Membranes from rigid block hexafluoro copolyaramides: Effect of block lengths on gas permeation and ideal separation factors
Introduction
Gas separation by membranes has been the focus of attention for some time and it has been clearly identified that the ideal membrane should have high gas permeability and a high separation factor. However, it is well known that for membranes prepared by polymeric materials there exists a trade off since those membranes that present high permeability present a low separation factor while those presenting low permeability possess high separation factors. Therefore, there is still interest in developing materials that will present high selectivity for a specific gas or gas pair with a high permeability above the upper bond value as discussed by Robeson and others [1], [2], [3]. The relationship between gas transport properties and the structure of different polymers has been studied in a systematical way for several rigid aromatic polymer families such as polysulfones [4], [5], [6], polycarbonates [6], [7] polyesters [8], [9], [10], polyimides [11], polynorbornene dicarboxyimides [12], [13] and polyamides [14], [15], [16], [17] among others. In these highly aromatic polymers, it has been established that bulky pendent groups from the main chain will increase gas permeability while maintaining the selectivity for several particular gas pairs by hindering structural rotations in amorphous rigid polymers [18], [19]. For some time now copolymerization has been recognized as an important tool for the development of materials with specific characteristics [20], [21]. Copolymerization offers the opportunity to prepare polymeric materials with properties tailored in the desired direction. Thus, the introduction of an appropriate second monomer in the polymer, or sometimes a third one, has been used to tailor material properties such as degree of crystallinity, thermal stability, elastic modulus, glass transition temperature and gas transport and gas separation properties [14], [15], [20], [21], [22], [23], [24], [25].
In particular for gas transport and separation properties there have been reports on the effect of random copolymerization on gas transport properties of aromatic rigid copolyamides [14], [15] and copolyesters [8], [9]. Random copolymerization of rigid aromatic copolymers with two different monomers results in properties such as glass transition temperature, Tg, elastic modulus, E, permeability coefficients and gas separation factors that can be predicted with a simple mixing rule involving the comonomers concentration present in the particular copolymer. Gas permeability coefficients and ideal gas separation factors, in particular, follow the additive rule that depends on comonomer concentration in the particular copolymer. Thus, using the additive rule it is possible to determine gas permeability and separation factors in random copolymers. On the other hand, it is not possible to enhance selectivity for a given gas pair, since the ideal separation factors fall in between those of the homopolymers. It is also seen that the trade off, which implies that an increase of gas permeability gives place to a decrease in selectivity, is maintained. A possible option to achieve higher permeability coefficients with higher separation factors is the use of block copolymers since they present differences in the microstructure that could favor the permeation of one gas over the other, opening the opportunity to tune up gas permeation and gas separation by controlling the microstructure of the copolymer phases. Gas permeability measurements in block copolymers have been reported in polystyrene–polybutadiene block copolymers with an oriented lamellar structure by Csernica et al. [26] who found that gas permeability coefficients depend strongly on the direction of soft to rigid lamellar microstructure. In this arrangement permeation was well described by a three phase model that incorporates the interfacial regions of the block copolymer [27]. A similar behavior in poly(styrene-b-ethylene oxide-b-styrene) triblock copolymers was observed by Patel and Spontak [28]; they observed a high affinity to CO2 attributed to the ethylene oxide block presence. The affinity to CO2 was also found in some polymers with ethylene oxide blocks by Okamoto et al. in segmented polyether imides with soft and rigid microphases [29]. Kim et al. [30] and Bondar et al. [31] also determine the affinity of polyether segments in polyamide-b-ethylene oxide block copolymers PEBAX towards CO2. Recently, Reijerkerk et al. [32] reported the use of blends and copolymers containing PEO and its strong ability to interact with CO2 to increase the separation of this gas from other gases. It also appears that poly(ether-b-amide) thermoplastic elastomers decrease their gas permeability significantly, up to 3.5 times, upon uniaxial orientation due to phase orientation as reported by Armstong et al. [33].
In all cases the reported gas transport properties involve the presence of a soft phase in the block copolymer. For applications such as oxygen enriched air for combustion engines, or separation of combustion gases there are a couple of studies: one from cardo-copolybenzoxazol membranes [34], and one from rigid block copolymers [35]. The large rigid blocks and cardo moieties help to increase permeability with little or no loss in selectivity for gas separation. This could be an interesting route to increase the productivity of high temperature membranes without losing the permeability.
In this work we describe the preparation of three different rigid block copolyaramides bearing two bulky hexafluoro (–CF3) groups and a lateral tert-butyl group, (–C-(CH3)3), and the second block without the lateral tert-butyl group. The effect of block length, keeping the comonomer concentration constant, on thermal properties as well as gas permeability coefficients and gas separation factors is analyzed. The polyamide blocks are built from two similar aramides, the first one that is obtained from the combinations of 4,4′-(hexafluroisopropylidene) dianiline and 5-tert-butylisophthalic acid, HFA/TERT, while the second block is built from 4,4′-(hexafluoroisopropylindene) dianiline and isophthalic acid, HFA/ISO. The resulting block copolyamides named HFA/TERT-b-HFA/ISO were prepared by solution polycondensation in two steps in a 50% molar ratio of HFA/TERT to HFA/ISO with three different block lengths (9:9, 12:12 and 18:18). The block composition was regulated by a stoichiometric imbalance based on the Carothers equation [36].
Section snippets
Reagents
The monomers 4,4′-(hexafluoroisopylidene) dianiline (HFA), the aromatic dicarboxylic acids: isophthalic acid (ISO), and 5-tert-butylisophthalic acid (TERT) were purchased from Aldrich Chemical Co. and were used without further purification. The chemical structures of these monomers are shown in Table 1. Anhydrous calcium chloride (Baker) was dried under vacuum at 180 °C for 24 h before use. The reagents N-methyl-2-pyrrolidinone (NMP, 99.5% Aldrich), pyridine (Py, 99% Aldrich), triphenyl phosphite
Rigid block copolyaramides characterization
The block copolyaramides and their homopolymer aramide membrane films were cast by evaporation of DMAc solutions. The resulting transparent slightly yellow films were tough and rigid. Fig. 1 shows the structure of the block copolyaramides HFA/TERT-b-HFA/ISO. In all block copolymers, the actual molar concentration of the blocks HFA/TERT and HFA/ISO was kept constant, n=m, while each block was prepared with 9, 12 and 18 aramide repeating units and combined with the other block with the same
Conclusions
The preparation of polymeric membranes formed by blocks of rigid HFA copolyaramides, one of them bearing a bulky lateral tert-butyl group, with increasing block length was shown to be possible. The results from dynamic mechanical and thermal properties indicate that the desired blocky structure was obtained in the final membrane materials. It was also found that block copolyaramide membranes present a density that is quite similar but slightly lower, as the length of the blocks that form the
Acknowledgments
This research was conducted under Grant 83295 from CONACYT (México's National Council for Science and Technology). The authors wish to thank Dr. Patricia Quintana and M.C. Daniel Aguilar for their help with the X-ray diffraction measurements.
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