Chi Hoon Park

Enhancement of proton transport by nanochannels in comb-shaped copoly(arylene ether sulfone)s.

We gratefully acknowledge support of this research by the WCU(World Class University) program, National Research Foundation(NRF) of the Korean Ministry of Science and Technology (no. R31-2008-000-10092-0). NRCC publication 52850.

Nanocrack-regulated self-humidifying membranes

The regulation of water content in polymeric membranes is important in a number of applications, such as reverse electrodialysis and proton-exchange fuel-cell membranes. External thermal and water management systems add both mass and size to systems, and so intrinsic mechanisms of retaining water and maintaining ionic transport in such membranes are particularly important for applications where small system size is important. For example, in proton-exchange membrane fuel cells, where water retention in the membrane is crucial for efficient transport of hydrated ions, by operating the cells at higher temperatures without external humidification, the membrane is self-humidified with water generated by electrochemical reactions. Here we report an alternative solution that does not rely on external regulation of water supply or high temperatures. Water content in hydrocarbon polymer membranes is regulated through nanometre-scale cracks (‘nanocracks’) in a hydrophobic surface coating. These cracks work as nanoscale valves to retard water desorption and to maintain ion conductivity in the membrane on dehumidification. Hydrocarbon fuel-cell membranes with surface nanocrack coatings operated at intermediate temperatures show improved electrochemical performance, and coated reverse-electrodialysis membranes show enhanced ionic selectivity with low bulk resistance.

Tuning microcavities in thermally rearranged polymer membranes for CO2 capture.

Microporous materials have a great importance in catalysis, delivery, storage and separation in terms of their performance and efficiency. Most microporous materials are comprised of inorganic frameworks, while thermally rearranged (TR) polymers are a microporous organic polymer which is tuned to optimize the cavity sizes and distribution for difficult separation applications. The sub-nano sized microcavities are controlled by in situ thermal treatment conditions which have been investigated by positron annihilation lifetime spectroscopy (PALS). The size and relative number of cavities increased from room temperature to 230 °C resulting in improvements in both permeabilities and selectivities for H(2)/CO(2) separation due to the significant increase of gas diffusion and decrease of CO(2) solubility. The highest performance of the well-tuned TR-polymer membrane was 206 Barrer for H(2) permeability and 6.2 of H(2)/CO(2) selectivity, exceeding the polymeric upper bound for gas separation membranes.

Phase separation and water channel formation in sulfonated block copolyimide.

We compared experimental and simulated data to investigate the phase separation and water channel formation of proton exchange membranes (PEMs) for fuel cell. Sulfonated block copolyimides (SPIs) were adopted as model polymers for experiments and simulations, and Nafion was used as a reference. Nafion and SPIs were observed to have different microscopic structures such as constituent atoms, backbone rigidity, and the locations of sulfonic acid groups, all of which significantly affect phenomenological properties at the macroscopic level such as density, water uptake, and proton conductivity. In particular, SPIs show much weaker microphase separation than Nafion, mainly due to the lower mobility of sulfonic acid groups and the existence of acceptable sites for hydrogen bonding even in hydrophobic segments, which impedes water channel formation for proton transport. As a result, the phase separation behavior and the resulting water channel formation are the major factors affecting macroscopic properties of PEMs such as water uptake and proton transport.

Thermal treatment effect on the structure and property change between hydroxy-containing polyimides (HPIs) and thermally rearranged polybenzoxazole (TR-PBO).

In this study, we report on the effect of thermal treatment on polyimide precursors (HPIs) and on the resulting thermally rearranged polybenzoxazole (TR-PBO) polymer membranes as investigated through the use of molecular dynamics (MD) simulations. For this purpose, we have first analyzed the structures of hydroxy-containing polyimides before thermal treatment and those of the thermally rearranged polybenzoxazoles after the thermal treatment, according to their temperature conditions. As expected, HPIs and TR-PBOs always show very limited motion of their polymer chains, indicated by the radius of gyration, due to their well-known thermal stability. In particular, the very rigid and stiff PBO linkages did not undergo significant change in their torsional angle distribution. On the other hand, in regards to intrachain movement, HPI chains were significantly affected by temperature. Their conformational changes were notably observed, which affected the distances between possible reaction sites, oxygen atoms in hydroxyl groups, and carbon atoms in the imido-ring. The free volume analysis, performed on both polymers and during thermal treatment, indicates that HPIs have a unimodal distribution of free volume areas, which partially coalesce in larger areas having, however, a relatively narrow size. Further, TR-PBO shows a bimodal cavity distribution, and after thermal treatment and TR reaction, the free volume structures in TR-PBO are maintained. The cavity size distributions determined by simulation were also consistent with free volume distributions determined by positron annihilation lifetime spectroscopy.

Modified Sulfonated Poly(arylene ether sulfone) Membranes Prepared via a Radiation Grafting Method for Fuel Cell Application

Polymer modifications involving crosslinking and grafting by radiation have been widely researched for use in biopolymers, hydrogels, heat-resisting electric wires, vulcanization, polymer recycling, gas separation, and pervaporation membranes because of their advantages over traditional chemical crosslinking and grafting methods, including a catalyst-free reaction, post-modification at room temperature for solid polymers, and short modification times and steps. Various recent studies have also utilized radiation modification techniques to prepare proton exchange membranes (PEMs). PEMs are membranes which have the ability to selectively transfer protons generated by electrochemical reactions from the anode to the cathode in fuel cells. For this purpose, polymers having strong acidic functional group as proton carriers (e.g. sulfonic acid groups) have generally been used. There are two main strategies to introduce sulfonic acid groups into polymers. One is to introduce the groups directly to a polymer (post-sulfonation or polymersulfonation) and the other is to perform polymerization with a sulfonated monomer (monomer-sulfonation). To date, preirradiation and post-sulfonation methods, in which sulfonic acid groups are introduced after crosslinking or grafting non-sulfonated polymers by irradiation, have been used in radiation-induced processes of PEMs in order to prevent unfavorable membrane damage such as decomposition of sulfonic acid groups. However, this method has structural limitations for PEMs because the types of target polymer for irradiation are restricted to perfluorinated or partially fluorinated polymers and aliphatic polymers. In addition, crosslinking and grafting agents should have sites available for post-sulfonation. Whereas, few studies have been conducted on radiation-induced sulfonated polymers (post-irradiation method) despite the advantage that various types of polymers, prepared with various sulfonated and nonsulfonated monomers, can be used as a matrix for radiation modification. In this study, we investigated a post-irradiation method for PEMs and tried to improve PEM performance by preparing a radiation-grafted sulfonated polymer. Sulfonated polyarylene ether sulfone (SPAES) was used as a polymer matrix for irradiation. Here, the sulfonated polymer with a perfluorinated backbone (e.g. Nafion) was excluded due to its serious decomposition resulting from the chain scission effect caused by irradiation. Due to its additional acid group, acrylic acid was used as a grafting agent to increase the proton conduction properties of the resulting polymer.