The trend of membrane structure evolution under shear and/or elongation flow fields of immersion precipitated spun tapes

The trend of membrane structure evolution under shear and/or elongation flow fields was investigated via immersion precipitated tape spinning of a model cellulose acetate (CA)/acetone/formamide/water precursor at shear rates of 231–1005 s−1 and/or draw ratios of 1–4. The effects of flow fields, tape thickness and material-based parameters were partitioned via comparing the tape structures with their corresponding cast membranes (relaxed flow fields effects), using scanning electron microscopy (SEM). The flow field application induced tape skin thinning, bulk macrovoid intensification, and microvoids size and ligament thickness increase. Nonetheless, it did not substantially change the macrovoids areal percent (MV%). In contrast, the MV% of the corresponding cast membranes was altered enormously (up to 50%) by their thickness decrease. The skin thicknesses of the cast and corresponding tape membranes were in the order of 1.1–7.2 and 0.25 μm, respectively. The membrane skin thickness decrease not only intensified macrovoid formation but also changed the sponge-like structure of some cast membranes into the finger-like structures in their corresponding tape spun membranes. The membrane precursor transition toward the two phase coexistent compositions of the CA/acetone/formamide phase diagram via diminution of solvent mixture quality raised its critical thickness (LC) of macrovoid elimination and its finger-like to sponge-like structure evolution. It also changed the trend of membrane structure evolution under the applied flow fields. The increase of spun tape membrane thickness lowered the flow field׳s contribution to the macrovoid formation while omitted them after crossing 3LC.