During the last two decades, significant improvements in the performance of polymeric materials for gas separation membranes have been made[1–6], and understanding of the relationships between the polymer structure and gas transport properties of polymeric membranes has been greatly advanced[2, 3]. Despite these advantages and progresses, polymeric membranes are restricted by the trade-off trend between gas permeability and selectivity, as shown by Robeson. Most of the researchers have paid special attention to the relationship between polymer structure and gas separation properties in order to improve membrane performance both in permeance and selectivity.
The permeability of a polymeric membrane is mainly controlled by the chain mobility, the packing density, and the free volume of the polymer structure. The introduction of rigid fillers having particle size close to the characteristic size of the macromolecules which form the selective polymer film can be the best technique to improve gas permeability by inhibiting molecular chain packing and increasing free volume. Therefore, mixed matrix membranes (MMM) defined as the synergistic combination of organic polymers with inorganic nanofillers (both permeable and impermeable) dispersed at the nanometer level have been studied as an alternative approach to solve the trade-off problem of polymeric membranes in gas separation[8, 9].
Proper material selection for both the polymer matrix and the inorganic phase is important in the development of MMM. It has been found that polymer properties as well as inorganic phase properties affect mixed matrix membranes morphology and thus influence the separation performance. Compared to the pure polymer membranes, many polymer-inorganic MMM show higher permeabilities without sacrificing or even improve gas selectivity. Pinnau and He reported an unexpected increase of gas permeability without loss of gas selectivity in a series of high-free-volume glassy polymers whereby inorganic nonporous nanoparticles, such as fumed silica or carbon black, were incorporated into the polymeric matrix. Some experimental studies used carbon nanotubes as inorganic nanofillers to fabricate MMM, and an improvement of the gas permeability compared to the neat polymer membranes was observed. For instance, Kim et al. reported on the addition of carbon nanotubes (CNTs) to poly(imide siloxane) membranes which resulted in increased O2, N2, and CH4 permeability. Cong et al. prepared the brominated poly(2, 6-diphenyl-1,4-phenylene oxide) composite membrane with single-walled CNTs or MWCNTs and found that the low concentration of CNTs addition increases the gas permeability without sacrificing the selectivity. Weng et al. fabricated the MWCNTs/PBNPI membrane. In their results both the permeabilities and the selectivities of H2, CO2, and CH4 improved significantly at high MWCNTs concentrations (>5 wt.%). Based on these investigations, one can conclude that the interaction between polymer matrix and nanotubes may disrupt the polymer chain packing thus enhancing gas diffusion due to introducing more free volume voids between the polymer chains and nanoscale defects on the polymer/nanofillers interface.
Tailoring the free volume cavities by controlling the macromolecule's size and shape of the microporous polymer directly influences gas transport properties. In particular, a novel class of high free volume, glassy, ladder-type polymers, referred to as polymer of intrinsic microporosity (PIM), is a potential candidate for highly effective gas separation membranes comprising the capability for gas permeability and selectivity optimization by changing the polymer chain packing. McKeown et al.[17, 18] were the first to report this new class of rigid ladder-type polydioxanes containing highly contorted chains. Among these, PIM-1, containing the contorted spirobisindane unit, has attracted the most attention due to its relative ease of synthesizing high molecular weight polymers and the combining outstanding permeability with relatively moderate but technically attractive selectivity[19, 20], especially for O2/N2 and CO2/CH4 pairs, which shows the upper bound trade-off introduced by Robeson. However, a major drawback in the practical use of PIM-1 is the significant decay of its gas permeability with time.
In the present work, MMM were fabricated by loading of functionalized MWCNTs (f-MWCNTs) as an inorganic dispersed phase and PIM-1 as a polymer matrix. The MWCNTs were chosen because they had been proven to be promising nanofillers in tailoring polymeric material suited to be prescribed for application even at low incorporation. To the best of our knowledge, so far there is no literature available on using MWCNTs combined with PIM-1 as polymer matrix for gas separation. However, it is well documented that for sufficient enhancement of MMM performance, the dispersion of MWCNTs in the polymer matrix should be very fine, which means that the surface interaction between the filler and the polymer matrices should be strong. In response to that, the MWCNTs were functionalized with poly(ethylene glycol) (PEG) as a spacer via ‘grafting to’ method to facilitate their dispersion in the PIM-1 matrix. The as-prepared MMM were characterized for their morphology using scanning electron microscopy (SEM). Gas permeability, permselectivity, and long term membrane stability were studied by pure gas permeation measurements.