NAMS 2002 Workshop - ICOM 2008

Polymeric Membranes II – 2
Wednesday July 16, 10:15 AM-10:45 AM, Moloka’i
Dehydration of Alcohols By Pervaporation Through Polyimide Matrimid®
Asymmetric Hollow Fibers with Various Modifications
L. Jiang (Speaker), National University of Singapore, Singapore
T. Chung, National University of Singapore, Singapore - chencts@nus.edu.sg
R. Rajagopalan, National University of Singapore, Singapore
Membrane development involving material selection and membrane fabrication is
the heart for the successful development of pervaporation system applied in high
temperature and corrosive environment. Among various polymers applied,
polyimide is promising material that has already adopted by some commercial
fiber producers for gas separation due to its good thermal and chemical stability.
Nevertheless, intensive investigation of its asymmetric membrane, a more
favorable structure, for pervaporation application is quite limited.
In this study, 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, 3propane
diamine (PDA) on the separation property of these fibers has been
investigated. It is found that an increase in the cross-linking degree results in an
increase in separation factor and a decrease in flux. This mainly arises from the
restricted polymer chain mobility and redistributed free volume size and number
induced by the crosslinking process. XRD characterization confirms a tighter
polymer networking in hollow fibers with the crosslinking modification. 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. 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 fill up and
seal the non-selective cracks (defects). Experimental 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. Comparison
between the dehydration of different alcohols reveals that a better separation
performance could be obtained for alcohols having a larger molecular crosssection.