Chapter 16
Permselectivities of Poly(amide imide)s and Similar Poly(ester imide)s as Dense Films and Thin-Film Composite Membranes Detlev Fritsch
Downloaded by COLUMBIA UNIV on June 23, 2013 | http://pubs.acs.org Publication Date: September 2, 1999 | doi: 10.1021/bk-1999-0733.ch016
Institut für Chemie, GKSS Forschungszentrum GmbH, Max-Planck-Strasse, D-21502 Geesthacht, Germany
Poly(amide imide)s (PAI's) are rarely tested as gas separation materials. We have recently discovered PAI's with and without 6F-containing groups which have reasonably high gas permeability and selectivity. To understand the influence on permselectivity of the amide and the ester bonds, PAI's and poly(ester imide)s (PEI's) differing only in this particular bond were synthesized, and their permeability and diffusivity to the gases He, H2, O , N , CO and CH was determined. As an additional variation in structure, 4 methyl-groups were introduced in the orthoposition to the ester bonds. The para-linked PAI exhibits lower permeability than the analogous meta-linked PAI but offers higher selectivity (e.g. about 40% increase for H /CH ). Gas permeability coefficients of PEI's are slightly higher than their PAI analogues. The introduction of 4 methyl-groups results in a marked increase in gas permeability (e.g. for O , permeability increases from 10.5 to 18.9 Barrer) and even a slight increase in selectivity. Due to the excellent film forming properties of the polymers, defect-free thin film composites with high fluxes and thicknesses as low as 200 nm were also prepared. 2
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Aromatic polyimides have been identified as interesting materials for gas separation applications (7-5). They combine high permeability at considerable selectivity with superior mechanical strength and long term durability even at elevated temperature. Within this class of polymers, however, permeability covers several decades. For example, in the case of oxygen, permeabilities from < 0.001 to > 100 Barrer have been reported. The highest permeability was observed in polymers with rigid structures bearing
© 1999 American Chemical Society
In Polymer Membranes for Gas and Vapor Separation; Freeman, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
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234 bulky alkyl substituents ortho to the imide ring. Additional functional groups such as ether, ester, and amide may be introduced to vary the properties. Poly(amide imide)s (PAFs) are rarely tested for their gas permeation properties (7, 3-7) even though they can be tailored to form competitive membrane materials (8). Some recently synthesized PAFs offer the chance to prepare similar poly(ester imide)s (PEFs) which differ only in this particular bond. The permeability and selectivity of these polymers to He, H2, O2, N , C 0 and CH4 permits a determination of the influence of this structural variable on gas transport properties. The basic gas permeation properties are not the only limiting factor in the use of polymers in gas separation. Film formation properties are as important as the gas permeability and selectivity of a polymer in determining its potential as a membrane material. Because the gas flux through a membrane depends most critically on thickness, the challenge is to make the membrane film - the active separation layer - as thin as possible while avoiding defects. The PAI and PEI polymers were, therefore, also tested for preparation and use as thin film composite membranes.
Downloaded by COLUMBIA UNIV on June 23, 2013 | http://pubs.acs.org Publication Date: September 2, 1999 | doi: 10.1021/bk-1999-0733.ch016
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Experimental Monomer and polymer syntheses of PAI were performed as reported previously (9). Diacids with preformed imide groups were reacted with diamines to yield the PAI in one step (9-77). The diphenol 2,2-bis(4-hydroxyphenyl)hexafluoropropane for the PEI synthesis was obtained from Aldrich. The analogous diol with 4 methyl groups ortho to the hydroxy moiety (2,2-bis(3,5-dimethyl-4-hydroxyphenyl)hexafluoropropane) was synthesized by condensing hexafluoroacetone hydrate with 2 moles of 2,6-dimethylphenol following a patent (72). PEFs were obtained from diacids and diols in dry N,Ndimethylformamide (DMF)/pyridine (1/1) and an excess (2.6 mole/mole diacid) of tosyl chloride (75). Reaction time was 3 to 5 h at 120 °C under a dry argon flux. Reduced viscosities (0.5% at 30°C in N-methylpyrrolidone (NMP)) of > 0.35 dL/g were required for the preparation of free-standing, non-brittle, methanol stable films. Solvent-free films about 30-90 pm thick were prepared (see ref. 8). Their gas permeation properties were measured with pure gases using a self-built vacuum time-lag apparatus. Permeate pressure increase with time was recorded at 30°C by two MKS baratron pressure sensors (10 mbar maximum (permeate), 5 bar maximum (feed)) which were connected to a computer. Feed pressure was varied from 0.2 to 1 bar. Permeate pressure was
