A model for wet-casting polymeric membranes incorporating nonequilibrium interfacial dynamics, vitrification and convection (original) (raw)
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Effect of evaporation step on macrovoid formation in wet-cast polymeric membranes
Journal of Membrane Science, 1994
A systematic study was carried out of the effect of the initial casting solution concentration, film thickness, and duration of the evaporation step on the occurrence and prominence of macrovoid pores in cellulose acetate membranes cast from acetone solutions via the dry/wet phase-inversion process. The duration of the evaporation step is shown to have a strong influence on macrovoid formation mainly through its influence on the solvent concentration gradient and casting solution viscosity at the instant of immersion into the precipitation bath. The results of this study are interpreted using predictive models for the evaporative casting process and for macrovoid formation.
Macrovoid Defect Growth during Evaporative Casting of Polymeric Membranes
2003
Macrovoid (MV) formation is a significant problem in evaporatively cast polymeric membranes. MVs are large, elongated or teardrop-shaped pores (~10−50 µm) that can impair membrane structural integrity. Although MVs have been extensively studied, there is no general agreement on the mechanisms governing MV growth. Recently, our research group has formulated the solutocapillary convection (SC) hypothesis, which contends that MV growth involves three principal forces: a Marangoni force generated by surfacetension gradients within the MV interface, a viscous drag force, and a gravitationally induced body force. Two sets of complementary experiments were conducted to test the SC hypothesis. Ground-based videomicroscopy flow-visualization (VMFV) was utilized to measure the flow velocities at the MV-casting solution interface and deep within the casting solution. The measurements were performed with casting solutions containing 10 wt% cellulose acetate (CA), 30 wt% H 2 O, 60 wt% acetone, and 200ppm TiO 2 particles for flow visualization, and the surface tension was controlled by surfactant addition. Qualitatively, the experiments indicated that MV growth occurs in three distinct phases: (1) a very rapid initial growth period, (2) a much slower growth phase, and (3) absorption of selected MVs into the expanding demixed region. The presence of tracer particles inside the MVs suggests the presence of a convective flow, which transfers the particles from the bulk solution to the MV interior. Although the VMFV experiments did not establish any surfactant effect on the interfacial velocities, a statistically significant effect on the MV number density was observed. In the second set of experiments, membranes were cast aboard a KC-135 aircraft under 0-g and 2-g conditions. Despite careful attention to the design and fabrication of the membrane casting apparatus (MCA), several problems were encountered, the most significant of which was the contamination of the casting solution by the activated carbon particles used for solvent absorption. Despite these difficulties, SEM analysis of uncontaminated membrane samples indicated that the MV morphology was strongly influenced by the solvent-nonsolvent ratio. However, dependence of MV size and number density on the magnitude of the buoyancy force could not be established since (in many cases) the MVs penetrated through the entire thickness of the cast membrane. Based upon the insights obtained from these experiments, a new MCA has been designed, which incorporates wider casting wells, deeper recesses for casting thicker membranes, and better isolation of the activated carbon. The new MCAs will be used in an upcoming KC-135 flight, and should enable complete quantitative evaluation of the SC hypothesis.
Dense polymer film and membrane formation via the dry-cast process Part I. Model development
1994
The dry-cast membrane-formation process is a major phase-inversion technique by which asymmetric membranes are manufactured. In this paper a fully predictive model which incorporates coupled heat and mass transfer is developed to describe the evaporation of both solvent and nonsolvent from an initially homogeneous polymer/solvent/nonsolvent system. This unsteady-state, one-dimensional, coupled heat-and mass-transport model allows for local film shrinkage owing to excess volume of mixing effects as well as evaporative solvent and nonsolvent loss. The model can predict composition paths into the ternary phase diagram which determine the onset of phase transition. The ternary phase diagram is predicted using the Flory-Huggins theory allowing for composition-dependent interaction parameters. The model is applied to the well-characterized cellulose acetate/acetone/water system for which suffrcient experimental data are available to permit determination of the friction coefficients in the ternary mass-transport model. The model is solved numerically using a software package based on the method of lines which is capable of handling moving boundary problems. The modeling studies indicate that for a given polymer/solvent/ nonsolvent/support system, the most influential parameters are the gas-phase mass transport, initial cast film thickness, and initial composition. Of particular importance, the model can predict the general morphological characteristics associated with the formation of dense polymer films and symmetric as well as asymmetric membranes.
Macrovoid growth during polymer membrane casting
2002
The Solutocapillary Convection (SC) hypothesis contends that macrovoid (MV) growth in dry-cast membranes is governed by a solutal-Marangoni convection-induced force caused by the rapid evaporation of volatile solvent from the liquid/gas interface, a viscous drag force, and a gravity-induced buoyancy force. Two different sets of experiments using the cellulose acetate-acetone-water system were conducted to test the SC hypothesis. Membranes were cast aboard a KC-135 aircraft that enabled short periods of microgravity (~0-g) as well as 2-g conditions. The studied process variables included the solvent/non-solvent (S:NS) ratio, surface tension, and the magnitude of the body force (buoyancy). SEM analysis of the resulting membrane morphologies indicated that the MV morphology was strongly influenced by the S:NS ratio. However, dependence of MV size and number density on the buoyancy force could not be established. In the second set of experiments, videomicroscopy flow-visualization (VMFV) was utilized to measure fluid velocities at the MV/casting-solution interface and in the bulk solution. The magnitude of the solutocapillary convection was controlled via surfactant additions. A comparison of the ratio of the edge to the bulk velocity for MVs made from surfactant-free and surfactant-containing casting solutions did not provide evidence of a statistically significant surfactant effect. However, the presence of the surfactant did affect the MV number density. In addition, the presence of tracer particles inside the MVs indicated that a convective flow enables their transfer from the bulk to the interior of the MV.
Journal of Membrane Science, 2006
A new model is developed for the evaporative or dry-casting process for polymeric membrane formation that incorporates convective transport owing to density changes; the latter effect has been ignored in all prior evaporative casting models. Densification inevitably occurs during the evaporative casting process owing to the removal of solvent that in turn causes the polymer molecules to assume a more
Journal of Membrane Science, 1994
In order to validate the dry-cast model developed by us, a multifaceted experimental approach was undertaken whereby three process variables can be followed independently in real time. Since it is extremely difficult to determine experimentally the concentration and temperature profiles within the cast polymer solution, experiments were designed so as to provide information on the coupled mass-and heattransfer processes by measuring other process variables. The experimental data-acquisition technique combined gravimetric, inframetric, and light-reflection analyses which provided information on the overall mass change, surface temperature, and the onset and duration of phase separation, respectively. Structural studies were conducted using scanning electron microscopy. These studies revealed that macrovoids or "fingers" can be formed in dry-cast membranes. A hypothesis for the formation of fingers based on the model predictions and experimental observations is proposed.
A diffusion-controlled model for wet-casting membrane formation
Journal of Membrane Science, 1991
A simplified analytical treatment of membrane formation mechanism, following the Paul method, is proposed using a casting solution immersed in a coagulation bath. This model assumes a discontinuous system consisting of parallel layers in the membrane with each layer proceeding rapidly to phase inversion. It allows the measurement of local composition variation at each layer depending on the flux ratio of solvent to nonsolvent. It is difficult to test the model unless a direct method of estimating the flux ratio at each layer is developed. However, some conclusions can be drawn about the applicability of the suggested model according to the type of membrane. For a homogeneous membrane, the flux ratio is independent of position. For an asymmetric membrane, the flux ratio has two values: the larger value for the skin layer and the other for the sublayer. Experimental results are compared with the values predicted by this pseudobinary mathematical model, and show fairly good agreement.
Modeling of asymmetric membrane formation by dry-casting method
Journal of Membrane Science, 2004
Many polymeric membranes are produced by phase inversion technique invented by Loeb and Sourirajan in 1962. The dry-casting method is one of the major phase inversion techniques in which a homogeneous polymer solution consisting of solvent(s) and nonsolvent(s) is cast on a support and then evaporation of the casting solution takes place under convective conditions. In this paper, we model membrane formation by the dry-casting method. The model takes into account film shrinkage, evaporative cooling, coupled heat, and mass transfer and incorporates practical and reliable diffusion theory as well as complex boundary conditions especially at the polymer solution/air interface. The predictions from the model provide composition paths, temperature, and thickness of the solution. By plotting the composition paths on the ternary phase diagram, we ascertain the general structural characteristics of the membranes prepared from particular casting conditions. The predictive ability of the model was evaluated by comparing the results with the experimental data obtained from gravimetric measurements for cellulose acetate (CA)-acetone-water system. In an attempt to illustrate the importance of diffusion formalism on the predictions, recently proposed multicomponent diffusion theory and its simplified forms were utilized in the model. The computational results show that the critical factor for capturing the accurate behavior of membrane formation is the diffusion formalism utilized in the model.
Macrovoid pore formation in dry-cast cellulose acetate membranes: buoyancy studies
Journal of Membrane Science, 2002
Experiments were conducted onboard a NASA KC-135 aircraft in order to assess the validity of two hypotheses proposed for the growth of macrovoid (MV) pores formed during the dry-casting of cellulose acetate (CA)/acetone/water casting solutions. The KC-135 aircraft provides the capability for greatly reducing the effective gravitational body forces that influence the buoyancy force on MVs. Buoyancy should have no effect on MV growth as proposed in the purely diffusive growth hypothesis but should influence MV growth via the solutocapillary convection hypothesis since the latter involves a balance between Marangoni, viscous drag, and buoyancy forces. CA membranes were cast in low-gravity (low-g) (KC-135) and normal-gravity (1-g) (ground-based control) from CA/acetone/water solutions as a function of the solvent/non-solvent (S/NS) ratio. Morphological analysis indicated that MV growth was enhanced in low-g only for the case in which the S/NS ratio = 2.0; no effect was observed for higher values of the S/NS ratio. These studies provide support for the solutocapillary convection hypothesis; however, the present data do not unambiguously demonstrate the occurrence of solutocapillary convection. Further research is required to provide such proof.