Dehydration of alcohols by pervaporation through polyimide Matrimid^® asymmetric hollow fibers with various modifications

Matrimid^® polyimide asymmetric hollow fibers have been fabricated and applied for pervaporation dehydration of isopropanol. The effectiveness of thermal annealing at high temperatures and/or chemical crosslinking using 1,3-propane diamine (PDA) on the separation property of these fibers has been investigated. It is found that an increase in the degree of cross-linking results in an increase in separation factor and a decrease in flux. Thermal annealing alone has failed to improve hollow fiber performance due to the cracks caused by inhomogeneous shrinkage in heating process. Nevertheless, appropriate application of thermal annealing as a pretreatment for crosslinking can produce fibers with the optimal performance. The pristine hollow fibers have a flux and a separation factor of 6.2 kg / m² h and 7.9, respectively; with this thermal treatment followed by crosslinking, the flux and separation factor change to 1.8 kg / m² h and 132, respectively. It is believed that the formation of charge transfer complexes (CTCs) within the polymer matrix during heat treatment not only assists polymeric chain packing and rigidification but also facilitates more efficient PDA crosslinking, thus results in higher size and shape discrimination in pervaporation. Apparently, PDA molecules could also seal the non-selective cracks (defects). These results indicate the combined thermal and chemical modification possibly is an effective method independent of the initial status of the hollow fiber (e.g. defective or defective free) in revitalizing and enhancing the membrane performance. XRD characterization confirms a tighter polymer networking in hollow fibers with the crosslinking modification. Comparison between the dehydration of different alcohols reveals that a better separation performance could be obtained for alcohols having a larger molecular cross-section.