(646f) Characterization and Gas Separation Performance of Two High Performance Polyimide Blend Membranes

^a ProcESS - Process Engineering for Sustainable Systems, Department of Chemical Engineering, KU Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium

^b Department of Chemical Engineering, Chemical and Petroleum Engineering School, Shiraz University, Shiraz 71345, Iran

^c Department of Chemical and Biological Engineering, Princeton University, Princeton, New Jersey 08544, USA

Abstract

The emission of greenhouse gases, generated by human industrial activities, is perhaps the most prominent environmental problem today. In recent years, CO₂ has attracted much attention because it is the most important human induced greenhouse gas and is believed to be responsible for approximately 64% of the enhanced greenhouse effect. Carbon Capture and Sequestration (CCS) is considered as a promising way to reduce the CO₂ concentration in the atmosphere. According to the IEAâ??s 2 °C scenario report, the CO₂ emissions should be decreased to 14 billion tons per year in 2050, and CCS should be responsible for approximately 20% of this reduction [1].

Among various carbon capture technologies, polymeric membrane-based technology is a potential separation method due to small operation units, low energy consumption, low capital and operating costs, lack of mechanical complexity, environmentally friendly, and compact process design. However, the major drawback of membrane gas separation which limits its application is the low CO₂ concentration and pressure of the flue gas, which requires the use of membrane with high selectivity to meet the specification of the International Energy Agency, a CO₂ recovery and purity of around 80% [2].

Generally, properties of membrane desired in gas separation are high transport performance, high mechanical strength, strong thermal and chemical stability, plasticization resistance, commercial availability, reasonable cost, along with high permselectivity. In general, polymeric materials do not simultaneously meet all of these criteria. For example, highly permeable polymers exhibit moderate to low selectivity values while materials with high resistance to harsh chemical environments or plasticizing gases are either difficult to process or are very expensive [3].

In recent years, gas separation research has focused on aromatic, glassy polymers and particularly on rigid, polymers with high glass transition temperature (Tg). Within several classes of aromatic glassy polymers, polyimides have been attracting much attention due to their outstanding set of physiochemical properties including superior thermal stability, remarkable chemical resistance, excellent thermo-oxidative stability, and good mechanical toughness [4]. Due to these unique characteristics, they are widely used as adhesives, coatings, films, fibers, foams. However, due to their high crystallinity (brittleness) and high cost, only few of them (P84, Matrimid, Ultem) are suitable for membrane manufacturing.

Blending of polymers with different features is considered an applicable method to develop new polymer with desired properties [5]. Synergistic interaction of different polyimides can impart superior properties to the blend over the constituting polymers [6]. In the study of polymer blends, polymer-polymer miscibility is crucial because the new properties depend on the degree of miscibility of the polymers at a molecular level. It is well known that in most miscible polymer blends, specific interactions such as dipole-dipole forces, hydrogen bonding and charge transfer complexes are mainly responsible for the miscibility [7]. It is interesting to note that polymer blends are usually incompatible because of unfavorable interactions between polymer segments. The interest in the use of polymeric blends as gas separation membranes is associated with the possibility of gas transport management through the morphological control of the cross-section of the material, which is mainly dependent on blend components, phases and interface characteristics [8, 9]. In this work, the miscibility behavior and intermolecular interactions of two high performance polyimides in compositions of 100/0, 80/20, 50/50, 20/80, 0/100 have been evaluated. The polymer blend systems have been characterized by different analytical techniques including optical microscopy, Differential Scanning Calorimetry (DSC), Fourier Transform Infrared Spectroscopy (FTIR), X-ray Diffraction (XRD) and rheological measurements. DSC results of two blend systems showed the existence of a single glass transition temperature (Tg) in each composition, suggesting the miscibility of the blends. In order to study the specific interactions between polymer blend systems, the Tgs of the polymer blends were estimated by theoretical equations and compared with experimental data. The empirical Tg values exhibited a positive deviation from the linearity, indicating the presence of specific interactions between polymer chains; this was confirmed by FTIR spectra. The interactions between studied polymer systems and four aprotic solvents were assessed on the basis of the difference between their solubility parameters. DMSO and DMAC had the highest and lowest affinity respectively. Moreover, the single and mixed gas separation properties of the blend films were measured for separation of CO₂ from N₂ by a lab-made gas permeation test setup at pressures 2, 4 and 6 bar for temperatures between 303 and 333 K. Comparison of the results with that of the pure membranes revealed that the blend membrane had a higher permeability to CO₂ and a lower permeability to the N₂ and therefore had a higher selectivity value of CO₂/N₂. Gas permeance of N₂, increased with increase in feed pressure irrespective of blend composition.

References

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