Gas transport properties of aromatic polyamides containing adamantyl moiety
Graphical abstract
Introduction
Membrane-based gas separation has been established as a superior, economical and efficient separation technique over other conventional methods [1], [2]. However, trade-off between permeability (P) and selectivity (α) of the polymeric membranes showed in the Robeson upper-bound relationship renders the growth of this technique [3]. High-performance polymer membranes find various attractive applications such as removal of carbon dioxide from natural gas for fuel efficiency and reduction of the pipeline corrosion by CO2 contaminant in the natural gas [4]. Other important applications for the membrane based gas separation are removal of hydrogen from mixtures with nitrogen or hydrocarbons in petrochemical processing applications, separation of air for oxygen enrichment in the combustion processes and medical application or nitrogen enrichment for food packaging [5]. However, large material cost for high output applications, difficulty in attaining high product purity, and the limited thermal and chemical stability of many polymers are the major drawbacks that limited the full development of membrane based separations [6]. Thus, high permeability along with high gas selectivity for different gas pairs (e.g., CO2/CH4, O2/N2, etc.) and superior physical and mechanical properties for the polymer membranes are of prime interest. All these requirements led to an intense research in the development of new membranes for separation processes with the participation of both industrial and academic laboratories [3], [7] around the world. Gas permeation through a polymer membrane proceeds by a solution diffusion mechanism [8]. In this process gas molecules first get absorbed on the polymeric surface and then move through the polymer membrane by jumping through the adjacent holes (FFV) in the polymer due to the disruption of the chain packing. The jump occurs by the transient opening of the leap channels of effective size. So, by careful selection of the structural elements which helps to control the FFV, segmental mobility and the interaction with the gas molecules in the polymeric materials, it is possible to obtain effective gas separation.
Several classes of polymers such as polysulfones [9], poly(arylene ether)s [10], [11], modified celluslose [12], polyimides [13], [14], [15], poly(arylketone)s [16], polyamides [17], [18], [19] have been investigated for gas separation applications. The improvement of gas permeability and selectivity due to the presence of pendant bulky trifluoromethyl groups in the polymer main chain is a well-established fact and also reported in previous publications from our group [11], [13], [14], [18], [19]. Among the various classes of polymers studied, it has been observed that polyimides are the most investigated candidates as the materials for gas separation applications. High gas selectivity for different gas pairs (e.g., CO2/CH4, H2/CH4, H2/CO2, O2/N2, etc.) along with excellent thermal stability, chemical resistance, film forming ability, mechanical strength and versatile chemistry made polyimides promising materials for gas separation applications [13], [14], [15]. However, polyamides as membrane materials were not explored much for gas separation applications despite their outstanding physical and chemical properties, similar to polyimides. The insolubility of the polyamides in common organic solvents, mostly due to the presence of strong hydrogen bonding, made it difficult to convert them into the thin films that are required for membrane based separation applications. Also this high degree of packing and low polymer chain mobility restricts gas transport through these polyamide membranes. There are several approaches that have been adopted to overcome the solubility issue of polyamides. These are mostly related to reduction of the hydrogen bonding and restriction of their close packing by introducing bulky substituent, kinks and bends in the polymer backbone [17], [19]. All together, a structural element which hinders the polymer chain packing and consequently increases the FFV improves the gas permeability as well as selectivity [20].
It is reported that the incorporation of the adamantyl (tricylo[3.3.1.1]decane) moiety into polymer backbone improves the glass transition temperature (Tg), thermal stability, chain rigidity, and solubility in comparison to their un-substituted polymer analogs [21], [22], [23]. Modification of polymer backbone by incorporating bulky cardo group for gas separation application has been reported in the literature [9], [15], [18], [19], [20], [23]. In our earlier work we have reported improvement of gas transport properties, primarily the gas selectivity after the inclusion of aliphatic cardo cyclohexylidene moiety in the polyamide main chain [19]. Bandyopadhyay et al. synthesized polyamides with tert-butyl groups and have reported improved gas permeability [18]. Sen et al. reported improvement in the O2/N2 selectivity of polyimides with cardo spiro-biindane moiety [15]. Contreras et al. prepared polynorbornenes containing dicarboximide side groups with adamantyl moieties and reported high O2/N2 permselectivity [20]. Pixton et al. reported improvement in gas permeability for the polysulfones containing adamantyl connector compared to the bisphenol connector [9]. Also Maya et al. incorporated adamantyl ester as a pendant group in their copolyimides and have reported improved gas permeability due to the adamantyl group [23].
To the best of our knowledge, so far there is no report on polyamides with pendant adamantyl group for gas separation application. In the present work we have designed and prepared a series of new polyamides containing bulky adamantyl group. Therefore, a new diamine monomer was designed and synthesized that led to a series of new polyamides on reaction with different aromatic dicarboxylic acids. Accordingly, this work deals with the detailed synthesis and characterization of a new series of polyamides including their gas transport properties.
Section snippets
Starting materials
3,5-Dihydroxybenzoic acid (Merck), ethanol (Merck),1-adamantane methanol (Sigma Aldrich), N,N′-dicyclohexylcarbodiimide (DCC) (Sigma Aldrich), 4-dimethylaminopyridine (DMAP) (Sigma Aldrich), SnCl2·2H2O (Merck), 4,4′-(hexafluoroisopropylidene)bis(benzoic acid) (Sigma Aldrich), 5-tert-butyl-isophthalic acid (Sigma Aldrich), isophthalic acid (Sigma Aldrich), terephthalic acid (Sigma Aldrich), tetrakis (triphenyl phosphine) palladium (0) (99%) (Acros organics), triphenyl phosphite (TPP) (Merck),
Monomer synthesis
The diamine monomer, 1-adamantylmethyl[3,5-bis-{2′-trifluoromethyl-4′-(4″-aminophenyl) phenoxy}]benzoate (7) was prepared starting with ethyl 3,5-dihydroxy benzoate (2) and 4-fluoro-4′-nitro-3-trifluoromethyl-biphenyl (1) following the reaction as depicted in Scheme 1. The intermediate fluorinated dinitro compound (3) was obtained with good yield by the aromatic nucleophilic substitution (SNAr) reactions of 1 and 2. But, when we used 3,5-dihydroxybenzoic acid in place of ethyl 3,5-dihydroxy
Conclusion
A new diamine (7) monomer containing the adamantine moiety was synthesized with high purity. The diamine monomer was used to prepare a series of novel poly(ether amide)s with high yield and high molecular weight. The polymers showed good solubility in various organic solvents due to the presence of pendant adamantyl moiety and –CF3 groups. A combination of high thermal and mechanical properties made these polyamides suitable for investigation of their gas transport properties. The membranes
Acknowledgments
D. Bera acknowledges IIT Kharagpur for providing him a research fellowship to carry out this work. The authors thank AvH Foundation for donation of the GPC instrument used in this work and Department of Science and Technology (DST), India for financial support as project sponsor (Grant no. SR/S3/ME/0008/2010) for this work.
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