NAMS 2002 Workshop - ICOM 2008

Polyacrylonitrile (PAN) UF membranes are mainly used for water treatment, but
have also been applied as support for thin film composite SRNF membranes [1].
Despite their relatively good chemical stability, e.g. in hexane and toluene, PAN
membranes can not be used in more aggressive solvents such as DMF, DMSO
and THF. Moreover, due to the poor solubility of PAN in most solvents, it is
practically not feasible by phase-inversion only to reduce the pore size of PAN
membranes into the range needed for NF selectivity. Different modification
techniques, such as heat treatment in the presence of ZnCl2, and low
temperature plasma grafting of styrene, have been applied to transform PAN
based UF membranes in (SR)NF membranes [1]. These modifications did
however not imply chemical stability in aprotic solvents. Partial hydrolysis of the
nitrile groups of PAN membranes under alkaline conditions is frequently applied
to render the surface of the membranes hydrophilic and charged [4]. Membrane
Products Kiryat Weitzman patented a procedure for the synthesis of composite
SRNF membranes composed of an interfacially cross-linked top layer on top of a
PAN support which is on its turn cross-linked through immersion in a base,
followed by heat treatment [5]. These modified membranes clearly showed
improved stability in aprotic solvents.
The synthesis of partially hydrolyzed, asymmetric, purely PAN based SRNF
membranes and their use for selective separations in aprotic solvents has
however never been reported. In the presented work, PAN UF membranes were
prepared by casting DMSO/THF based PAN solutions and immersing the
obtained films in de-ionized water. These membranes were then immersed (1-60
min) in a concentrated (1-10 wt./vol.%) aqueous base solution (NaOH or
NaOCH3) at elevated temperatures (25-90°C). Hydrolysis resulted in a partial
conversion of the nitrile groups into carboxyl, amidine, acrylamide and other
functional groups [6], as observed via ATR-IR. This chemical change was
accompanied by an average decrease of the pore diameter in such a way that
selectivities in the NF range were obtained. While a minimal degree of crosslinking
was required for chemical stability in aprotic solvents, the membranes
completely dissolved in the base medium under more stringent hydrolysis
conditions due to their increased hydrophilicity. Effective cross-linking could thus
only be achieved in a relatively small stability window. Small dyes with MW
ranging from 300 to 1000 Da were successfully separated from DMF, NMP,
DMAc, DMSO and THF at high permeabilities. The cross-linked membranes
showed moreover excellent long-term stabilities.
References
[1] P. Vandezande et al., Chem. Soc. Rev. 2008, 37, 365.
[2] K. Vanherck et al., accept. for public. in J. Membr. Sci. (2008)
[3] Y.H. See Toh et al., J. Membr. Sci. 2007, 301, 3.
[4] Z. Wang et al., J. Membr. Sci. 2007, 304, 8.