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).     

Modeling of multilayer drying of polymer films

A model was developed to predict the drying behavior of multilayer polymer films on inert substrates. The model considers simultaneous heat and mass transfer controlled by complex thermodynamic and transport properties of polymer solutions. Key components of the model are the incorporation of the free volume theory to predict diffusivities in each polymer layer, the use of heat and mass transfer coefficients to describe complex transport phenomena in the gas phase, the incorporation of exact equilibrium boundary conditions at polymer–polymer interface, and the use of the Flory–Huggins theory to describe both liquid–liquid and vapor–liquid equilibria. The model can be applied to guide processing, product formulation, scale‐up, and oven design. As an example, the model is applied to simulate the drying of a two‐layer coating of poly vinyl acetate (in toluene) over polystyrene (also in toluene) on a polyester substrate. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1665–1675, 1999     

Electrochemically produced responsive hydrogel films: influence of added salt on thickness and morphology

We report on electrochemically prepared hydrogel layers of poly-N-isopropylacrylamide (pNIPAm) and on the influence that the supporting electrolyte has on their thickness and morphology. Ions that are destabilizing in the Hofmeister sense increase the thickness. The effect correlates well with the ion's tendency to lower the lower critical solution temperature (LCST) of pNIPAm films. AFM micrographs show small-scale globules. When the films were produced in the presence of a destabilizing salt (such as ammonium sulfate) one also observes larger features, resembling wrinkles. We attribute the globules to nucleated growth of surface-attached microgels, whereas the wrinkles presumably are produced by the collapse of hydrogen bubbles underneath a well-crosslinked film. Adding a chain transfer agent to the reactant solution reduces the lateral heterogeneities.     

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.     

Metal nanoparticles on polymer surfaces: 4. Preparation and structure of colloidal gold films

The process of the enlargement of gold hydrosol nanoparticles adsorbed on the surfaces of glassy polymers (polystyrene and poly(2-vinylpyridine)) in mixed aqueous solution of chloroauric acid and hydroxylamine is studied. It is established that the character of this process depends on the intensity of metal-polymer interaction and the density of nanoparticle packing in an initial monolayer. At a high coverage of a poly(2-vinylpyridine) surface by seeding gold particles, their rather uniform growth is observed, whereas, at low coverage, the enlargement of adsorbed particles, as well as the nucleation and growth of new particles take place. At the same time, new Au nanoparticles are not formed on the polystyrene surface in the enlargement process, even at low coverages by preliminarily deposited seeding hydrosol particles. Adsorbed gold particles can also be enlarged after their preliminary incorporation into the polystyrene surface layer. Such an incorporation (partial embedding) is ensured by the annealing of a system at a temperature between surface (T_g) and bulk glass transition temperatures. In this case, the T_g value can be considerably decreased (up to room temperature) by the addition of small amounts of a homologue with a much lower molecular mass in the polystyrene matrix. Lateral conductivity of colloidal Au films formed on a poly(2-vinylpyridine) surface by the enlargement of adsorbed seeding particles is measured. According to these measurements, contacts providing the formation of conductive channels are formed in the process of nanoparticle enlargement.__________Translated from Kolloidnyi Zhurnal, Vol. 67, No. 2, 2005, pp. 149–160.Original Russian Text Copyright © 2005 by Dementeva, Kartseva, Bolshakova, Vereshchagina, Ogarev, Kalinina, Rudoy.     

Comparison of USAXS information with RMSA prediction for the effect of added salt on colloidal crystal structure

Charged colloidal particles form so‐called colloidal crystals in dispersions because of electrostatic interparticle interactions between them. We performed a systematic and quantitative investigation of the effect of salt on the interparticle distance in colloidal crystals with the ultra‐small‐angle X‐ray scattering (USAXS) technique. We accumulated a great deal of significant information about the mechanism of the colloidal crystal formation by USAXS. We found that the interparticle distance in colloidal crystals shows a maximum as a function of the added salt concentration. The maximum position of the interparticle distance was located around κa = 1.3 (κ⁻¹ is the Debye length, and a is the particle radius). The behavior of the decrease of the interparticle distance with the increase of the added salt concentration at κa > 1.3 could be explained not quantitatively but qualitatively with the effective hard‐sphere theory. Thus, it was suggested that the dependence of the interparticle distance on the salt concentration at κa > 1.3 was reasonable according to the classical theories, such as the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. In this article, we advance this theoretical approach with the classical theory, that is, with the assumption of the only repulsive force for the interparticle interaction, to elucidate the origin of the curious behavior of the interparticle distance. We estimated the structure factor by the rescaled mean spherical approximation (RMSA) with a Yukawa‐type interparticle interaction potential. A comparison between the prediction of the RMSA data and USAXS experimental data was performed. In this analysis, the charge renormalization procedure was applied. Although the origin of the behavior of the interparticle distance at κa < 1.3 is still a mystery, we concluded that the behavior at κa > 1.3 could be interpreted as a result from the DLVO manner. The effective surface charge number was on the order of several percentages of the analytical surface charge number. This experimental fact certainly proves the validly of our USAXS observation quantitatively and also predicts the existence of novel factors for κa ≤ 1.3, the real origin of colloidal crystal formation, which have never been taken into account previously. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 78–90, 2001     

A linear dichroism study of colloidal silver in stretched polymer films

The visible absorption spectrum of silver colloids incorporated in poly vinyl alcohol films has been resolved into five Gaussian bands of different polarization. The energy positions and relative intensities of the bands can be explained using an exciton model in which dipole-dipole coupling occurs between silver particles in a linear chain arrangement. An important conclusion that stems from the analysis is that surface-enhanced Raman scattering occurs only when the exciting light is polarized parallel to the main axis of the linear chain colloidal aggregate.     

Autostratification in drying colloidal dispersions: experimental investigations

In films cast from a colloidal dispersion comprising two particle sizes, we experimentally examine the distribution of particles normal to the substrate. The particle concentrations at various positions in the film are determined through atomic force microscopy and NMR profiling. The results are compared to a previously derived diffusional model. Evidence for diffusional driven stratification is found, but the importance of other flows is also highlighted. The conditions that enhance particle stratification are found to be a colloidally stable dispersion, low initial volume fractions, a low concentration of the stratifying particle, and for the Peclet numbers of the two components to straddle unity.     

Temperature dependence of electro-optical characteristics of polymer dispersed liquid crystal films

Polymer dispersed liquid crystal (PDLC) films consist of microdroplets of a liquid crystal dispersed in a polymer matrix. Their applications are based on the electrically controllable light scattering properties of the liquid crystal droplets. The effects of temperature on the electro-optical properties of PDLC films have been rarely investigated. In this work, we studied the light transmission on varying the temperature and frequency. It was observed that the transmission at a fixed voltage decreased with increasing temperature above 43°C, independent of frequency. We examined possible origins of this unusual dependence of the transmission on the temperature. It was concluded that conductivity effects due to free ions newly created at high temperatures could be responsible for the unusual behaviour observed.     

Temperature dependence of electro-optical characteristics of polymer dispersed liquid crystal films

Polymer dispersed liquid crystal (PDLC) films consist of microdroplets of a liquid crystal dispersed in a polymer matrix. Their applications are based on the electrically controllable light scattering properties of the liquid crystal droplets. The effects of temperature on the electro-optical properties of PDLC films have been rarely investigated. In this work, we studied the light transmission on varying the temperature and frequency. It was observed that the transmission at a fixed voltage decreased with increasing temperature above 43°C, independent of frequency. We examined possible origins of this unusual dependence of the transmission on the temperature. It was concluded that conductivity effects due to free ions newly created at high temperatures could be responsible for the unusual behaviour observed.     

Influence of salt on colloidal lithography of albumin

We investigate the influence of salt on colloidal lithography of biomolecular patterns. Albumin labeled with fluorescein isothiocyanate (FITC) was adsorbed on polyelectrolyte-coated glass substrates covered by negatively charged colloids using fluorescence microscopy. After removing the colloids, a well-defined albumin pattern remains, and we study how the pattern changes upon adding salt to the protein solution. The proposed method is simple and cheap and can be used to create stable one- and two-dimensional biomolecular arrays.     

Auto-stratification in drying colloidal dispersions: a diffusive model

The mechanism by which the particles in a drying film come into close packing during solvent evaporation has an important role to play in the final film morphology. During drying the particles can develop non-uniform concentrations across the vertical height of the film, depending on their diffusion rate. By applying the principles of classical diffusion mechanics to a hard sphere system, a theory for this novel method of stratification during drying of a two component film has been derived. The model is dependent on the particle Peclet numbers and when one is above unity and the other below, maximum stratification is observed.     

Lattice Monte Carlo simulations of a charged polymer chain: effect of valence and concentration of the added salt

The configurational properties of a single polyelectrolyte chain accompanied by counterions and added salt are simulated using the cooperative motion algorithm on the face-centered cubic lattice. In particular, a greater emphasis is put on the effect of valence z(s) and concentration of the added positive (negative) salt ions n(s) on the polymer behavior. This is achieved by inspecting two families of systems with widely varying numbers n(s) of monovalent (z(s)=1) or multivalent (z(s)=4) salt ions at two fixed reduced temperatures T*=0.5, 1. The calculations indicate that especially at the lower temperature the addition of some amount of multivalent salt has a tremendous impact on chain conformations compared to the situation with monovalent salt. Even for relatively low concentrations of the former, the mean radius of gyration (1/2) and the mean end-to-end distance (1/2) decrease sharply, i.e., the polymer exists in strongly collapsed forms. This reduction of polymer size is also accompanied by a drop in the system inner energy e* and the effective mean charge per monomer q*. The analysis of various pair-correlation functions g(ab)(r) indicates that the latter effect-caused by condensation of ions onto the chain-is dominated by the multivalent ones. Furthermore, it is found that for z(s)=4, the uncondensed salt ions tend to group themselves into small clusters.     

Autostratification in drying colloidal dispersions: effect of particle interactions

When particles differing in size or charge are mixed and cast, vertical segregation is an inevitable phenomenon in the produced films. Apart from the Peclet number, which is the ratio of evaporation to diffusion rates, particle interactions play a crucial role in determining the distribution of particles in the dried films. Trueman et al. (1) developed a model for vertical segregation of particles during drying. Their numerical solution assumed that the chemical potentials were determined entirely by entropy. We report the effect of particle interactions in various systems: (i) charged particles with different Peclet numbers and (ii) charged particles with the same Peclet numbers. An experimental study has also been carried out for particles with Peclet numbers straddling unity; the experimental results conform with the behavior predicted theoretically.     

Plasmonic films based on colloidal lithography

This paper reviews recent advances in the field of plasmonic films fabricated by colloidal lithography. Compared with conventional lithography techniques such as electron beam lithography and focused ion beam lithography, the unconventional colloidal lithography technique with advantages of low-cost and high-throughput has made the fabrication process more efficient, and moreover brought out novel films that show remarkable surface plasmon features. These plasmonic films include those with nanohole arrays, nanovoid arrays and nanoshell arrays with precisely controlled shapes, sizes, and spacing. Based on these novel nanostructures, optical and sensing performances can be greatly enhanced. The introduction of colloidal lithography provides not only efficient fabrication processes but also plasmonic films with unique nanostructures, which are difficult to be fabricated by conventional lithography techniques.     

Heterogeneous diffusion in thin polymer films as observed by high-temperature single-molecule fluorescence microscopy

Single-molecule fluorescence microscopy was used to investigate the dynamics of perylene diimide (PDI) molecules in thin supported polystyrene (PS) films at temperatures up to 135 °C. Such high temperatures, so far unreached in single-molecule spectroscopy studies, were achieved using a custom-built setup which allows for restricting the heated mass to a minimum. This enables temperature-dependent single-molecule fluorescence studies of structural dynamics in the temperature range most relevant to the processing and to applications of thermoplastic materials. In order to ensure that polymer chains were relaxed, a molecular weight of 3000 g/mol, clearly below the entanglement length of PS, was chosen. We found significant heterogeneities in the motion of single PDI probe molecules near T(g). An analysis of the track radius of the recorded single-probe molecule tracks allowed for a distinction between mobile and immobile molecules. Up to the glass transition temperature in bulk, T(g,bulk), probe molecules were immobile; at temperatures higher than T(g,bulk) + 40 K, all probe molecules were mobile. In the range between 0 and 40 K above T(g,bulk) the fraction of mobile probe molecules strongly depends on film thickness. In 30-nm thin films mobility is observed at lower temperatures than in thick films. The fractions of mobile probe molecules were compared and rationalized using Monte Carlo random walk simulations. Results of these simulations indicate that the observed heterogeneities can be explained by a model which assumes a T(g) profile and an increased probability of probe molecules remaining at the surface, both effects caused by a density profile with decreasing polymer density at the polymer-air interface.     

Formation of ordered mesoporous films from in situ structure inversion of azo polymer colloidal arrays

This work shows that mesoporous polymeric films with spherical and elliptical pores can be obtained by in situ structure inversion of the azo polymer colloid arrays through selective interaction with solvent. The epoxy-based azo polymer contained both the pseudo-stilbene-type azo chromophores and the hydrophilic carboxyl groups. The colloidal spheres of the azo polymer were prepared by gradual hydrophobic aggregation of the polymeric chains in THF-H2O media, induced by a steady increase in the water content. Ordered 2D arrays of the hexagonally close-packed colloidal spheres were obtained by the vertical deposition method. After the solvent (THF) annealing, the ordered 2D arrays were directly transformed to mesoporous films through the sphere-pore inversion. Under the same condition, the 2D arrays composed of the ellipsoidal colloids, which were obtained by the irradiation of a polarized Ar+ laser beam on the colloidal sphere arrays, could be transformed to films with ordered elliptical pores. To our knowledge, this is the first example to demonstrate that mesoporous structures can be directly formed from the colloidal arrays of a homopolymer through structure inversion. This observation can shed new light on the nature of self-assembly processes and provide a feasible approach to fabricate mesoporous structures without the infiltration-removal step. By exploring the photoresponsive properties of the materials, mesoporous film with special pore structure and properties can be expected.     

Heterogeneous fluorine-containing surface macro-, micro- and nanostructures in polymer films and their applications

The methods of heterogeneous macro-, micro- and nanostructure formation on the surfaces of polymer films via their modification with gaseous fluorine have been considered. The successive fluorination and sulfonation of polyolefin films allows the synthesis of heterounit bi- and polyfunctional molecular fragments, thereby providing controlled chemical structuring at the nanolevel within a macromolecular segment. The combined use of the above methods of polymer-surface modification and intermediate film deformation under uniaxial tension results in the formation of surface microzones with fluorinated and sulfonated structures. Fluorinated and sulfonated macro- and microzones are formed via mechanical fracture of the fluorinated layer before sulfonation or via shielding of the polymer surface before sulfonation according to a given pattern through photolithographic methods. The practical applications of polymer films with heterogeneous surface structures are discussed.     

Conversion of colloidal aggregates into polymer networks on aging

After long-term aging, surfactant-mediated colloidal aggregates of sulfonated polyaniline (S-PANI) and poly(vinylidene fluoride) (PVF₂) converted into three-dimensional polymer networks, whereas colloidal crystals prepared from pure PVF₂ remained unaltered. A model, where the surfactant tails anchored from the colloidal particles interdigitate with time resulting in coalescence of the particles to form the network morphology, has been proposed. X-ray photoelectron spectroscopy (XPS) revealed higher relative abundances of carbon atoms on the surface of the polymer networks than those of the colloidal aggregates, which adequately supports the proposed model.     

Photoactive additives for cross-linking polymer films: Inhibition of dewetting in thin polymer films

In this report, we describe a versatile photochemical method for cross-linking polymer films and demonstrate that this method can be used to inhibit thin polymer films from dewetting. A bifunctional photoactive molecule featuring two benzophenone chromophores capable of abstracting hydrogen atoms from various donors, including C-H groups, is mixed into PS films. Upon exposure to UV light, the bis-benzophenone molecule cross-links the chains presumably by hydrogen abstraction followed by radical recombination. Photoinduced cross-linking is characterized by infrared spectroscopy and gel permeation chromatography. Optical and atomic force microscopy images show that photocrosslinked polystyrene (PS) thin films resist dewetting when heated above the glass transition temperature or exposed to solvent vapor. PS films are inhibited from dewetting on both solid and liquid substrates. The effectiveness of the method to inhibit dewetting is studied as a function of the ratio of cross-linker to macromolecule, duration of exposure to UV light, film thickness, the driving force for dewetting, and the thermodynamic nature of the substrate.