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In this paper we describe the development of a real-time, noninvasive technique using infrared (IR) thermography for measurement of the temperature of polymer solutions cast as thin films in which evaporation of a volatile solvent occurs. The technique requires the accurate determination of a single surface radiation property, the emittance, and its dependence on the casting solution composition. We report results obtained for the cellulose acetate (CA)-acetone system for which IR measurements indicate a relatively rapid and significant temperature decrease followed by a gradual temperature recovery toward the initial temperature. In general the thin film temperature response can be described by the parameters ΔT max , the maximum degree of cooling; t min , the time at which T min is reached; and the recovery temperature, T rec , at a time equal to 2 × t min . For 155 and 258 μm thick films evaporatively cast from a 15 wt% CA solution, IR thermography indicated maximum temperature decreases of 18 and 26°C, respectively. The characteristics of the thin film temperature curves for these two cases compare favorably with those predicted by a first-principles model recently developed by the authors. Based upon these results, the general applicability of the infrared technique for the study of dense film- and membrane-formation processes is described, and the significance of thin film temperature measurements with respect to membrane morphology is discussed.

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 from 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 sufficient 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.

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 heat-transfer 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 fomred in dry-cast membranes. A hypothesis for the formation of fingers based on the model predictions and experimental observations is proposed.

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

An inherent characteristic of many polymer-based systems used in membrane manufacture is the tendency to form numerous defects such as macrovoids and pinholes. Such defects are manifested as large scale surface pores. In this research note we discuss the potential use of electric fields to influence the mechanisms by which these defects are formed during the precipitation step in the wet-cast phase-inversion process for polymeric membrane formation. Several electrical field phenomena including surface-induced-polarization, electrohydrodynamic convection, and enhanced transport can potentially be used to alter polymeric membrane morphology if the time scale for these electric field effects is short enough. We report preliminary experimental results corresponding to the formation of tubular membranes from a polysulfone-polyvinylpyrrolidone blend in a N,N-dimethylacetamide solution precipitated in water under the influence of an a.c. electric field. This study suggests that the electric field effects can be operative on the short time scale of membrane formation. Based upon the statistically significant differences in the frequency distribution of defect number versus defect area which were obtained, the results indicate that the presence of a 2.2 kV/cm a.c. electric field decreases the average number density of defects while increasing the total defect area. These results suggest that further experiments with more sophisticated equipment would permit the electric field effects to be optimized in order to reduce the number and the size of the surface defects.