Mesoporous carbon membranes from polyimide blended with poly(ethylene glycol)

Influence of Mixed Imide Composition and Thermal Annealing on Ionic Liquid Uptake and Conductivity of Polyimide-Poly(ethylene glycol) Segmented Block Copolymer Membranes

Understanding the impact of different bridging groups in the two-step polymerization of poly(ethylene glycol) (PEG)-incorporated polyimide (PI) materials is significant. It is known that the proton exchange membranes (PEMs) used in industry today can experience performance degradation under rising temperature conditions. Many efforts have been devoted to overcoming this problem by improving the physical and mechanical properties that extend the hygrothermal life of a PEM. This work examines the effect of oxygenated and fluorinated bridging anhydrides in the production of PI-PEG PEMs. It is shown that the dianhydride identity and the amount incorporated in the synthesis influences the properties of the segmented block copolymer (SBC) membranes, such as increased ionic liquid uptake (ILU), enhanced conductivity and higher Young's modulus favoring stiffness comparable to Nafion 115, an industrial standard. Investigations on the ionic conductivity of PI-PEG membranes were carried out to determine how thermal annealing would affect the material's performance as an ion-exchange membrane. By applying a thermal annealing process at 60 °C for one hour, the conductivities of synthesized segmented block copolymer membranes values were increased. The effect of thermal annealing on the mechanical properties was also shown for the undoped SBC via measuring the change in the Young's modulus. These higher ILU abilities and mechanical behavior changes are thought to arise from the interaction between PEG molecules and ethylammonium nitrate (EAN) ionic liquid (IL). In addition, higher interconnected routes provide a better ion-transfer environment within the membrane. It was found that the conductivity was increased by a factor of ten for undoped and a factor of two to seven for IL-doped membranes after thermal annealing.

The Phase Structural Evolution and Gas Separation Performances of Cellulose Acetate/Polyimide Composite Membrane from Polymer to Carbon Stage

Blending and heat-treatment play significant roles in adjusting gas separation performances of membranes, especially for incorporating thermally labile polymers into carbon molecular sieve membranes (CMSMs). In this work, cellulose acetate (CA) is introduced into polyimide (PI) as a sacrificial phase to adjust the structure and gas separation performance from polymer to carbon. A novel result is observed that the gas permeability is reduced, even when the immiscible CA phase decomposes and forms pores after heat treatment at 350 °C. After carbonization at 600 °C, the miscible CA has changed without contribution, while the role of the immiscible CA phase has changed from original hindrance to facilitation, the composite-based CMSM at a CA content of 10 wt.% shows highest performances, a H₂ permeability of ~5300 Barrer (56% enhancement) with a similar H₂/N₂ permselectivity of 42. The structural analyses reveal that the chain interactions and phase separation behaviors between CA and PI play critical roles on membrane structures and gas diffusion, and the corresponding phase structural evolutions during heat treatment and carbonization determine gas separation properties.

A brief review on carbon selective membranes from polymer blends for gas separation performance

The development of carbon membranes for the separation of various gases has gained interest among researchers due to their superior performance in gas separation. The preparation of carbon membranes by blending materials has many advantages including time and cost effectiveness for tuning the properties of the membranes. Here we review the recent research progress that has been made in the context of breakthroughs and challenges in the development of carbon membrane materials. In addition, we provide information regarding carbon membrane fabrication in terms of the selection of precursors and additives, carbon membrane process conditions, and coating conditions that influence the performance of gas separation of the resulting carbon membranes. The perspectives and future research directions for carbon membranes are also presented.

Molecular-sieving capabilities of mesoporous carbon membranes

The size-sieving properties of a mesoporous carbon membrane were studied via molecular permeation and cyclic voltammetry experiments. Two phenomena, simple diffusion and electrochemically aided diffusion, were investigated. Molecular diffusion through the membrane was caused by a concentration gradient across the membrane and was facilitated by electrosorption of ions under an externally applied electric field. The diffusion of molecules transported through the membrane was characterized by the values of permeability and apparent diffusion coefficient in the membrane. Because larger molecules are more restricted in terms of penetrating the pores, the size-based selectivity of the mesoporous carbon membrane could be readily observed. For example, in the two-component permeation experiment, a high selectivity (alpha=56.9) of anilinium over Rhodamine B was found. It is inferred that the diffusive transport of the larger Rhodamine B molecules with a more extensive retardation comes from the competitive mechanism between the two kinds of molecules in accessing the pore. A series of voltammetric experiments involving a mesoporous carbon membrane immersed in various electrolytes with ions of different sizes allowed the observation of ion-exclusion phenomena. It was found that the size effect is significant for electrochemically aided diffusion and electrosorption processes. The number of cations inside the pores of the membrane decreases with increasing cation size. This phenomenon is due to the size-exclusion effect, which could be demonstrated by the values of electrical double-layer capacitance for sodium, magnesium, and tetrahexylammonium cations, at potentials ranging from negative values to the point of zero charge, corresponding to 86.7, 73.1, and 50.0 F/g, respectively. The findings of this work manifest that the relationship between the pore size and the dimensions of the molecules determines the transport and sorption behavior of nanoporous carbon materials.

Nanoporous structured submicrometer carbon fibers prepared via solution electrospinning of polymer blends

A facile means for obtaining submicrometer carbon fibers with a nanoporous structure is presented. A mixture of polyacrylonitrile (PAN) and a copolymer of acrylonitrile and methyl methacrylate (poly(AN-co-MMA)) in dimethylformamide was electrospun into submicrometer fibers with a microphase-separated structure. During the followed oxidation process, the copolymer domains were pyrolyzed, resulting in a nanoporous structure that was preserved after carbonization. The microphase-separated structure of the PAN/poly(AN-co-MMA) electrospun fibers, the morphology, and porous structure of both the oxidized and the carbonized fibers were observed with scanning electron microscopy and transmission electron microscopy. The carbon fibers have diameters ranging from several hundred nanometers to about 1 microm. The nanopores or nanoslits throughout the fiber surface and interior with diameters of several tens of nanometers are interconnected and oriented along the longitudinal axis of the fibers. This unique nanoporous morphology similar to the microphase-separated structure in the PAN/poly(AN-co-MMA) fibers is attributed to the rapid phase separation, solidification, as well as the stretching of the fibers during electrospinning. The pore volume and pore size distribution of the carbonized fibers were investigated by nitrogen adsorption and desorption.