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.