Design of gas separation membranes derived of rigid aromatic polyimides. 1. Polymers from diamines containing di-tert-butyl side groups

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Abstract

A series of polyimides has been obtained from the experimental diamine 1,4-bis(4-aminophenoxy)2,5-di-tert-butylbenzene (TBAPB) and three commercial dianhydrides, i.e. pyromellitic dianhydride (PMDA), 3,3′,4,4′-byphenyltetracarboxylic dyanhidride (BPDA) and 4,4′-hexafluoroisopropyliden diphthalic anhydride (6FDA), following classical polyimidation methods. Analogous polyimides with diamines 2,2-bis(4-aminophenyl)hexafluoropropane (6FpDA) and 1,4-bis(4-aminophenoxy)benzene (APB) have been also prepared for comparative purposes. All polyimides showed high thermal stability, with decomposition temperatures above 490 °C, and glass transition temperatures higher than 270 °C. The rigid, rod-like structure of PMDA, combined with bulky diamines TBAPB and 6FpDA, yielded polyimides with high fractional free volume (FFV), close to that of 6FDA–6FpDA. The high FFV of PMDA–TBAPB contrasts with its structure that has fairly large degree of short range order as evidenced by X-ray diffraction. The gas permeation properties were greatly dependent on the dianhydride moiety and, as a rule, they compared well with those of polyimides used for gas separation. The polymers from PMDA, PMDA–TBAPB and PMDA–6FpDA, showed the best gas productivity values. By means of molecular modelling calculations, it has been observed that monomer TBAPB preferably adopts a contorted, rotation-restricted conformation, what can explain the special characteristics observed in the reported polyimides.

Research highlights

▶ Well-designed monomers with conveniently placed bulky groups enhance gas properties. ▶ Monomers with high rotational barriers enhance gas separation properties. ▶ Rod-like structure of PMDA–TBAPB produces very high FFV. ▶ PMDA–TBAPB polyimide displays gas permeabilities similar to those of 6FDA–6FpDA.

Introduction

Over the past decades, investigation on polymer membranes for gas separation has focused on aromatic, glassy polymers, and particularly on rigid, high glass transition temperature (Tg) polymers, such as polyimides [1], [2]. Aromatic polyimides have achieved growing importance as high performance materials, thanks to their excellent balance of mechanical and thermal properties; however, despite their undeniable favourable characteristics they did not deserve special attention as membrane-forming materials until the boost of thermoplastic and soluble polyimides, which took place in the eighties of the last century [3], [4]. In this regard, commercial and experimental dianhydrides and diamines of the most varied chemical structures have been used as condensation monomers to attain novel soluble aromatic polyimides [5], [6], [7], [8]. In this time, many soluble, processable polyimides have been reported and evaluated as materials in applications related with advanced technologies, but most of them have remained as experimental polymers, and only a very few have achieved technical significance [4], [9], [10].

Regarding gas permeability, penetration of small molecules is severely restricted through polymer matrixes with strong interchain attraction forces and high degree of molecular packing, as it is the case of wholly aromatic classical polyimides. This is especially true for commercial polyimides as Kapton® and Upilex®. The permeability of these materials to gases is rather low in comparison with other glassy polymers such as polycarbonates or polysulfones [11], [12], [13]. Much more favourable for this application are technical, soluble polyimides, such as Matrimid® and Ultem®, particularly Matrimid®, which offers permeability to oxygen about 2 barrers, and O2/N2 selectivity above 6 [14], [15].

Thus, much of the research work is being addressed to investigate new polyimides that exhibit higher permeability to specific gases without greatly impairing their inherent good selectivity. The challenge at this respect is to manipulate their chemical structure to achieve the most favourable balance of transport properties. The majority of the chemical modifications attempted have been directed to enhance diffusivity through an increment of the fractional free volume (FFV). In this regard, fluorine-containing polyimides, mainly those having the group hexafluoroisopropylidene, have reached special importance as they provide a favourable balance of permeability and selectivity [16], [17].

The most orthodox way to improve the gas separation properties of a polymer material is to increase FFV by introducing bulky groups and to increase at the same time the rigidity of the main chain, because the use of structures with high rigidity results in a strong size sieving ability. Thus, the approach followed in this work has consisted of using diamines with bulky side groups conveniently placed to produce an increase in both FFV and rigidity, improving the gas permeation properties. According to this premise, the diamine 1,4-bis(4-aminophenoxy) 2,5-di-tert-butylbenzene (TBAPB) should be a good candidate to develop membranes with both high permeability and selectivity. The presence of tert-butyl groups in ortho positions in the central ring should bring about both effects. The bulky and contorted conformation of diamine TBAPB is shown in Fig. 1. This work should provide new relationships to design and create new structures with tailor-made properties, decreasing the gap between classical materials and new ones, as are, for instance, the novel polymers with intrinsic microporosity (PIMs), ladder-like polymers with spiranic moieties and excellent gas separation properties [18], [19], [20], [21].

Taking into consideration these antecedents, TBAPB has been made to react with various commercially available aromatic dianhydrides to yield a series of polyimides having improved gas permeability and correct permselectivity. The election of dianhydride monomers has been done to achieve this goal at a maximal extent, without loss of the positive characteristics of polyimides, particularly without significantly lowering transition temperatures and mechanical properties. These polyimides have been reported by Liaw and Liaw [22] and Yang and Hsiao [23], but they showed only the effect of the diamine on mechanical and optical properties. Herein we examine the effect of both tert-butyl groups and dianhydride on the gas permeation, diffusion and separation performance. A comparison is also made of analogous polyimides derived from diamine 2,2-bis(4-aminophenyl)hexafluoropropane (6FpDA). Furthermore, a theoretical study has been carried out by molecular modelling to calculate properties directly related to permeability, such as torsional mobility and rotational energy barriers.

Section snippets

Materials

Pyromellitic dianhydride (PMDA, from Merck), 3,3′,4,4′-byphenyltetracarboxylic dyanhidride (BPDA, from Chriskev), 4,4′-hexafluoroisopropyliden diphthalic anhydride (6FDA, from Chriskev) and 2,2-bis(4-aminophenyl)hexafluoropropane (6FpDA, from Chriskev) were sublimated just before use. 1,4-Bis(4-aminophenoxy)benzene (APB, from Criskev) was purified by recrystallization from ethanol.

1,4-Bis(4-aminophenoxy)2,5-di-tert-butylbenzene (TBAPB) was synthesized in two steps, according to the previously

Results and discussion

As mentioned above, all the polyimides derived from diamines APB and TBAPB and also polyimide PMDA–6FpDA, were synthesized by a conventional two-stage procedure involving the formation of poly(amic acid) followed by thermal imidization. FTIR measurements were carried out to check the progress of the imidization. Fig. 3 shows the FTIR spectra of the polyimide films. The presence of characteristic imide group absorptions in the regions 1778 and 1720 (Cdouble bondO asym. and sym. stretching, respectively),

Conclusions

By introducing a bulky group conveniently placed in an aromatic diamine it has been possible to increase in a synergistic way the rigidity of the molecular chain and the fractional free volume of polyimides. The reaction of the bulky, contorted diamine TBAPB with rigid dianhydrides PMDA and 6FDA yielded polyimides with very high FFV, close to that of 6FDA–6FpDA.

The presence of tert-butyl groups in diamine TBAPB prevented the efficient packing of the polymer chains. Moreover, in PMDA–TBAPB, the

Acknowledgements

The Financial support provided by CSIC (Project PIE 200860I066) and Spanish Ministerio de Ciencia e Innovación (Project MAT-2007-62392 and program Consolider, project MULTICAT) is gratefully acknowledged.

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