(160f) Parallel-Aligned Graphene Oxide/Pebax Ultrathin Composite Membrane for Gas Separation

The application of graphene oxide (GO) in membrane offers a substantial opportunity to realize efficient separation at the molecular level. The separation mechanism for most of the existing GO membranes is based on the intergalleries between GO nanosheets. As a result, current research focus is to assemble GO into stacked laminates for membrane fabrication. However, the pure GO membranes are usually very brittle, and it is difficult to control the laminate structures (alignment, interlayer space) over a large surface area.

A promising alternative is to blend GO into a polymer matrix for gas separation membrane fabrication. Shen et al. [1] applied the favorable hydrogen bonding between Pebax and GO to assemble GO into laminate structures. The flat sheet membrane has molecular sieving spacing and straight diffusion pathways, allowing efficiency gas transport through the membrane. In another line of research, Berean et al. [2] blended GO into PDMS matrix. The formation of interfacial voids provides fast gas transport channels. However, most of the existing GO/polymer composite membrane are relatively thick, and the formation of thin GO/Polymer films are preferable for practical gas permeation. However, it is difficult to control the orientation of the GO laminates within the ultrathin layer, and the randomly orientated GO laminates would create non-selective defects.

In this work, we applied the dip coating technique to fabricate the GO/Pebax ultrathin coating layer on a porous PVDF membrane. Dip coating process is easy to control and scale-up, and the resultant membrane exhibited significantly improved mechanical properties. We then postulated the surface tension, capillary effect and gravity during the dip-coating process might facilitate the GO sheets alignment within the thin selective layer. The SEM and TEM images confirmed the presence of the shear-aligned GO laminate structures. Due to the confinement of Pebax polymer chains, GO nanosheets formed stacked laminates with intergallery distance of 0.35 nm, which is beneficial to the separation of CO₂ (0.33 nm) and N₂ (0.36 nm).

The gas separation results show the incorporation of GO within Pebax significantly improved the CO₂ permeance by 90 %, while the CO₂/N₂ selectivity was relatively unchanged. However, when the GO loading within the selective layer was too high, the aggregation of GO led to the loss of selectivity and permeance. Such composite membranes exhibited ideal stability over an extended testing period, indicating the aging effect was negligible. We further investigated the formation mechanism of the aligned structure, and discovered the thickness of the liquid layer was the key to the alignment: higher withdrawn speed led to thicker liquid film on a polymeric membrane, which required longer evaporation time. The constant convection force towards the liquid surface during the prolonged evaporation process has been discovered to lead to the heterogeneous GO structures [3]. Conclusively, the shear-aligned GO laminates can be achieved within a thin polymer layer by a facile dip coating process. This technique provides a facile approach to realize the large-scale application of GO/polymer membrane for gas separation.

[1] J. Shen, G. Liu, K. Huang, W. Jin, K. R. Lee, N. Xu, Angew. Chem. Int. Ed. 2015, 54, 578-582. Angew. Chem. 127, 588-592.

[2] K. J. Berean, J. Z. Ou, M. Nour, M. R. Field, M. M. Y. A. Alsaif, Y. Wang, R. Ramanathan, V. Bansal, S. Kentish, C. M. Doherty, A. J. Hill, C. McSweeney, R. B. Kaner, K. Kalantar-zadeh, J. Phys. Chem. C, 2015, 119, 13700-13712.

[3] C.-H. Tsou, Q.-F. An, S.-C. Lo, M. De Guzman, W.-S. Hung, C.-C. Hu, K.-R. Lee, J.-Y. Lai, J. Membr. Sci. 2015, 477, 93-100.