Qinnan Zhang

Chemical and Biomolecular Engineering

Faculty Advisor: Dr. Ruilan Guo

Toward a better understanding of iptycene-based polyimide membranes: structure, microporosity and gas separation performance

Membrane technology for gas separation has been commercialized for more than 30 years and can be applied in many industries, such as removal of carbon dioxide from natural gas and biogas (CO2/CH4), production of nitrogen from air (N2/O2) and hydrogen recovery from petrochemical plants (H2/CH4, H2/ N2). Membrane separation shows strong growth potential due to its high energy-efficiency, low operating cost and small footprint comparing to conventional gas separation processes [1]. Ideally, the membranes for industrial application should have both high permeability (P) and high selectivity (α). High permeability allows small membrane area for reduced cost, while high selectivity improves purity of desired products. However, there is an intrinsic trade-off between permeability and selectivity for polymeric membranes, where highly permeable membranes always show low selectivity and vice versa. The trade-off has been illustrated by Robeson in permeability-selectivity upper bound plots and is now used as an empirical criterion to gauge performance of membranes [2]. Majority of current gas separation membrane studies have been directly to explore new rigid polymer structures that show high permeability-selectivity combinations to overcome the limitation. Among all these works, polyimides are drawing lots of attention because of their good thermal and chemical stability, as well as their decent gas separation performance. The rigid aromatic polyimide backbones can pack tightly to create small interchain spacing, i.e., fractional free volume (FFV), delivering moderate to high selectivity. However, the permeability of existing polyimides is still relatively low making them less desirable for industrial implementation. In this regard, it is in pressing need to explore new macromolecular design of polyimide structure that features high chain rigidity with large fractional free volume for fast gas diffusion, and narrow free volume size distribution to maintain high selectivity [3]. Introducing bulky, shape-persistent building block, such as iptycene moieties, into polymer backbone represents a promising macromolecular design to produce high performance membrane materials. Iptycenes are a family of rigid, three-dimensional molecules with phenyl rings attached to the central hinge, where triptycene and pentiptycene are the simplest members of the family with three and five phenyl rings, respectively [4]. In our group’s previous research on iptycene-based polyimides, it has been shown that the incorporation of rigid iptycene moieties in polymer backbone can effectively enhance the fractional free volume as well as control the molecular cavity architecture, leading to much improved separation performance relative to commercial Matrimid® polyimide [5,6]. However, the ether bond in these custom-synthesized iptycene-based monomers prevents further improvement in gas permeability of these iptycene-based polyimides [7]. In this project, a new triptycene diamine is designed wherein the polymerizable amine groups are directly positioned on triptycene skeleton without using spacer moieties containing ether bond. This new monomer design will significantly improve the backbone rigidity and introduce non-planar, contorted structure as the heterocylic imide rings are directly connected to triptycene building block. As a result, high gas permeability is expected for the polyimides prepared from this new triptycene diamine due to the large fractional free volume induced by inefficient chain packing. Based on this new triptycene diamine, a series of novel iptycene-containing polyimides will be prepared by condensation reaction using various commercial and custom synthesized dianhydrides. The choice of the dianhydride monomers is carefully made to finely tune the size and size distribution of free volume-based microcavities of the membranes. Custom-synthesized dianhydride monomers containing bulky units, such as triptycene and pentiptycene, are of particular interest which tend to restrict the chain packing and further enhance the gas transport performance. More importantly, the iptycene units in polyimide backbone can potentially construct “hourglass”-like ultrafine micropores in membranes to boost size sieving effect, enhancing selectivity without sacrifice permeability [8]. Membranes of the comparable polymers will be fabricated and characterized to estalish the property-structure relationships for this new family of polyimide membranes. Overall, this study will deliver a broad set of polyimides with varied iptycene-containing diamine-dianhydride pairs to examine the effect of iptycene units on physical properties, gas transport properties and separation efficiency of the membranes. This in turn can contribute to a better understanding of the role of iptycene moieties in manipulating free volume architecture and affecting gas separation performance.

References: [1] R.W. Baker, B.T. Low, Gas Separation Membrane Materials: A Perspective, Macromolecules. 47 (2014) 6999–7013. [2] L.M. Robeson, The upper bound revisited, J. Memb. Sci. 320 (2008) 390–400. [3] C. Li, S.M. Meckler, Z.P. Smith, J.E. Bachman, L. Maserati, J.R. Long, B.A. Helms, Engineered Transport in Microporous Materials and Membranes for Clean Energy Technologies, Adv. Mater. 30 (2018) 1704953. [4] T.M. Swager, Iptycenes in the design of high performance polymers, Acc. Chem. Res. 41 (2008) 1181–1189. [5] S. Luo, Q. Liu, B. Zhang, J.R. Wiegand, B.D. Freeman, R. Guo, Pentiptycene-based polyimides with hierarchically controlled molecular cavity architecture for efficient membrane gas separation, J. Memb. Sci. 480 (2015) 20–30. [6] J.R. Wiegand, Z.P. Smith, Q. Liu, C.T. Patterson, B.D. Freeman, R. Guo, Synthesis and characterization of triptycene-based polyimides with tunable high fractional free volume for gas separation membranes, J. Mater. Chem. A. 2 (2014) 13309–13320. [7] S. Luo, J.R. Wiegand, B. Kazanowska, C.M. Doherty, K. Konstas, A.J. Hill, R. Guo, Finely Tuning the Free Volume Architecture in Iptycene-Containing Polyimides for Highly Selective and Fast Hydrogen Transport, Macromolecules. (2016). [8] J.R. Weidman, R. Guo, The Use of Iptycenes in Rational Macromolecular Design for Gas Separation Membrane Applications, Ind. Eng. Chem. Res. 56 (2017) 4220–4236.