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…     

A model for the drying process during film formation in waterborne acrylic coatings

Establishing drying mechanisms during film formation in waterborne acrylic coatings is a technologically important problem, however complex, and still poorly understood. A model for the prediction of evaporation kinetics is proposed in this paper, where films are supposed to dry normally with respect to the film surface, and a drying front separates a top dry region from a bottom wet region. The model accounts for the competition between water evaporation and particle diffusion that determines the degree of vertical homogeneity, but also for the competition between water evaporation and particle deformation that ultimately establishes the rate-determining step in film formation processes. The model was validated by performing gravimetric water-loss experiments on latexes of acrylic polymers of various composition, various particle size and stabilizing systems, under different environmental temperatures and humidity, and various initial film thicknesses in order to evaluate the effect of the different factors that can in principle influence the film formation process.     

Time-resolved in situ small-angle X-ray scattering study of silica particle formation in nonionic water-in-oil microemulsions

The formation of silica particles by the ammonia-catalyzed hydrolysis of tetraethyl orthosilicate (TEOS) in the polyoxyethylene (5) nonylphenyl ether (NP-5)/cyclohexane/water microemulsion system was investigated by time-resolved small-angle X-ray scattering (SAXS). The SAXS data could be modeled as a combination of two species where one describes the silica-particle containing microemulsion droplets and the other the reverse droplets. The analysis allowed the determination of the evolution of the system of particles of silica and reverse droplets. A model of nucleation and growth of the silica particles is confirmed and the volume fraction versus time data for the silica particles is in agreement with first order kinetics with respect to TEOS concentration. Moreover to describe the long time evolution of the system, a correlation among the silica particles has been taken into account by introducing a structure factor with a local silica volume fraction eta = 0.1. This high local density is 2 orders of magnitude larger than the global silica fraction and can be explained in terms of depleting interaction.     

PEG和PVP改性溶胶-凝胶二氧化硅减反膜的对比

聚乙二醇(PEG)和聚乙烯吡咯烷酮(PVP)对溶胶-凝胶二氧化硅减反膜的结构和激光损伤阈值(LIDT)有不同的影响.动态光散射、透射电镜(TEM)以及小角X射线散射实验表明:PEG能够促进二氧化硅颗粒的生长并增加团簇生长的一致性.然而,在PVP改性的溶胶中,Si―OH与PVP之间氢键作用制约了二氧化硅颗粒的生长.此外,多重分形谱(MFS)分析表明,PEG可以提高氧化硅薄膜的均匀性,PVP却降低了薄膜的均匀性,因此,PEG改性的氧化硅薄膜的抗激光损伤性能增强,而PVP改性的氧化硅薄膜的抗激光损伤性能被削弱.固体29Si魔角旋转核磁共振(MASNMR)结果表明,PEG能够提高Si―O四面体缩聚程度并改善薄膜的表面分形结构,PVP则相反,导致了PEG改性的氧化硅薄膜与PVP改性的氧化硅薄膜具有不同的抗激光损伤性能.     

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.     

Drying Study of Siloxane-PPG Nanocomposites

This is a study of the structural transformations occurring in hybrid siloxane-polypropyleneglycol (PPG) nanocomposites, with different PPG molecular weight, along the drying process. The starting materials are wet gels obtained by the sol-gel procedure using as precursor the 3-(trietoxysilyl)propylisocyanate (IsoTrEOS) and polypropylenglycol bis(2-amino-propyl-ether) (NH₂-PPG-NH₂). The shrinkage and mass loss measurements were performed using a temperature-controlled chamber at 50°C. The nanostructural evolution of samples during drying was studied in situ by small angle x-ray scattering (SAXS). The experimental results demonstrate that the drying process is highly dependent on the molecular weight of polymer. After the initial drying stage, the progressive emptying of pores leads to the formation of a irregular drying front in gels prepared from PPG of high molecular weight, like 4000 g/mol. As a consequence, an increase of the SAXS intensity due to the increase of electronic density contrast between siloxane clusters and polymeric matrix is observed. For hybrids containing PPG of low molecular weight, the pore emptying process is fast, leading to a regular drying front, without isolated nanopockets of solvents. SAXS intensity curves exhibit a maximum, which was associated to the existence of spatial correlation of the silica clusters embedded in the polymeric matrix. The spatial correlation is preserved during drying. These results also reveal that the structural transformation during drying is governed by capillary forces and depends on the entanglement of polymer chains.     

Temperature and relative humidity dependency of film formation of polymeric latex dispersions

Thermogravimetric analysis and a synchrotron small-angle X-ray scattering technique were employed to characterize the structural evolution of a polymeric latex dispersion during the first three stages of film formation at different temperatures and relative humidities. Three intermediate stages were identified: (1) stage I*, (2) stage I**, and (3) stage II*. Stage I* is intermediate to the conventionally defined stages I and II, where latex particles began to crystallization. The change of drying temperature affects the location of the onset of ordering, whereas relative humidity does not. Stage I** is where the latex particles with their diffuse shell of counterions in the fcc structure are in contact with each other. The overlapping of these layers results in an acceleration of the lattice shrinkage due to a decrease of effective charges. Stage II* is where the latex particles, dried well above their T(g), are deformed and packed only partially during film formation due to incomplete evaporation of water in the latex film. This is because of a rapid deformation of the soft latex particles at the liquid/air interface so that a certain amount of water is unable to evaporate from the latex film effectively. For a latex dispersion dried at a temperature close to its minimum film formation temperature, the transition between stages II and III can be continuous because the latex particles deform at a much slower rate, providing sufficient surface area for water evaporation.     

Orientation of platelets in multilayered nanocomposite polymer films

The orientation of platelets in micro-meter-thick polymer-clay nanocomposite films was investigated with small-angle neutron scattering (SANS), small-angle X-ray scattering (SAXS), and wide-angle X-ray diffraction (WAXD). The films with various clay contents (15–60% by mass fraction) were prepared by a layer-by-layer approach from polymer-clay solutions that led to the formation of a high degree of orientation in both polymer and clay platelets. Shear-induced orientation of polymer-clay solutions is compared with the orientation of polymer-clay films. SANS, SAXS, and WAXD, with beam configurations in and perpendicular to the spread direction of the film, were used to determine the structure and orientation of platelets. In all films, the clay platelets oriented preferentially in the plane of the film. The observed differences in semidilute solutions, with clay surface normal parallel to the vorticity direction, versus bulk films and with clay surface normal parallel to the shear gradient direction at clay mass fractions of 40 and 60%, were attributed to the collapses of clay platelet during the drying process. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 3237–3248, 2003     

Slow drying of a spray of nanoparticles dispersion. In situ SAXS investigation

Nanoparticles confined in droplets of less than a picoliter are forced to organize in submicronic dry grains through solvent evaporation. The evolution of structures of the grains and the constituent nanoparticles during the slow drying process are investigated in situ with small-angle X-ray scattering (SAXS) for the first time. The scattering results have been explained on the basis of the equilibrium thermodynamics of the droplets in the drying tube. We demonstrate that this technique is really efficient in describing the internal arrangement of the nanoparticles inside the drying droplets. Distinction between an almost homogeneous repartition of the nanoparticles in droplets and formation of core shell like particles even in strongly polydispersed droplets can be made using SAXS.     

Effect of soluble polymer binder on particle distribution in a drying particulate coating

Soluble polymer is frequently added to inorganic particle suspensions to provide mechanical strength and adhesiveness to particulate coatings. To engineer coating microstructure, it is essential to understand how drying conditions and dispersion composition influence particle and polymer distribution in a drying coating. Here, a 1D model revealing the transient concentration profiles of particles and soluble polymer in a drying suspension is proposed. Sedimentation, evaporation and diffusion govern particle movement with the presence of soluble polymer influencing the evaporation rate and solution viscosity. Results are summarized in drying regime maps that predict particle accumulation at the free surface or near the substrate as conditions vary. Calculations and experiments based on a model system of poly(vinyl alcohol) (PVA), silica particles and water reveal that the addition of PVA slows the sedimentation and diffusion of the particles during drying such that accumulation of particles at the free surface is more likely.     

三甲基氯硅烷对纳米多孔二氧化硅薄膜的修饰

以正硅酸乙酯为先驱体,采用溶胶-凝胶法,结合旋转涂胶、超临界干燥工艺在硅片上制备了纳米多孔SiO2薄膜.用三甲基氯硅烷(TMCS)对该SiO2薄膜进行了表面修饰,采用FTIR、TG-DTA、AFM和椭偏仪等方法研究了TMCS修饰前后薄膜的结构、形貌、厚度与介电常数等性能.超临界干燥后的SiO2薄膜含有Si－O－Si与Si－OR结构,呈疏水性.在空气中250 ℃以上热处理后SiO2薄膜因含有Si－OH而呈吸水性. TMCS修饰后的SiO2薄膜在温度不高于450 ℃时可保持其疏水性和多孔结构. SiO2薄膜经TMCS修饰后基本粒子和孔隙尺寸增大,孔隙率提高,介电常数可降低至2.5以下.     

Fabrication of Hollow or Macroporous Silica Particles by Spray Drying of Colloidal Dispersion

In this study, hollow silica particles were fabricated by atomizing the dispersion of silica nanocolloids synthesized by modified Stober method and self-organization of the particles by spray drying technique. Rapid evaporation of the droplet containing the silica nanoparticles resulted in the formation of hollow microparticles at high evaporation temperature due to hydrodynamic instability of the droplet. Similar strategy was adopted for the fabrication of macroporous silica particles by the sol spray drying of the hetero-colloidal dispersion of polystyrene nanospheres and commercial silica nanoparticles. The morphologies of the porous particles were observed by scanning electron microscope with varying drying temperature. As a demonstrative purpose, the results using emulsion droplets as confining geometry was compared with the porous particles obtained from spray dryer. Collectively, spray drying was found to be more efficient manner to prepare the porous materials with continuous way in the view of production efficiency and time.     

Microstructure formation in dip-coated particulate films

The microstructure-dependence of dip-coated particulate thin films on the stability of an aqueous silica sol used as coating bath is studied. Different stability conditions are adjusted in the sol by changing electrolyte concentration and pH value. Care was taken to avoid pronounced aggregation of the particles before the coating process. The characterization of the stability behavior gives clear evidence of a non-DLVO contribution at low pH values that is attributed to hydration forces. Structural evolution of the particulate network during film formation is studied using a dialysis accumulation procedure. The viscosity of the accumulated sol is measured as a function of shear rate and related to the drying characteristic of the coating process. Atomic force microscopy (AFM), small-angle X-ray scattering (SAXS) and N2 sorption are used to obtain information on the surface and volume structure of the dip-coated films. The structure of coatings is found to distinctly vary with stability parameters. This is attributed to changing interactions during the first drying stage. Finally, the influence of coating structures on the light transmission properties is determined. A comparison between the extinction of the uncoated and the coated substrate revealed a difference of up to +/-50% in dependence on the microstructure.     

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.     

Pore-scale modelling and tomographic visualisation of drying in granular media

Spatio-temporal evolution of liquid phase clusters during drying of a granular medium (realised by random packing of cylindrical particles) has been investigated at the length-scale of individual pores. X-ray microtomography has been used to explicitly resolve the three-dimensional spatial distribution of the solid, liquid, and gas phases within the wet particle assemblies. The propagation of liquid menisci through the granular medium during drying was dynamically followed. The effect of contact angle on the degree of dispersion of the drying front has been studied by observing drying in a layer of untreated (hydrophilic) and silanised particles; the drying front was found to be sharper in the case of the silanised (less hydrophilic) particles. This observation was confirmed by direct numerical simulations of drying in a digitally encoded porous medium identical in structure to the experimental one. The simulations also revealed that the average gas-liquid interfacial area in a given porous microstructure strongly depends on the contact angle.     

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.     

Role of capillary stresses in film formation

Stresses generated during film formation were deduced from the deflection of a copper cantilever coated with a drying latex. Experiments with particles of varying radii and glass transition temperatures (Tg) focused on conditions for which capillary stresses normal to the film deform the particles to close the voids. Soft particles (low Tg) formed continuous films, but hard ones (high Tg) produced fascinating arrays of cracks. For both soft and rigid particles, the lateral stresses were tensile and scaled on the surface tension divided by the particle radius. Clearly, tensile stresses in the plane of the film responsible for cracking arise from the same capillary pressure that drives compression in the normal direction. Solving the model (Routh & Russel 1996, 1999) for lateral flow of the fluid dispersion prior to close packing and deformation of the solid beyond close packing yields volume fraction, film thickness, and stress profiles for comparison with observations for both film-forming and film-cracking cases.     

Transmission electron microscopy observations of composite polymer colloid nucleated by functionalized silica

In this work, functionalized nanometric silica particles were engaged in emulsion polymerization of ethyl acrylate. The morphological characterization of this composite material was performed by transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS). We were particularly interested in the state of encapsulation of the silica particles and their distribution in the latex film. Initialy, we successively studied both components of the composite: polymer beads and silica particles differ by their size and also by their contrast. In addition, it was possible to perfrom dark field TEM to study this system of two amorphous phases because their respective amorphous halos are not too close. Hence, we investigated the colloid material in aqueous media and after film formation. Although no ideal encapsulation is observed in the colloid in aqueous media, the distribution of silica in the latex film is good. SAXS results are in good agreement with TEM observations.     

Migration and precipitation of soluble species during drying of colloidal films

The evaporation-induced convection resulted in a transport of dissolved species, a water-soluble polymer (carboxymethylcellulose) and dissolved CaCO(3), to the drying front of silica and CaCO(3) dispersions where the material eventually precipitates. Scanning electron microscopy and chemical analysis showed that the concentration of carboxymethylcellulose, CMC, is highest in the centre of the dried silica film and decreases towards the perifery. The colloidal films of the monodisperse silica particles displayed a high degree of structural order even at high concentrations of the non-adsorbed polymer CMC, which suggests that any depletion induced interparticle attraction is insufficient to affect the assembly of the colloidal crystal. The CaCO(3) particles are slightly soluble and we found that rod-like crystals reprecipitated in the centre of the particle films on top of the polyacrylate-coated particles. Addition of CMC disturbs the formation of distinct crystal shapes which was attributed to a complexation of Ca(2+) in solution.     

Atypical Film-Forming Behavior of Soluble Tetra-3-Nitro-Substituted Copper Phthalocyanine

Thin films of metal phthalocyanines (MPc) are known to exhibit excellent physical properties but poorly controlled morphologies. Therefore, the present work seeks to understand the film growth mechanism of a model compound for potentially usable MPc, specifically, copper tetra(3-nitro-5-tert-butyl)phthalocyanine (CuPc*). The Langmuir-Schaefer (LS) technique was applied to prepare a series of CuPc* films under different processing conditions. The film growth was examined by Brewster angle microscopy (BAM) on the water surface and small-angle X-ray scattering (SAXS) from the solid films. Neutron reflectometry (NR) measurements of the water uptake into the films and computer simulation of hydrated CuPc* were performed to substantiate an idea of colloidal MPc-water aggregates as nanoscale precursors of smooth solid films. This idea appears fruitful in terms of materials chemistry.