Pentiptycene-based ladder polymers with configurational free volume for enhanced gas separation performance and physical aging resistance

Abstract

Polymers of intrinsic microporosity (PIMs) have shown promise in pushing the limits of gas separation membranes, recently redefining upper bounds for a variety of gas pair separations. However, many of these membranes still suffer from reductions in permeability over time, removing the primary advantage of this class of polymer. In this work, a series of pentiptycene-based PIMs incorporated into copolymers with PIM-1 are examined to identify fundamental structure–property relationships between the configuration of the pentiptycene backbone and its accompanying linear or branched substituent group. The incorporation of pentiptycene provides a route to instill a more permanent, configuration-based free volume, resistant to physical aging via traditional collapse of conformation-based free volume. PPIM-ip-C and PPIM-np-S, copolymers with C- and S-shape backbones and branched isopropoxy and linear n-propoxy substituent groups, respectively, each exhibited initial separation performance enhancements relative to PIM-1. Additionally, aging-enhanced gas permeabilities were observed, a stark departure from the typical permeability losses pure PIM-1 experiences with aging. Mixed-gas separation data showed enhanced CO2/CH4 selectivity relative to the pure-gas permeation results, with only ∼20% decreases in selectivity when moving from a CO2 partial pressure of ∼2.4 to ∼7.1 atm (atmospheric pressure) when utilizing a mixed-gas CO2/CH4 feed stream. These results highlight the potential of pentiptycene’s intrinsic, configurational free volume for simultaneously delivering size-sieving above the 2008 upper bound, along with exceptional resistance to physical aging that often plagues high free volume PIMs.

Microporous Pentiptycene-Based Polymers with Heterocyclic Rings for High-Performance Gas Separation Membranes

“…Physical aging behavior was preliminarily investigated on the TR-350-50-450 film by measuring the pure-gas permeability after it had been aged for 5 months under ambient conditions. Physical aging, which involves polymer chain relaxation overtime to reach a thermodynamic equilibrium state with tighter chain packing, generally reduces permeability with some gains in selectivity. , PPBO exhibited promising aging resistance with decently preserved permeability and marked enhanced selectivity upon aging (Figure ), similar to other iptycene-containing polymers, such as iptycene-based polyimides, triptycene-based PBOs, and pentiptycene-based PIMs . As shown, the 5 month aged TR-350-50-450 film maintained 72% H 2 permeability; at the same time, the H 2 /CH 4 selectivity of the aged sample exceeded the fresh film selectivity by 70%, moving the aged film even further beyond the 2015 H 2 /CH 4 upper bound.…”

“…Here, pentiptycene units are introduced for the first time as the building block to prepare a series of microporous PBO-based TR polymer membranes to further explore the potential of iptycene-containing TR polymers for gas separation applications. As a natural extension to triptycene, bulkier pentiptycene unit, composed of five arene rings fused in a H-shape scaffold, can disrupt chain packing more efficiently once introduced into the PBO backbone, leading to higher permeability than its triptycene counterparts, as demonstrated in our previous studies on pentiptycene-based polyimides ,, and pentiptycene-PIM . Furthermore, pentiptycene has a higher fraction of IFV elements, which will not only augment the overall free volume but also improve further the size-sieving behavior of the TR membranes, enabling higher permeability–selectivity combinations.…”

“…One of the recent and arguably most effective strategies to improve the selectivity of PIMs and TR polymers is to introduce shape-persistent building blocks or structural units with intrinsic molecular microporosity, such as iptycene moieties, to boost the size-sieving capability. ,,− Iptycenes are [2,2,2]-ring systems composed of fused aromatic rings, with the simplest configurations being triptycene and pentiptycene, containing three and five arene rings, respectively . Providing internal free volume (IFV) elements intrinsic to the molecular configurations that have dimensions comparable to the kinetic diameters of many gas molecules, iptycene units have been proven to be capable of generating finely tailored free volume architecture, therefore enabling superior size-sieving properties. ,− Previous study in our group has successfully demonstrated the first iptycene-containing TR polymers, i.e., triptycene-based PBOs (TPBOs), showing superior separation performance exceeding the 2015 H 2 /CH 4 upper bound and the CO 2 /CH 4 and O 2 /N 2 2008 upper bounds, representing arguably the most selective TR polymers. , …”

Machine learning enables interpretable discovery of innovative polymers for gas separation membranes

“…Current approaches generally involve tuning chemical groups to increase affinity and solubility toward a desired gas or incorporating greater free volume to increase overall diffusivity. When assembling a new polymer, typically a desired enhancement is targeted (i.e., higher CO 2 affinity, higher overall permeability, and aging resistance), and a chemical group that is likely to achieve that enhancement is incorporated into the polymer chemistry (12)(13)(14)(15). For achieving higher permeability, polymers of intrinsic microporosity (PIMs) have been extensively studied during the past two decades (16,17).…”

Facilitated transport membrane with functionalized ionic liquid carriers for CO2/N2, CO2/O2, and CO2/air separations

“…24 Recent progresses on the S-D membranes focused on increasing (1) the solubility selectivity toward CO 2 over other gases by the incorporation of highly polar or ionic components; [25][26][27][28][29] and (2) the diffusivity selectivity for CO 2 sieving by rigid polymeric backbones with high free volume. [30][31][32][33][34] There is usually a tradeoff between gas permeance and selectivity of a membrane, as described by the Robeson upper-bound. [35][36][37] The S-D type membranes are often implemented as multi-stage membrane systems to achieve the desired separation, which inevitably drives up the energy consumption.…”