Effect of air-gap distance on the morphology and thermal properties of polyethersulfone hollow fibers

By using 30/70 polyethersulfone/NMP (N-methyl-2-pyrrolidone) solutions as an example, we have determined the role of air-gap distance on nascent fiber morphology, performance, and thermal properties. An increase in air-gap distance results in a hollow fiber with a less layer of fingerlike voids and a significant lower permeance. For the first time we have reported that the T_g of a dry-jet wet-spun fiber prepared from one-polymer/one-solvent systems is lower than that of a wet-spun fiber, and T_g decreases with an increase in air-gap distance. These interesting phenomena arise from the fact that different precipitation paths take place during the wet-spinning and dry-jet wet-spinning processes. Wet-spun fibers experience vigorous and almost instantaneous coagulations; it results in hollow fiber skins with a long-range random, unoriented chain entanglement, but loose structure. Dry-jet wet-spun fibers first go through a moisture-induced phase separation process and then a wet-phase inversion process; it results in external fiber skins with a short-range random, compact, and slightly oriented or stretched structure. As a result, the outskin of wet-spun fibers have a greater free volume and a higher first T_g than that of the dry-jet wet-spun ones. Both SEM (scanning electronic microscope) photomicrographs and DSC (differential scanning calorimeter) analyses support our conclusion.