Formation and characterization of hollow-fiber membranes for gas separation

The formation of asymmetric hollow fibers for applications as state-of-the-art gas separation membranes involves an understanding of how numerous processing variables affect a wide variety of essential morphological properties. Several of these morphological features are large in scale compared to those features controlling the separation of gaseous species. These large-scale properties are described in this work as macroscopic if they can be characterized using optical microscopy. Properties requiring scanning electron microscopy for sufficient characterization are described as microscopic.; Macroscopic and microscopic properties can be characterized quickly compared to the submicroscopic features controlling permeation; therefore, general frameworks aiding in establishing and controlling these large-scale features have been developed in this work. If these large-scale properties are not sufficiently controlled, several detrimental effects can occur, including fiber breaks, oscillations in fiber diameter, and large void regions in the microporous morphology (i.e., macrovoids). These detrimental features not only influence permeation properties but also mechanical integrity. Examples of catastrophic fiber deformation resulting from the implementation of excessive transmembrane pressure differences are provided.; The development of a practical framework for the establishment and control of submicroscopic features is currently inhibited by the lack of satisfactory characterization techniques. To characterize large surface areas of dehydrated fiber, permeation experiments analogous to actual membrane applications are often performed. This approach imposes rather large transmembrane pressure differences and as a result, the mean free path of gaseous species in a given asymmetric morphology can range from tens to hundreds of angstroms. By imposing relatively small transmembrane pressure differences, as demonstrated in this work, the mean free path is more uniform throughout a given asymmetric morphology. This novel approach serves as a first step toward understanding the fundamentals of gas permeation in these complex asymmetric morphologies. Moreover, it also aids in the development of permeation techniques to characterize the subtle submicroscopic differences, which control final membrane permeation properties.