Spin coating of thin and ultrathin polymer films

The spin coating of thin (> 200 nm thick) and ultrathin (< 200 nm thick) polymer films is examined in several solvents of varying volatility over a broad range of polymer solution concentrations and spin speeds. Experimentally measured film thicknesses are compared with a simple model proposed by Bornside, Macosko, and Scriven, which predicts film thickness based on the initial properties of the polymer solution, solvent, and spin speed. This model is found to predict film thickness values within 10% over the entire range of conditions explored, which gave film thicknesses from 10 nm to 33 μ:m. The model underpredicts film thickness for cases in which a very volatile solvent is used or the initial concentration of polymer is high, while overpredicting film thickness for cases in which a low volatility solvent is used or the initial polymer concentration is very low. These deviations are a consequence of how the model decouples fluid flow and solvent evaporation.     

Thin-Film-Composite Gas Separation Membranes: On the Dynamics of Thin Film Formation Mechanism On Porous Substrates

Abstract

A wide range of gas separations of interest in energy applications are carried out using membranes. Growing attention has been paid to the technology of making thin-film-composites(TFCs) membranes. Understanding the polymer solution and substrate property is key to successfully preparing TFCs membranes. This paper reports on some fundamental issues of coating hollow fibers with polymer from solution by dip coating. Polymeric porous hollow fibers with varying porosities and permeances were coated with polymer solutions of different viscosities in a continuous process. In addition, the fibers were coated dry and by presoaking in the coating solvent. It was found that the thickness of the coating on the low permeance/porosity/wet and dry fibers could be approximated by the Deryaguin model (h/R = 1.33 (Ca) ^0.67). For dry fibers, as the fiber porosity increased, the measured coating thickness was significantly underestimated by the Deryaguin equation. It is believed that the pores in the fiber allow rapid capillary suction of the solvent into the fiber walls and the bore, thus increasing the solution viscosity near the fiber wall, resulting in an increase in the coating thickness. Significant differences in the rate of solvent uptake were observed in these fibers by wicking experiments on a microbalance to support the above hypothesis.     

Optimization of operating conditions in a single‐zone drier for two‐layer polymer coatings

In industrial dryers, hot air is blown over wet coatings either from top or from both top and bottom sides to remove the solvent. Drying of multilayer coatings is a complicated process that involves simultaneous heat and mass transfer, transport, and thermodynamic behavior of polymer solutions. Airflow and its temperature are important operating conditions in a dryer. We report optimization of the operating conditions to minimize residual solvent without inducing blisters for two‐layer coatings using mathematical modeling. Our results reveal that optimal air flow on the bottom side of the coating is always greater than or equal to that on its top side—an agreement with the common notion of “backside drying.” Furthermore, the effect of coating thickness, residence time, individual layer thickness, initial solvent concentration, and reversal of the two layers on the optimal conditions is discussed. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Guidelines for dryer design based on results from non‐Fickian model

In polymer solution coatings below the glass transition temperature of the pure polymer, the coating can go undergo a glass transition and develop stresses during drying. When stresses develop, a non‐Fickian model accurately describes solvent mass transport in drying polymer coatings. The non‐Fickian model includes the solvent transport due to both stress and concentration gradients. This article presents a non‐Fickian model, which predicts a lower residual solvent than does the corresponding Fickian model. We showed in an earlier article that the non‐Fickian model predicts trapping skinning (higher residual solvent under more intense operating conditions) at higher drying gas‐flow rates. In this article, the non‐Fickian model was used to investigate how the gas‐flow rate, dry film thickness, and substrate thickness affect the residual solvent for a single‐zone dryer. This work recommends guidelines for choosing gas‐flow rates, gas temperatures, and substrate thickness to minimize the residual solvent. The model predictions show that, at any gas temperature, the residual solvent is minimum at an intermediate gas‐flow rate. The trapping skinning effect is less evident in thicker coatings and substrates. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 477–486, 2003     

Fluidized bed film coating of cohesive Geldart group C powders

The difficulty of coating cohesive Geldart group C powders in a conventional fluidized bed is attributed to strong inter-particle force between fine particles leading to poor fluidization behavior. Dry coating approach involving deposition of nanosize particles on the surface of group C powders is considered to reduce the interparticle force and improve the fluidization behavior of fine powders. Polymer film coating at an individual particle level is achieved on these pre-coated fine powders in a commercially available spouting fluidized bed (MiniGlatt). The effect of operating conditions such as inlet air temperature, polymer concentration, polymer weight ratio, water percentage in solvent and spray rate of coating solution on the quality of film coating are investigated. Experimental results demonstrate that the quality of film coating goes down as polymer concentration in coating solution goes higher, whereas the lower inlet air temperature is found to enhance polymer film generation and coating quality. It is also observed that the higher polymer weight ratio promotes agglomeration without affecting the coating quality to a great extent. An optimum water ratio in acetone-water solvent as well as spray rate can be optimized to achieve superior coating quality with acceptable agglomeration ratio.Graphical abstractDry coating approach involving deposition of nanosize particles on the surface of group C powders is considered to reduce the interparticle force and improve the fluidization behavior of fine powders. Polymer film coating at an individual particle level is achieved on these pre-coated fine powders in a commercially available spouting fluidized bed (MiniGlatt). The effect of operating conditions such as inlet air temperature, polymer concentration, polymer weight ratio, water percentage in solvent and spray rate of coating solution on the quality of film coating are investigated. Experimental results demonstrate that the quality of film coating goes down as polymer concentration in coating solution goes higher, whereas the lower inlet air temperature is found to enhance polymer film generation and coating quality.

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Figure: SEM images of Aluminum-1 particles coated to investigate the effect of polymer concentration. (a, b) Spray rate 2.11 ml/min, polymer concentration 4%, inlet air temperature 40 °C, polymer weight ratio 4% and water in solvent 0%; (c, d) spray rate 2.11 ml/min, polymer concentration 16%, inlet air temperature 40 °C, polymer weight ratio 4% and water in solvent 0%.     

Design of Binary Polymeric Coatings for Minimizing the Residual Solvent,Part I: Experimentation

Several binary polymeric coatings of poly(styrene)–tetrahydrofuran have been dried under quiescent drying conditions. Coatings are made using four different solutions of polymer having 5.01, 9.75, 10.06, and 15.14 wt% poly(styrene) initially. Experiments were performed for several different thicknesses for each polymer solution. Residual solvent remaining was lowest in the thicker coating. Results show that the thicker coating should be applied once rather than layer by layer to minimize the remaining residual solvent. The next layer is applied after complete drying of the previous layer. This practice should continue until the desired thickness is obtained. For instance, coatings of initial thickness 1010 µ m and final thickness 61 µm have residual solvent which is lower than the residual solvent in dried coatings of nearly the same final thicknesses using the layer-by-layer technique.     

Modeling,simulation and fabrication of coated structures using the dip coating technique

Using the dip coating technique, we fabricate erbia-coated quartz fibers and glass slides. Further we present a thin film model of the dip coating technique applied to the glass slides. The model includes evaporation of the solvent and a bulk reaction term to simulate the creation of the erbium oxide that forms the coating. Evolution equations for the solvent and coating thicknesses are developed and solved numerically. We study how the solvent evaporation rate, the bulk reaction rate, the dynamic viscosity, the angle of the inclined substrate, the characteristic depth of the solution thickness, the initial profile of the solution thickness, and the length of substrate affect the coating thickness. The model is calibrated using the experimental results to provide a tool for predicting the coating thickness.     

丙烯酸聚氨酯涂料成膜过程(Ⅰ)溶剂扩散系数的估算

采用傅里叶变换红外光谱（FTIR）测定了不同时间下丙烯酸聚氨酯涂料体系成膜聚合物的固化度及固化反应速率常数,并将该体系按二元聚合物-溶剂体系进行简化处理,在Vrentas-Duda自由体积理论模型的基础上,利用Di-Benedetto方程关联了溶剂的扩散系数与涂膜的固化度,建立了涂膜中溶剂扩散系数的预测模型.结果表明,本涂膜体系中溶剂扩散系数是溶剂浓度、干燥时间的函数.     

Spin and dip coating of light-emitting polymer solutions: Matching experiment with modelling

This paper reports experimental observations on spin and dip coating of light-emitting polymer (LEP) solutions where both the process conditions as well as the solution properties are factors influencing thickness and uniformity of thin LEP films. In terms of spin coating, which is a typical process for the manufacture of polymer light-emitting diodes (PLEDs), a number of process variables including spin speed were systematically explored. A matching series of dip-coating experiments was also carried out with the retraction speed as a primary variable. Modifications of existing models for both spin and dip coating were developed to include solvent evaporation and the effect of solution viscosity change during evaporation. Both models were found to give reasonable agreement with the major observed trends for final film thickness as a function of process conditions tested in this paper.     

Mathematical modelling of simultaneous solvent evaporation and chemical curing in thermoset coatings: A parameter study

A mathematical model, describing the curing behaviour of a two-component, solvent-based, thermoset coating, is used to conduct a parameter study. The model includes curing reactions, solvent intra-film diffusion and evaporation, film gelation, vitrification, and crosslinking. A case study with a polyisocyanate and a polyol in an organic solvent, catalysed by an organotin compound, is selected for analysis. The transient effects on reactant conversion values and solid content of the film of the following parameters are studied: initial solvent concentration and film thickness, wind velocity, and bulk air concentration of solvent. Simulations of solvent evaporation are compared to experimental data from a previous investigation. As part of the parameter study, mechanisms of this complex coating system are discussed.     

Polycarbonate adsorption on platinum: Thermal curing effects

Adsorption of polycarbonate (I) from dilute chloroform solution on to ‘clean’ platinum flags was investigated through the use of Iodide-125 radio tracer. Coverage versus concentration plots produced a logarithmic dependence of coverage on polymer concentration. A new method for clinging solvent removal was employed which increased the reproducibility of the coating. Both the effect of molecular weight and thermal stability of the polymer were investigated.     

Stress development in drying coatings

As a solvent‐cast polymeric coating dries, each part reaches a concentration at which it solidifies and develops elastic modulus. Thereafter, as further solvent departs, that part shrinks out‐of‐plane, but not in‐plane, if the coating adheres to its substrate. Hence, it develops in‐plane elastic stress. If the stress grows large enough, the stress‐free state may yield, which reduces the final stress level. A theoretical model of diffusion and mass transfer, large shrinkage‐induced deformation, and elastic stress, together with yielding and postyielding viscous deformation, was developed to predict stress evolution in one‐dimensional drying of polymer coatings. Concentration varies only perpendicularly to the substrate, the coating shrinks only in that direction, and the stress varies only in that direction but is in‐plane isotropic. The predictions are compared with measurements of evolving stress in various solvent‐cast polymer coatings and aqueous gelatin coatings by a cantilever‐deflection method. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 1000–1013, 2001     

Effect of Casting Solution on Morphology and Performance of PVDF Microfiltration Membranes

The effects of preparation‐influencing parameters such as polymer concentration, thickness of casting solution, and type of solvent on morphology and performance of poly(vinylidene difluoride) (PVDF) microfiltration membranes for the treatment of emulsified oily wastewater were investigated. Flat‐sheet membranes were prepared from a casting solution of polymer and additive in various solvents by immersing the prepared films in nonsolvent‐containing mixtures of water and 2‐propanol. The membranes were characterized using scanning electron microscopy. Increasing the polymer concentration and membrane thickness significantly affected the pore size, leading to permeate flux decrease. An attempt was made to correlate the effect of the solvent on membrane morphology and performance employing solubility parameters between solvent and nonsolvent).     

Formation and characterization of perfluorocyclobutyl polymer thin films

Perfluorocyclobutyl (PFCB) polymers are a new class of materials that show promise as selective layer materials in the development of composite membranes for gas separations, such as carbon dioxide/methane (α_{pure gas} = 38.6) and oxygen/nitrogen (α_{pure gas} = 4.8) separations. In many of the flat sheet applications, a thin film of the selective layer that is free of major defects must be coated onto a support membrane. A focus of this study was to elucidate the impacts of solvents, polymer concentration, and dip‐coating withdrawal speed on PFCB thin film thickness and uniformity. An extension was proposed to the Landau–Levich model to estimate the polymer film thickness. The results show that the extended model fits the thickness‐withdrawal speed data well above about 55 mm/min, but, at lower withdrawal speeds, the data deviated from the model. This deviation could be explained by the phenomenon of polymer surface excess. Static surface excesses of polymer solutions were estimated by applying the Gibbs adsorption equation using measured surface tension data. Prepared films were characterized by ellipsometry. Refractive index was found to increase with decreasing film thickness below about 50 nm, indicating densification of ultrathin films prepared from PFCB solutions below the overlap concentration. Atomic force microscopy was used to characterize surface morphologies. Films prepared from tetrahydrofuran and chloroform yielded uniform nanolayers. However, films prepared using acetone as solvent yielded a partial dewetting pattern, which could be explained by a surface depletion layer of pure solvent between the bulk PFCB/acetone solution and the substrate. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Über die strukturbildung in polymerfilmen durch phasentrennung bei der filmbildung aus lösungen in löser-nichtlöser-gemischen

Phase separation during film formation of a thermoplastic copolymer from solution in mixtures of high-volatile solvents and low-volatile non-solvents was studied. The parameters determining the void volume of the microcellular films formed by phase separation are the non-solvent fraction, polymer concentration of the initial solution and difference in volatility between solvent and non-solvent. The microcellular structure depends on the time at which phase separation occurs during film formation.     

Mass transport and modeling of emulsion phase-inversion coating of hydrogel beads

A new coating method named emulsion phase-inversion coating for preparing microcapsules containing hydrogel beads was reported in our previous work. In this paper, we suggest a numerical model for predicting the coating process. By defining suitable dimensionless parameters, the governing equation and its associated initial and boundary conditions are discretized using Euler's differential equation and the system of resulting differential equations is simultaneously solved using MATLAB for the dimensionless coating thickness and water content in beads. Parametric studies show that the dimensionless coating thickness depends on the dimensionless critical concentration, an internal property of the hydrogel beads, which can be calculated using the dimensionless final coating thickness. In addition, the apparent diffusion coefficient of water in coating layers can be easily obtained by fitting our experimental data to the simulated results. The comparison experiments show that there is a good agreement between the predicted and experimental coating thickness with the fitted apparent diffusion coefficient. The model also reveals that the apparent diffusion coefficient when using Polyacrylnitrile (PAN) is far smaller than that when using Polysulphone (PSf) as the coating polymer, resulting in a relatively thinner and more hydrophilic coating layer, which suggested that we could modulate the physiochemical properties of the coating layers by changing the coating polymer.     

Photocurable epoxy/cubic silsesquioxane hybrid materials for polythiourethane: Failure mechanism of adhesion under weathering

A new inorganic–organic hybrid coating containing epoxy‐functionalized cubic silsesquioxane (CSSQ) has been developed, which can be polymerized cationically by UV radiation. This solvent‐free solution can be used as hybrid coating for polythiourethane (PTU) substrate. The surface properties of the coating film were determined by adhesion and scratch resistance. The excellent adhesion of coating films on the substrate was observed at the initial stage before weathering, but deteriorated after exposure to the sunshine. The low viscosity of hybrid coating solution (～ 15 mPa s) leads to fast curing and the formation of hybrid coating film during the photopolymerization reaction. The adhesion failure was evaluated by atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FTIR), and X‐ray photoelectron spectroscopy (XPS) analyses. AFM images showed that the surface is smooth at the initial stage, but a texture surface was developed after weathering. The shrinkage of the hybrid film due to the increase in crosslinking density by postpolymerization would affect the surface roughness after weathering. XPS analysis indicated that the adhesion failure occurred by photodegradation of the PTU substrate during weathering. The weathering resistance was significantly improved by adding UV absorbers, which protected the polymer substrate from the photodegradation. The advantages of the hybrid coating include fast cure speed, solvent‐free formulation, and improved surface properties of the coating film. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Correlations for zero-shear and shear-rate dependent viscosity of polymeric solutions

Solution viscosity is known to be sensitive to the nature of solvent and the polymer concentration. The magnitude of such effects, however, is not quantitatively understood. Correlations to predict zero-shear and shear-rate dependent viscosity of polymer solutions from polymer solution parameters are presented. Polymer domain volume is found to be the controlling parameter for the zero-shear viscosity of dilute polymer solutions, that is solutions with insignificant entanglements. At higher concentrations, where polymer-polymer interactions become significant, it is found that, except for the magnitude of the solvent viscosity, the nature of the solvent is of no consequence. In this region zero-shear viscosity is correlated as a function of the parameter cM^5/8, where C and M represent concentration and molecular weight, respectively. A relaxation time such as Bueche's time constant satisfactorily represents the solvent effects on the non-Newtonian behavior of polymer solutions by incorporating the measured zero-shear and solvent viscosities.     

Ultra-high-modulus fibers from solution spinning

Ultra-high-modulus fibers such as Du Pont PRD-49 (initial modulus up to ～1000 g/den) and Monsanto X-500 (initial modulus up to ～600 g/den) are spun from solutions. Both polymers are characterized by a high intrinsic rigidity of individual molecular chains and considerable orientation along the fiber axis. The thermodynamics of solution for rigid and semirigid macromolecules is critically reviewed in order to illustrate conditions under which spontaneous formation of highly oriented fibers is expected. In the case of semirigid polymers, the free energy of (random) mixing pure solvent and parallellized polymer may, according to Flory, become positive for some critical value of a “flexibility parameter.” Formation of an ordered phase for semirigid polymers is not, however, observed by lowering temperature or increasing polymer concentration. In the case of rod-like polymers, still according to Flory, at some critical value of polymer concentration (which decreases with the axial ratio of the macromolecule) the isotropic solution of rods undergoes phase separation with formation of a partly ordered solution. This theoretical prediction is satisfactorily verified by data. While Du Pont fibers are spun from this anisotropic solution, Monsanto's X-500 only yields an isotropic solution at room temperature up to the limit of polymer concentration at which crystallization occurs. This inability of X-500 to form anisotropic solutions at the expected critical concentration is attributed to a partial degree of flexibility. Mechanical properties and orientation of fibers spun from the anisotropic solution appear to be superior to those obtained by spinning from isotropic solution, according to Du Pont's results. When a polymer has a partial degree of flexibility, alteration of physico-chemical variables such as solvent type, solvent composition, temperature, and polymer concentration may still be used in order to increase its rigidity. Theoretical arguments and data supporting this contention are discussed. Moreover, alteration of these variables may also be used to alter the crystallization temperature, allowing formation of the anisotropic solution to occur at a high enough polymer concentration. This expectation was verified in the case of X-500. Finally, the all important role of mechanical orientation of solutions is emphasized. According to Hermans, under high enough shear stress, the difference between the isotropic and the anisotropic solution vanishes. In line with these consideration, drawing techniques are particularly useful in order to achieve almost-perfect orientation and theoretical moduli.     

The effect of additives,solvent type,and polymer concentration on macromolecule dimensions

An equation is derived to estimate the hydrodynamic volume occupied by a typical linear polymer molecule, knowing only the polymer molecular weight and concentration of polymer in the solution. This equation may be applied to solutions above a minimum polymer concentration. Accuracy of the equation is determined by comparing it to an established method of calculating macromolecule dimensions. Above dilute solution conditions, the volume pervaded by a macromolecule is affected mainly by the polymer molecular weight and concentration of polymer in the solution. Chain dimensions are generally unaffected by the type of polymer, solvent, or additive in the solution.