单分散聚苯乙烯乳液高温成膜过程的形态观察

单分散乳液指微粒具有相同化学组成、粒径及界面性质等特征的分散体系 ,因其颗粒均一 ,结构可调 ,赋予了其很多独特性质 ,广泛应用于计量、电子、生物、分析、医学、化工和信息等领域 .同时 ,单分散微粒体系作为研究原子或分子结晶过程的模型物 [1] ,在凝聚态物理中具有重要作用 .单分散乳液在一些条件下能排列成最大密堆积规整结构 ,从而赋予乳胶膜更优异的性能 [2 ] .因此 ,研究单分散乳液的成膜过程 ,在基础理论和实际应用中具有重要意义 .软的乳胶微粒玻璃化温度在室温附近 ,它们在常温下就可形变融合成膜 ,此时水蒸发速度较慢 ,乳液中…     

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.     

The influence of sodium ethene sulphonate comonomer on the film formation process of poly(vinyl acetate) dispersions

Various latex dispersions from vinyl acetate/sodium ethene sulphonate (sodium vinyl sulphonate) copolymers, stabilised by a constant amount of Hostapal BV, a surfactant with poly(ethylene oxide) groups, were investigated by a variety of solid and liquid state nuclear magnetic resonance methods. In order to investigate the influence of sodium ethene sulphonate on the film formation process, the serum and polymer were analysed separately. The stoichiometric monomer composition of the copolymer in the aqueous phase and in the hydrophobic particles was obtained. The ionic comonomer is enriched at the particle surface via its proximity to the applied surfactant by two-dimensional exchange NMR. For investigations of the film formation process, latex dispersions were prepared and dried to form spatially homogeneous films at different defined solid contents. Depending on the chemical composition of a chosen dispersion, NMR allows the investigation of the drying process of the water. The drying process is a function of the ionic strength of the dispersion and the hydrophilicity of the polymer. It is correlated to the drying mechanism of the water within the film. A not fully dried film contains external water outside the particles, water at ionic and non-ionic groups at surfactants in the polymer water interface and, additionally, water in the swollen and mobilised polymer. The distribution of water to these environments is markedly changed by the ionic comonomer, especially close to the end of the drying process.     

Ceramic encapsulated latex composites

This work reports the encapsulation of latex particles in Al2O3-polystyrene (PS) composite films. These films were prepared from PS particles in Al2O3 dispersion at room temperature in various latex contents. Composite films were annealed at elevated temperatures in 10 min time interval above the glass transition temperature (Tg) of polystyrene. Transmitted photon intensities, I(tr) were monitored after each annealing step. AFM micrographs were also used to observe the physical changes of the composite films during annealing. It was observed that latex particles are encapsulated above a critical Al2O3 content of 33 wt% which corresponds to the critical occupation probability of p(c) = 0.33 at which the film obey the site-percolation model with a critical exponent of 0.45. Below p(c), it was seen that complete latex film formation process took place, where transparency of the film was increased by annealing.     

Cracking in drying latex films

Thin films of latex dispersions containing particles of high glass transition temperature generally crack while drying under ambient conditions. Experiments with particles of varying radii focused on conditions for which capillary stresses normal to the film deform the particles elastically and generate tensile stresses in the plane of the film. Irrespective of the particle size, the drying film contained, simultaneously, domains consisting of a fluid dispersion, a fully dried packing of deformed spheres, and a close packed array saturated with water. Interestingly, films cast from dispersions containing 95-nm sized particles developed tensile stresses and ultimately became transparent even in the absence of water, indicating that van der Waals forces can deform the particles. Employing the stress-strain relation for a drying latex film along with the well-known Griffith's energy balance concept, we calculate the critical stress at cracking and the accompanying crack spacing, in general agreement with the observed values.     

Latex film formation studied with the atomic force microscope: Influence of aging and annealing

The surface structure of latex dispersion films was examined with an atomic force microscope. All measurements were done in air on latex films having a minimum film formation temperature of 12°C and a glass transition temperature of 18°C. One aim of this study was to follow structural changes during film formation. Three minutes after spreading the film, its surface layer dried. Afterwards, the structure of the film did not change anymore. Only after 4 months could structural changes be observed: Though individual latex particles could be identified, the particles partly melted into one another.After annealing films at 50° or 60°C for 4 h, the latex particles partly melted into one another, but individual particles could still be identified. When annealing at or above 80°C, no individual latex particles were visible anymore. With increasing temperature the film roughness decreased from 3 nm without annealing to 0.8 nm at 100°C annealing temperature. In addition, islands of 2–4 nm thickness appeared on the film surface. These islands could be scraped off the film by increasing the force between tip and sample, indicating that they are composed of surfactant which was squeezed out of the film.     

A model for surfactant distribution in latex coatings

The presence of surfactants in dried latex films can adversely affect the adhesive, water-resistant, and gloss properties, so investigating the surfactant distribution in latex coatings is of prime industrial relevance. Here we present a model that predicts the distribution of surfactant in a latex coating during the solvent evaporation stage. The conservation equation for surfactant during solvent evaporation is solved in the limit of infinite particle Peclet numbers, a dimensionless quantity giving the measure of relative magnitudes of evaporative to diffusive fluxes. A parametric analysis using the model reveals that the surfactant adsorption isotherm is the determining physical parameter. The model always predicts surfactant excesses at the top surface and either excess or depletion at the bottom surface depending on the isotherm. Uniform distributions are predicted for low surfactant Peclet numbers. Attenuated total reflection Fourier transform infrared spectroscopic probes on film surfaces conform to the behavior predicted by the model.     

Hollow‐particle latexes: Preparation and properties

Hollow‐particle latexes were prepared according to the following stages: (1) the preparation of the methyl methacrylate–methacrylic acid (MAA)–ethylene glycol dimethacrylate copolymer ( I ) latex, (2) the preparation of a shell ( II ) based on polystyrene or styrene–acrylonitrile–divinyl benzene copolymer polymerized onto copolymer ( I ) particles, and (3) the neutralization of the core ( I ) carboxyl groups with a base (NH₄OH or NaOH) at temperatures close to the glass‐transition temperature of the polymer ( II ). The neutralization resulted in the expansion of the particles and formed water‐filled hollow particles. The microspheres had an overall diameter of 460–650 nm and a hollow diameter of 300–450 nm. Rheological studies and particle size measurements by transmission electron microscopy and dynamic light scattering of the copolymer ( I ) latex indicate that the maximum particle swelling occurred at an approximately equimolar MAA/base ratio. It was found that even without the neutralization of the MAA units, a small hollow formation in the latex particles occurred during stage 2 because one volume of the copolymer ( I ) retained about 8 volume parts of water. It was also discovered that the final hollow‐particle geometry after neutralization depends on the shell copolymer thickness and type as well as on the conditions during stage 3, that is, the time, temperature, base type, and concentration. The opacifying ability of the synthesized hollow particles was investigated in latex coatings. The opacifying ability values were generally in agreement with the hollow‐particle geometry. The only exception was related to the copolymer ( I )/copolymer ( II ) ratio. The maximum hollow volume was obtained at this value equal to 1/8, whereas the highest opacifying ability was observed at 1/10. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 1435–1449, 2001     

Void closure and interdiffusion processes during latex film formation from surfactant-free polystyrene particles: a fluorescence study

This study reports a steady state fluorescence (SSF) technique for studying film formation from surfactant-free polystyrene (PS) latex particles. The latex films were prepared from pyrene (P)-labeled PS particles at room temperature and annealed at elevated temperatures for 5-, 10-, 20-, and 30-min time intervals above the glass transition (T(g)) temperature of polystyrene. During the annealing processes, the transparency of the film changed considerably. Scattered light (I(sc)) and fluorescence intensity (I(0P)) from P were measured after each annealing step to monitor the stages of film formation. Evolution of transparency of latex films were monitored using photon transmission intensity, I(tr). Scanning electron microscopy (SEM) was used to detect variation in the physical structure of annealed films. Onset temperature for film formation, T(0), void closure, T(v), and healing temperatures, T(h), were determined and corresponding activation energies were measured. Void closure and interdiffusion stages were modeled and related activation energies were determined.     

Modeling of drying in films of colloidal particles

The process of film formation on a solid substrate from polymer colloid dispersion during solvent evaporation has been investigated by means of the Monte Carlo simulation method. Colloid particles are modeled as hard spheres. Time evolution of the colloid density distribution and coverage of the solid substrate are studied. Both density and structure of colloid film is shown to depend strongly on the evaporation rate. At a low evaporation rate, the coexistence of hexagonal and tetragonal domains of dried colloid monolayer has been observed. The results of monolayer structure are in good agreement with the confocal scanning laser microscopy observations of Dullens et al. (2004).     

Drying dip-coated colloidal films

We present the results from a small-angle X-ray scattering (SAXS) study of lateral drying in thin films. The films, initially 10 μm thick, are cast by dip-coating a mica sheet in an aqueous silica dispersion (particle radius 8 nm, volume fraction ?(s) = 0.14). During evaporation, a drying front sweeps across the film. An X-ray beam is focused on a selected spot of the film, and SAXS patterns are recorded at regular time intervals. As the film evaporates, SAXS spectra measure the ordering of particles, their volume fraction, the film thickness, and the water content, and a video camera images the solid regions of the film, recognized through their scattering of light. We find that the colloidal dispersion is first concentrated to ?(s) = 0.3, where the silica particles begin to jam under the effect of their repulsive interactions. Then the particles aggregate until they form a cohesive wet solid at ?(s) = 0.68 ± 0.02. Further evaporation from the wet solid leads to evacuation of water from pores of the film but leaves a residual water fraction ?(w) = 0.16. The whole drying process is completed within 3 min. An important finding is that, in any spot (away from boundaries), the number of particles is conserved throughout this drying process, leading to the formation of a homogeneous deposit. This implies that no flow of particles occurs in our films during drying, a behavior distinct to that encountered in the iconic coffee-stain drying. It is argued that this type of evolution is associated with the formation of a transition region that propagates ahead of the drying front. In this region the gradient of osmotic pressure balances the drag force exerted on the particles by capillary flow toward the liquid-solid front.     

Glass transition and film formation of VeoVa/vinyl acetate latices; role of water and co-solvents

The physical forces causing deformation of latex particles during the film formation process have been witley studied. However, the forces resisting particle deformation are still poorly characterized. It is clear that the extent of particle deformation is dependent on the viscoelastic nature of the polymer. In an emulsion, the latex particles will normally contain water, surfactants and “free” monomers which lead to plasticization of the polymer. Although this effect has been recognized, so far it has been studied only on films that had been dried and then partially or completely swollen by water. In this work, plasticization of the emulsion polymers by water and co-solvent has been quantified via differential scanning calorimetry investigation directly on the aqueous latex dispersions. More specifically, the plasticizing effect of water on VeoVa/vinyl acetate copolymer latices and its influence on minimum film-forming temperature (MFFT) has been studied. A linear correlation has been found between T_g and MFFT for the wet latices. This new direct method should help to improve our understanding of the forces resisting latex film formation. Additionally, the homogeneous distribution of the hydrophobic and hydrophilic monomers (VeoVa and vinyl acetate respectively) in the latex particles was verified via a ¹³C-NMR (nuclear magnetic resonance) study performed directly on the latices. This study confirmed that no significant core/shell type of morphology had influenced latex film formation.     

Mechanical properties of films from polymer latices

The mechanical behavior of latex films is governed by their macromolecular nature as well as by their origin from particles dispersed in an aqueous medium. When monomers of different polarity are copolymerized in emulsion copolymerization, a heterogeneous distribution of the polar groups in the latex and the film can occur, owing to the different water solubilities of the comonomers. Films from these latices in many cases show a two-phase morphology, first, consisting of the main polymer within the particles and, second, a phase which is concentrated in the interphase between the original particles and which has a strong influence on the mechanical properties of the films. Films from latices with crosslinked particles behave like homogeneous networks in the linear viscoelastic range, i.e. at small strains. Structured networks are found when latex films are interparticularly crosslinked during or after film formation, e.g. by polar bifunctional monomers or metal salts. Tensile tests of films show that the mechanical strength of latex films develops in the last stage of film formation by interdiffusion and entaglement formation across particle boundaries.     

Latex Film Formation in the Environmental Scanning Electron Microscope

Summary: Environmental scanning electron microscopy (ESEM) was used to study the film formation mechanisms and extent of coalescence of three acrylic latex compositions with different glass transition temperatures (T_g), here defined as standard-low T_g, standard-high T_g (both carboxymethyl cellulose- stabilised) and novel (stabilised with a novel polysaccharide derived from agricultural waste). The ESEM analysis revealed that the microstructure of the standard – low-T_g system consisted of individual particles in dispersion and upon evaporation a continuous film formed, whereas in the case of the standard - high T_g latex particle deformation was not observed, but particle aggregation resulted in the formation of crystal-like structures that have formed via the formation of stacking faults. However, in the case of the novel system the microstructure consisted of individual particles and clusters and during evaporation a discontinuous film formed with voids present within its structure and some of the clusters accumulating on the surface of the specimens.     

Study of bulk morphology of polymer latex films using laser confocal fluorescence microscopy

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Particle adsorption in evaporating droplets of polymer latex dispersions on hydrophilic and hydrophobic surfaces

Stain patterns formed by drying up of droplets of polymer latex dispersion on hydrophilic and hydrophobic surfaces were examined in light of the mechanism of particle adsorption in evaporating droplets. On hydrophilic surfaces, the volume of droplets decreased with time, keeping the initial outline of contact area, and circular stain patterns were formed after the dry-up of droplets. By the microscopic observation of particles in the droplets, it was found that a large portion of the particles were forced to adsorb on the outline of the contact area where a microscopic thin water layer was formed because of hydrophilicity of the surface. On hydrophobic surfaces, on the other hand, the contact area of droplets decreased as evaporation proceeded, while no particle was adsorbed on the surface at the early stages. The particles in the droplets started to aggregate when the concentration of particles reached a critical value, and the aggregates adsorbed on the surface forming tiny spots after the dry-up. Time evolutions of contact angle, contact area and volume of the droplets were analyzed in light of differences in the adsorption mechanisms between hydrophilic and hydrophobic surfaces. Received: 14 January 1998 Accepted: 1 May 1998     

Copper in Acid Chloride Solutions: Electrochemical Behavior by Quartz Microgravimetry and Voltammetry

The cathodic deposition of copper on a gold electrode and its subsequent anodic dissolution in an acid chloride solution, where two successive stages of a one-electron transfer are distinguishable because of the stability of chloride complexes of Cu(I), is studied by voltammetry and quartz microgravimetry. The formation of a film of an intermediate compound of Cu(I) during the deposition and dissolution of copper is revealed experimentally. Techniques for identifying the intermediate solid species are suggested. During a cathodic polarization, a film of intermediate compound CuCl forms at a reduced concentration of chloride ions in the Cu(II)/Cu(I) process, while during the anodic dissolution of the copper deposit formed during the cathodic polarization the intermediates appear in the Cu(0)/Cu(I) process, the concentration of chloride ions notwithstanding. The change in the electrochemical behavior of the system caused by a decrease in the concentration of chloride ions is explained.     

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.     

The Film Formation of Polymer Particles in Drying Thin Films of Aqueous Acrylic Latices

The aim of this study is to determine the factors that contribute to the process of film formation of binder particles in drying aqueous dispersion coatings, based on acrylic polymers. It is known that concentrated latices of uniform size show iridescent, colored light patterns. These colors are caused by interparticle interference, and they are only present when the latex particles are ordered in a regular structure. The interparticle interference can be characterized by measuring the transmission as a function of wavelength of the incident light. It appeared that the changes of the interparticle interference of a drying latex film can be related to changes in the interparticle distance and displacement. It was also found that the interparticle distance becomes "negative" upon coalescence of the latex particles. This means that from this point on, the change in interparticle interference is directly related to the indentation or deformation of the latex particles. It became clear that the coalescence process differs from deformation mechanisms accepted in the literature. It seems that the deformation of the particles follows a biaxial mechanism. This means that the particles deform only in one direction, perpendicular to the film surface. Copyright 2000 Academic Press.     

Different types of water in the film formation process of latex dispersions as detected by solid-state nuclear magnetic resonance spectroscopy

 The properties of polymer films prepared from latex dispersions are influenced by the drying or film formation process. In order to investigate this process, various systems of aqueous latex dispersions were dried until a specific solid content was reached. The samples investigated were based on vinyl acetate, vinyl acetate/ethylene and pure acrylics employing different surfactants and polyelectrolytes as stabilisers of the dispersions. The role of water in these partially dried films was investigated using ¹H and ²H solid-state NMR spectroscopy. Different types of water could be distinguished in the spectra. The drying latex films were found to contain interfacial external water, water at ionic and nonionic groups at surfactants in the polymer/water interface and also water inside the swollen polymer. These different types of water were examined separately using various NMR techniques. Received: 22 October 1999/Accepted in revised form: 19 November 1999