latex dosages required to achieve saturated adsorption on the cement surface obtained from zeta potential measurements correspond well with those determined in the sedimentation test. Electron microscopy photographs confirm that the charged latex polymers adsorb selectively on surface areas of hydrating cement showing opposite charge. This way, domains of organic latex polymers exist on the cement surface. They provide adhesion between the inorganic cement matrix and the organic polymer film formed later on by particle coalescence as a result of cement hydration and drying.     

Permeation through homopolymer latex films

The helium permeabilities of homopolymer films cast from polymer solutions and latex dispersions have been measured as a function of time. The permeability coefficients of most polymer latex films started off at a value which was considerably higher than that of the corresponding solvent-cast film, but then dropped at a rate dependent on the polymer concerned. The permeability coefficients levelled off at a value which was closer to, but still higher than that of the equivalent solvent-cast film. Solvent-cast film permeabilities remained constant for the period of several months over which time they were examined. The reduction in the permeability of latex films is attributed to ageing processes occurring within the film after casting. The fact that latex film permeability coefficients are always higher than those of solvent-cast films suggests that latex films never become completely homogeneous. The effect on permeability of the latex type and characteristics, and preparation and storage conditions has been examined. The polymer molecular weight, particle size, and surface-change density did not appear to exert any influence, within experimental error. However, the time-dependent permeability behavior was affected by the film preparation and storage temperatures.     

Skim and cream natural rubber particles: colloidal properties, coalescence and film formation

Cream and skim fractions of freshly tapped natural rubber latex have been studied using atomic force microscopy and scanning electric potential microscopy to elucidate the topology and charge properties in film formation. Elemental distribution maps of the particles have also been obtained using electron energy-loss imaging in a low-energy transmission electron microscope. The two rubber fractions are obtained by centrifugation. The cream fraction is stable while rapid coagulation occurs in the skim fraction. After removal of the coagulum, no further coagulation occurs and the remaining skim rubber particles are stable. The rubber particles from the cream rubber particles contain higher amount of adsorbed protein-phospholipid materials compared to those in the "self-cleaned" skim fraction. This difference in membrane property has a significant impact on the spreading of the cis-1,4-polyisoprene cores, their coalescence and film formation behavior. Coalescence of cream particles appears to be hindered by the membrane materials, forming a rough film that retains the topology of individual particles. Skim particles coalesce more readily, forming relatively smooth films.     

Measurement of interaction forces between lignin and cellulose as a function of aqueous electrolyte solution conditions

The interaction between a lignin film and a cellulose sphere has been measured using the colloidal probe force technique as a function of aqueous electrolyte solution conditions. The lignin film was first studied for its roughness and stability using atomic force microscopy imaging and quartz crystal microbalance measurements, respectively. The film was found to be smooth and stable in the pH range of 3.5-9 and in ionic strengths up to and including 0.01 M. This range of ionic strength and pH was hence used to measure the surface force profiles between lignin and cellulose. Under these solution conditions, the measured forces behaved according to DLVO theory. The force-distance curves could be fitted between the limits of constant charge and constant potential, and the surface potential of the lignin films was determined as a function of pH. At a pH greater than 9.5, a short range steric repulsion was observed, indicating that the film was swelling to a large extent but did not dissolve. Thus, lignin films prepared in this manner are suitable for a range of surface force studies.     

Interaction of toluene,hexadecane, and water with the surfaces of random copolymers of styrene and butadiene

Molecular dynamics simulations have been used to study the interaction of toluene, hexadecane, and water with the surfaces of thin films of a random copolymer of styrene and butadiene (SB) and a random copolymer of styrene, butadiene, and acrylonitrile (SBA). Qualitatively different behavior is seen with the three small molecules. The two hydrocarbons remain on the thin films of the copolymers, but water has little affinity for the surface. Toluene has a stronger tendency to penetrate into the film than does hexadecane, which remains at the surface of the thin film. The attractive interaction of the hydrocarbons with the surfaces is dominated by van der Waals interactions. The energy of the interaction of toluene with SB is slightly smaller than the heat of vaporization of toluene at the temperature of the simulation. When expressed on a weight basis, the interaction of toluene with SB is larger than the interaction of hexadecane with the same surface. Hexadecane interacts slightly more strongly with SB than with SBA.     

Hydrodynamics of dielectric fluid films

We introduce a general hydrodynamic model to study the stability of lipid films against thermal fluctuations. As one novel aspect the model accounts before all for a complete intrinsic surface rheology of the film interfaces. Thus the rheological behaviour of the surface adsorbed lipids is modelled which screen the hydrophobic film interior against the aqueous exterior. For coloured films we demonstrate first the influence of electrical forces on the dynamics and film stability. For that we perform a linear stability analysis on a simplified mechanically symmetric film with i) symmetric surface charge distribution and ii) linear electric potential drop across the film. Based on the complete film model we then categorize the complete set of solutions of the linearized equations of motion and we study the growth rates of unstable film modes. Finally we discuss the stability properties of a black film after introducing a repulsive mechanism due to the steric hindrance of the interfacial lipids.     

Electrophoretic motion of a spherical particle in a converging-diverging nanotube

A charged spherical particle is concentrically positioned in a converging-diverging nanotube filled with an electrolyte solution, resulting in an electric double layer (EDL) forming around the particle's surface. In the presence of an axially applied electric field, the particle electrophoretically migrates along the axis of the nanotube due to the electrostatic and hydrodynamic forces acting on the particle. In contrast to a cylindrical nanotube with a constant cross-sectional area in which the electric field is almost uniform, the presence of a converging-diverging section in a nanotube alters the electric field, perturbs the charge distribution, and induces a pressure gradient and a recirculating flow that affect the electrostatic and hydrodynamic forces acting on both the particle and the fluid. Depending on the magnitude of the surface charge density along the nanotube's wall, the particle's electrophoretic motion may be significantly accelerated as the particle transverses the converging-diverging section. A continuum model consisting of the Nernst-Planck, Poisson, and Navier-Stokes equations for the ionic concentrations, electric potential, and flow field is implemented to compute the particle's velocity as a function of the particle's size, the nanotube's geometry, surface charges, electric field intensity, bulk electrolyte concentration, and the particle's location. When the particle is negatively charged and the wall of the nanotube is uncharged, the particle migrates in the direction opposite to that of the applied electric field and the presence of the converging-diverging section significantly accelerates the particle's motion. This, however, is not always true when the nanotube's wall is charged with the same sign as that of the particle. Once the ratio of the surface charge density of the nanotube's wall to that of the particle exceeds a certain value, the negatively charged particle will not translocate through the tube toward the anode and does not attain the maximum velocity at the throat of the converging-diverging section. One can envision such a device to be a nanofilter that allows molecules with surface charge densities much higher than that of the wall to go through the nanofilter, while preventing molecules with surface charge densities lower than that of the wall from passing through the nanofilter. The induced recirculating flow may be used to enhance mixing and to stretch, fold, and trap molecules in nanofluidic detectors and reactors.     

Electrophoretic motion of a spherical particle with a symmetric nonuniform surface charge distribution in a nanotube

The electrophoretic motion of a spherical nanoparticle, subject to an axial electric field in a nanotube filled with an electrolyte solution, has been investigated using a continuum theory, which consists of the Nernst-Planck equations for the ionic concentrations, the Poisson equation for the electric potential in the solution, and the Stokes equation for the hydrodynamic field. In particular, the effects of nonuniform surface charge distributions around the nanoparticle on its axial electrophoretic motion are examined with changes in the bulk electrolyte concentration and the surface charge of the tube's wall. A particle with a nonuniform charge distribution is shown to induce a corresponding complex ionic concentration field, which in turn influences the electric field and the fluid motion surrounding the particle and thus its electrophoretic velocity. As a result, contrary to the relatively simple dynamics of a particle with a uniform surface charge, dominated by the irradiating electrostatic force, that with a nonuniform surface charge distribution shows various intriguing behaviors due to the additional interplay of the nonuniform electro-osmotic effects.     

Preparation and characterization of emulsifier-free core–shell interpenetrating polymer network-fluorinated polyacrylate latex particles

The emulsifier-free core–shell interpenetrating polymer network (IPN) fluorinated polyacrylate latex particles with fluorine rich in shell were prepared by emulsifier-free seeded emulsion polymerization with water as the reaction medium. The fluorinated copolymer could be fixed on the particle surface due to the formation of interpenetrating polymer network. The resultant core–shell particles were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), dynamic light scattering (DLS) analysis, Fourier transform infrared (FTIR) spectrometry, X-ray photoelectron spectroscopy (XPS) analysis and thermogravimetric analysis (TGA). The core–shell particles possessed very narrow monomodal particle size distributions. XPS analysis of the latex film displayed that perfluoroalkyl groups had the tendency to enrich at surface and there was a gradient concentration of fluorine in the structure of the latex film from the film–air interface to the film–glass interface. In addition, compared with the latex film of crosslinked polyacrylate prepared under the same condition, the emulsifier-free core–shell IPN-fluorinated polyacrylate latex film showed better thermal stability, higher contact angle and lower water uptake.     

Preparation of epoxy-acrylic latex based on bisphenol F epoxy resin

Bisphenol F based epoxy-acrylic latex with different amount of epoxy resin was successfully prepared by semi-continuous seeded emulsion polymerization. The resulting composite latexes had a narrow size distribution of about 105 nm in diameter. The DSC result showed that the epoxy resin and polyacrylate were grafting copolymerization. The FTIR spectra showed that the epoxy group had been introduced into the epoxy acrylic latex system, and the composite latex could be crosslinked with epoxy hardener at room temperature. The crosslinked composite latex film exhibited a high Tg compared to epoxy-acrylic latexes. The surface of the films with the epoxy resin was regular, and diffused into the polyacrylate phase in the epoxy-acrylic latexes films. Since the curing reactions occurred before latex particle coalescence stage, the surfaces of the cured epoxy-acrylate latex films had a number of interface particle. Compared with the acrylic latex, the thermal stability of the epoxy-acrylate latex was increased, and the stability of the cured film increased with increasing epoxy content.     

The effect of metallic cations on the electrophoretic properties of a pretreatedHevea latex

High-ammonia latex concentrate prepared from doubly-centrifuged fieldHevea latex was exhaustively dialysed to remove any residual water-soluble non-rubber constituents. The electrophoretic mobilities of the dialysed latex in the presence of various metallic cations were investigated as a function of electrolyte concentration. The mobility decreased with increasing concentration of the cations Na⁺, Ba²⁺, Mg²⁺ and Ca²⁺ in a manner consistent with the effect of simple electrolyte on compression of the electric double layer. Anomalous behaviour was noted for the divalent ion copper, in that it reversed the charge of the latex particles at a concentration even lower than that of uranyl ions. Multivalent cations (lanthanum, cerium and thorium) had a profound influence on the latex particles where very low charge reversal concentrations were observed. It is believed that strong adsorption of hydrolysed species from the metallic ions was responsible for reversing the charge of the originally negative latex particles. These experiments indicated that the efficiency with which the cations reversed the charge of the latex particle surface was in the order: lanthanum > cerium > copper > thorium > uranyl > calcium > magnesium > barium > sodium. The number of cation binding sites on the latex particle surface and the chemical free energies of cation adsorption were calculated. It was found that the interaction of the latex particle with the hydrolysable metallic cations was much more stronger than that with the simple divalent cations and that this intercation was comparable to that of biological surfaces.     

Effect of cosolvent NMP on properties of cationic fluorocarbon emulsifier-free emulsion

Cationic fluorocarbon emulsifier-free emulsion was prepared via a novel method of adding organic cosolvent N-methylpyrrolidone (NMP), which was characteristic of the reaction transferring from solution to emulsion polymerization system. The emulsion stability was analyzed by the coagulation and precipitation ratio. The properties of particles were characterized by dynamic laser light scattering, surface potential particle size analyzer, and transmission electron microscopy, respectively. The latex film surface properties and immersion behaviors were investigated by contact angles. The results showed that, with NMP increasing from 5 to 30 %, the emulsion stability became better. Also, particle sizes decreased from 264.0 to 95.5 nm, and their distribution transformed from multidispersion to monodispersion. Zeta potentials increased gently. Meanwhile, the latex film surface performance was slightly improved, and the more the content of NMP, the less the influence of the cure temperature on surface property. There was less mobility of fluorinated groups in the films with more NMP used.

Influence of electrostatic and chemical heterogeneity on the electric-field-induced destabilization of thin liquid films

A numerical model for thin liquid film (<100 nm) drainage in the presence of an external electric field is developed. Long-wave theory is applied to approximate and simplify the governing equations. A spatiotemporal film morphology evolution equation thus obtained is then solved using a combination of finite difference to resolve the spatial dimensions and an adaptive time step ODE solver for the temporal propagation. The effect of fluid properties, namely, viscosity and surface tension, on the film drainage time is observed for a homogeneous electric field, which leads to random dewetting spots. Electrically heterogeneous fields, achieved by modeling electrodes with various periodic patterns, are explored to identify their effect on the drainage time and behavior. Finally, the chemical heterogeneity of the substrate is coupled with the periodic electric heterogeneity to understand the implications of combined heterogeneity. It is observed that the introduction of any heterogeneity results in faster drainage of the film when compared to that of the homogeneous field. In all cases, the thin film is drained, leaving submicrometer-scale structures at the interface. Well-controlled surface patterns are found on the application of periodic heterogeneity. This study effectively demonstrates the immense potential of electrically induced thin film drainage as a means for faster de-emulsification and for the creation of ordered submicrometer-scale surface patterns on soft materials.