(414s) Layer-By-Layer Assemblies As Size-Selective Polymeric Membranes

The production of inexpensive, high purity hydrogen remains a critical challenge to improving the sustainability of fossil fuels and for realizing renewable and clean energy sources capable of displacing fossil fuels. Polymeric gas separation membranes combining low production costs with robust mechanical properties have found broad industrial application for O₂ and N₂ enrichment of air, upgrading of natural gas and hydrogen recovery from ammonia. In the absence of structure (i.e. in amorphous glassy or rubbery films), increasing the free volume of size-selective polymer membranes improves the permeability at the cost of reducing the permselectivity. This translates to an upper bound in separation performance achievable by dense, homogeneous and amorphous polymeric films. ^[1,2]  The authors have recently demonstrated that layer-by-layer (LbL) deposition techniques provide a robust, low-cost means to introduce size-selective functionality via manipulation of polymer processing conditions.^[4]

Extension of the LbL assembly technique to gas purification membranes promises the ability to tune overall gas permeabilities and permselectivities through manipulation of film nanostructure by varying deposition conditions.  In this presentation, the authors report the use of a branched polyethylenimine (PEI) / poly (acrylic acid) (PAA) system is used for achieving breakthroughs in light gas separations. This PEI/PAA assembly appears to have a “scrambled salt” structure, resulting in a highly interpenetrating network of high density which demonstrates size-selective gas separations in excess of Robeson’s ‘upper boundary,’ ^[1,2] which we attribute to the structure imparted by a high degree of ‘ionic-crosslinking.’ Details of materials analysis and separation measurements will be presented. The potential of the LbL approach to gas separation coatings to impact to several other significant applications of social and industrial importance will also be discussed.

[1] L. M. Robeson, J. Membr. Sci. 1991, 62, 165.

[2] L. M. Robeson, J. Membr. Sci. 2008, 320, 390.

[3] D. Kim, P. Tzeng, K. Barnett, Y-H. Yang, B.A. Wilhite, J. Grunlan, Adv. Mater. 2014, 26, 746-751.