Tailoring the molecular sieving properties and thermal stability of carbonized membranes containing polyhedral oligomeric silsesquioxane (POSS)-polyimide via the introduction of norbornene
Graphical abstract
Introduction
Polyimide has excellent mechanical strength and both heat and chemical resistance, and its rigid chemical structure is suitable for use in molecular separation membranes. Thus, polyimide membranes have already been used for a long time and in a variety of applications [1]. However, polyimide membranes continue to show low gas permeability by comparison with microporous inorganic membranes due to their dense polymer networks, and polyimide membranes have a problem with membrane degradation via plasticization [2]. Gas diffusion through a polymer membrane is considered a phenomenon whereby gas molecules move through a free volume that is not occupied by the van der Waals volume of the polymer chain [3,4]. Thus, gas permeability could be greatly improved by designing a polymer matrix with a chemical structure that would increase the free volume formed by thermal vibration, which would depend on the rigidity of the main chain and on the local thermal motion of the pendant unit.
The gas permeability through polyimide membranes has been improved with the introduction of cross-linking structures via UV cross-linking, thermal cross-linking, and chemical crosslinking [[5], [6], [7], [8]]. UV cross-linking of polyimide membranes derived from 3,3′4,4′-benzophenonetetracarboxylic dianhydride (BTDA) or BTDA/fluorinated hexafluoroisopropylidene-diphtalic anhydride (6FDA) with 2,4,6-trimethyl-1,3-phenylenediamine (DAM) have shown increased permselectivity due to the change in diffusivity rather than to the solubility of the coefficient [5]. Enhanced resistance to plasticization under high CO2 pressure and insolubility in strong solvents has been confirmed by thermal cross-linking of carboxylic acid containing a 6FDA-based polyimide membrane [6]. Thermal annealing of cross-linked polyimide membranes by ethylenediamine (EDA) also improved the plasticization resistance to higher CO2 pressures [7]. The coupling effects of EDA induced cross-linking and the thermal annealing accelerated the formation of charge transfer complexes (CTCs), which densified the polyimide structure and resulted in decreased permeability with increased CO2/CH4 selectivity [7]. Katrien et al. [8] reviewed the membrane properties of cross-linked polyimides via UV cross-linking, thermal cross-linking, and chemical cross-linking, and concluded that all of them were effective in improving not only the permselectivity in gas separation, pervaporation (PV), and nanofiltration (NF) but also in suppressing plasticization under high CO2 pressure. On the other hand, in many cases, the introduction of a cross-linking structure was not as effective for improving permeability due to densification of the polymer matrix.
Carbonization of a polyimide matrix was applied to tailor a microporous structure for gas separation [[9], [10], [11], [12], [13], [14]]. Carbonized molecular sieving (CMS) membranes derived from a 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA), 4,4′-oxydianiline (ODA) polyimide showed O2 permeance of 3.0 × 10−9 mol m−2 s−1 Pa−1 with an O2/N2 permeance ratio of 10 [10]. The separation properties of C2H4/C2H6 and C3H6/C3H8 were dramatically enhanced by carbonization of BPDA-pp’ODA polyimide [9,11] at 500–700 °C under N2, as were the hydroxyl functionalities of intrinsically microporous polyimides (PIM-6FDA-OH) [14] at 500–800 °C under N2. Studies have suggested that carbonization of a polyimide matrix is quite effective in improving membrane performance to a level that exceeds the upper boundaries of the trade-offs mentioned in the use of polyimide membranes [15].
Porosity-tailored CMS membranes derived from polyimide incorporated multi-wall carbon nanotubes (MWCNTs) and showed 2–4 times higher CO2 permeability than that without MWCNTs [16]. Also, carbon-segment formation by doping poly(p-phenylene oxide) (PPO) was effective in controlling the pore size and pore size distribution, which resulted in increases in H2 permeability ranging from 565 to 1448 Barrer and H2/N2 selectivity that ranged from 17 to 172 [17]. Since the microporous structure of a carbonized membrane depends on the selection of a polymer precursor, concentration of the casting solution, and the carbonization conditions (temperature, ramping rate, atmosphere, etc.) [18], much attention has been devoted to optimizing the preparation conditions.
Hybridization with porous materials (mixed matrix membranes (MMMs)) has also been extensively investigated to enhance polyimide membrane performance [19]. Metal-organic frameworks (MOFs) [[20], [21], [22], [23], [24]], graphene oxide (GO) [25], zeolite [23,26], and polyhedral oligomeric silsesquioxane (POSS) [27,28] have been utilized as dispersed porous media in incorporation with a polymer matrix. The incorporated porous nanoparticles establish a direct permeation path and/or modify the polymer matrix structure to increase the free volume. Although this innovative concept has drawn a great deal of attention, difficulties remain, and these include the weak interaction between a polymer matrix and porous nanoparticles and control of the dispersibility of nanoparticles within a thin polymer layer. Thus, chemically modified porous nanoparticles are desirable for homogeneous dispersion on a nanometer scale in a polymer matrix.
Polyhedral oligomeric silsesquioxane (POSS) has a cage structure consisting of a unit of (RSiO3/2)8 (R = H or organic groups) and a well-defined nanometer-sized structure, and is one of the most attractive nano building blocks (NBB) for use in the design of chemically incorporated porous nanocomposites [29,30]. POSS can be effectively incorporated into polymers by copolymerization and grafting [31,32]. Superior water flux in reverse osmosis (RO) desalination experiments using a 2000 ppm sodium chloride (NaCl) aqueous solution and compaction resistance is shown by homogeneous dispersions of POSS nanoparticles in a cellulose acetate (CA) membrane, which is accomplished by covalently linking a POSS nanoparticle pendant to a CA matrix [32]. No published studies, however, have yet described the gas permeation properties of POSS-Polyimide (PI) membranes.
In the present study, two types of POSS-polymers were utilized in the fabrication of gas-separation membranes. Fig. 1 shows the molecular structures of the POSS-polymers used in this work. One was POSS-polyimide-phenyl (POSS-PI-Ph) that consists of only polyimide in a polymer network, and the other was POSS-polyimide-phenyl-norbornene (POSS-PI-Ph-Norbornene) that consists of polyimide (main chain) and norbornene (pendant). Single-gas permeation properties were evaluated to discuss the effect that organic units exert on network pore size. Heat treatment was conducted for both types of membranes to evaluate the thermal stability and gas permeation properties of these POSS- derived carbonized membranes. The activation energy and gas-permeance ratios for these POSS-derived membranes were utilized to qualitatively discuss the gas-permeation properties, and the results using organosilica and polymer (polyimide, polyamide, polysulfone) membranes were compared.
Section snippets
Fabrication of POSS-derived polymer membranes
Samples of POSS-derived polymers (POSS-PI-Ph and POSS-PI-Ph-Norbornene) had a molecular weight of approximately 20,000 g mol−1, and were kindly supplied by NIPPON SHOKUBAI CO. LTD. The calculated weight percentages of polyimide-phenyl (PI-Ph) and POSS for POSS-PI-Ph polymers were 78 and 22%, respectively, while those of PI-Ph, POSS, and norbornene for POSS-PI-Ph-Norbornene polymers were 18, 22, and 60%, respectively. The detailed synthesis procedure of POSS-derived polymer is described in
Effect of heat treatment on the structure of POSS-derived gels/powders
Fig. 3 shows the TG curve under a He atmosphere of POSS-PI-Ph and POSS-PI-Ph- Norbornene gels. It should be noted that there was no appreciable weight loss for each sample at temperatures below 200 °C. POSS-PI-Ph showed no weight loss for temperatures ranging from 200 to 450 °C, and a dramatic weight loss of approximately 0.6 was observed between 450 and 550 °C. Residual weight was approximately constant at temperatures ranging from 550 to 1000 °C. When a norbornene unit was introduced into the
Conclusions
Two types of POSS polymers (POSS-PI-Ph and POSS-PI-Ph-Norbornene) were utilized for the fabrication of highly permeable gas-separation membranes. POSS-PI-Ph and POSS-PI-Ph-Norbornene separation layers for selective gas separation were successfully formed on a porous intermediate layer. Single-gas permeation properties were evaluated to discuss the effect of organic units on network pore size. POSS-derived membranes calcined at 250–350 °C under N2 showed approximately the same He/N2 and CO2/N2
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