Macrovoid growth during polymer membrane casting

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.

While the KC135 and VMFV experiments have provided new and intriguing data regarding the formation and growth of MVs, the results did not provide complete statistical evidence for the influence of solutocapillary convection on the MV growth process. Indeed, results from both the KC135 and VMFV experiments indicate that MV growth is a complex process and emphasize the need for additional studies that employ more sophisticated approaches. Given the unique potential advantages of the microgravity environment, we believe that the extended periods of low gravity available on the International Space Station would provide an appropriate environment for additional flight and VMFV experiments that could overcome many of the limitations inherent in the present study. Such experiments are planned over the next few years.