A morphological and structural study of Ultem/P84 copolyimide dual-layer hollow fiber membranes with delamination-free morphology

We have studied dual-layer Ultem/P84 hollow fiber membranes with various morphologies by using dual coagulation baths and different spinneret designs in this work. The effects of first external coagulant and bore-fluid chemistry as well as air-gap distance on the outer and inner layer morphology of the dual-layer hollow fibers have been investigated systematically. It is found that dual-layer hollow fiber membranes spun with a longer air gap show a larger size closed-cell structure compared to those spun at a shorter air gap possibly due to the partial phase inversion induced by water vapor at 65% relative humidity. In addition, the outer layer of hollow fibers spun using water or methanol in the first coagulation bath shows mostly an open-cell structure, whereas those spun using ethanol or 2-propanol exhibit mostly a closed-cell structure. To fulfill the delamination-free requirement for an ideal dual-layer hollow fiber for pressure-driven separation processes, two novel methods have been proposed in this work: (1) the addition of aluminium oxide (Al₂O₃) nanoparticles in the inner layer followed by heat treatment; and (2) the introduction of early convective premixing with the aid of an indented and heated dual-layer spinneret. The first method has reduced the degree of shrinkage of the inner layer during heat treatment and thus lowers the heat-treatment temperature to avoid any delamination, e.g., from 175 °C for 1 h to 150 °C for 2 h. The second method facilitates interlayer molecular diffusion and thus eliminates delamination during the spinning process. No post-heat treatment is needed.