Vivek P. Khare

Macrovoid pore formation in dry-cast cellulose acetate membranes: buoyancy studies

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.

Investigation of the specific role of chemical structure on the material and permeation properties of ultrathin aromatic polyamides

Introduction of crosslinking into a polyamide structure results in significant improvement in the permeation performance of the corresponding thin film composite (TFC) membranes. In order to identify the source of this improvement, a series of measurements were conducted to ascertain the mechanical, structural, and chemical properties of two polyamides — polymetaphenylene trimesamide (a network polyamide), and polymetaphenylene isophthalamide (a linear polyamide). Permeation measurements revealed that the network polyamide evidences higher water flux as well as salt rejection compared to the linear polyamide. The higher water flux was attributed mainly to the higher hydrophilicity (arising from the hydrolysis of unreacted carboxylic acid groups in the structure) of the network polyamide. Hydrophilicity was found to overwhelm other factors such as the film thickness and chain mobility. On the other hand, the high rejection was found to be due to a combination of high hydrophilicity, greater negative charge, and greater rupture strength of the network polyamide.

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.

Development of A Technique for the In-Situ Measurement of the Mechanical Properties of Ultra-Thin Interfacially Polymerized Films

Crosslinked polyamide barrier layers made by a self-limiting interfacial polymerization (IP) process are extremely thin (= 1000 A), relatively defect-free and possess excellent perm selective properties which enable their commercial application in thin film composite (TFC) membranes. Little information has been reported in the open literature concerning the physical as well as mechanical properties of these polyamide IP films due to the substantial difficulties in making direct measurements on such thin crosslinked materials. Consequently, the development of IP-TFC membranes for commercial separations has been largely a trial-and-error process. We are developing a novel experimental technique, Pendant Drop Mechanical Analysis (PDMA) that utilizes the capabilities of the pendant-drop tensiometer for in-situ measurements of the mechanical behavior of the IP films. The current effort focuses on relating the mechanical behavior of the IP films to their structure, which in turn will be related to the permselective performance. Preliminary PDMA results indicate significant differences in the mechanical behavior of the IP films as a function of composition and contact time. These differences can be related to changes in the network characteristics.