Structure Manipulation in Triptycene-Based Polyimides through Main Chain Geometry Variation and Its Effect on Gas Transport Properties

Two new triptycene-based polyimides, 6FDA-1,4-trip_<i>ortho</i> and 6FDA-2,6-trip_<i>para</i>, were synthesized to investigate the effect of varying polymer backbone geometry on chain packing and gas transport properties. Changing the imide linkage geometry from <i>para</i> to <i>ortho</i> reduced gas permeabilities by ∼48% due to more efficient chain packing of the asymmetric <i>ortho</i> structure, which is demonstrated by decreased <i>d</i>-spacing and fractional free volume. Varying the triptycene orientation from the 1,4- to 2,6-connection also caused a decrease in permeability (e.g., 29% decrease for <i>P</i>_CO2). This is likely the result of reduced chain mobility, as evidenced by increased <i>T</i>_g, and a shift in free volume distribution toward smaller cavities, as supported by smaller <i>d</i>-spacing. Physical aging studies show that the equilibrium specific volume of these isomeric polymers is similar, as evidenced by nearly identical gas transport properties exhibited by all aged samples

Conformational Transition of Poly(<i>N</i>‑isopropylacrylamide) Single Chains in Its Cononsolvency Process: A Study by Fluorescence Correlation Spectroscopy and Scaling Analysis

Fluorescence correlation spectroscopy (FCS) has been adopted to investigate the conformational transition of poly­(<i>N</i>-isopropylacrylamide) (PNIPAM) single chains with moderate molecular weights in the cononsolvency process. A practical approach of performing accurate FCS measurements with the presence of the refractive index mismatch was developed. The practical and reliable FCS calibration facilitates the acquisition of the hydrodynamic radius (<i>R</i>_H) of PNIPAM single chains with the change of the water–ethanol composition. By using the synthesized PNIPAM samples covering a range of degrees of polymerization (<i>N</i>), the scaling analysis in the relationship of <i>R</i>_H ∼ <i>N</i>^ν exhibits a progressive, re-entrant change of the scaling index (ν) between good solvent (0.57) and poor solvent (∼1/3) condition, which is a reflection of a re-entrant conformational transition of the polymers. Furthermore, the highly asymmetrical feature of the cononsolvency process of single PNIPAM chains was unveiled, which indicates a much stronger effect or interaction of the ethanol molecules to the PNIPAM chain. Comparisons of the present results with previous reports provided new information to the mechanism model of the PNIPAM cononsolvency

Highly Proton Conducting Polyelectrolyte Membranes with Unusual Water Swelling Behavior Based on Triptycene-containing Poly(arylene ether sulfone) Multiblock Copolymers

Multiblock poly­(arylene ether sulfone) copolymers are attractive for polyelectrolyte membrane fuel cell applications due to their reportedly improved proton conductivity under partially hydrated conditions and better mechanical/thermal stability compared to Nafion. However, the long hydrophilic sequences required to achieve high conductivity usually lead to excessive water uptake and swelling, which degrade membrane dimensional stability. Herein, we report a fundamentally new approach to address this grand challenge by introducing shape-persistent triptycene units into the hydrophobic sequences of multiblock copolymers, which induce strong supramolecular chain-threading and interlocking interactions that effectively suppress water swelling. Consequently, unlike previously reported multiblock copolymer systems, the water swelling of the triptycene-containing multiblock copolymers did not increase proportionally with water uptake. This combination of high water uptake and low swelling behavior of these copolymers resulted in excellent proton conductivity and membrane dimensional stability under fully hydrated conditions. In particular, the triptycene-containing multiblock copolymer film with the longest hydrophilic block length (i.e., BPSH100-TRP0-15k-15k) had a water uptake of 105%, an excellent proton conductivity of 0.150 S/cm, and a volume swelling ratio of just 29% (more than 42% reduction compared to Nafion 212)

Facile Synthesis of a Pentiptycene-Based Highly Microporous Organic Polymer for Gas Storage and Water Treatment

Rigid H-shaped pentiptycene units, with an intrinsic hierarchical structure, were employed to fabricate a highly microporous organic polymer sorbent via Friedel–Crafts reaction/polymerization. The obtained microporous polymer exhibits good thermal stability, a high Brunauer–Emmett–Teller surface area of 1604 m² g^–1, outstanding CO₂, H₂, and CH₄ storage capacities, as well as good adsorption selectivities for the separation of CO₂/N₂ and CO₂/CH₄ gas pairs. The CO₂ uptake values reached as high as 5.00 mmol g^–1 (1.0 bar and 273 K), which, along with high adsorption selectivity values (e.g., 47.1 for CO₂/N₂), make the pentiptycene-based microporous organic polymer (PMOP) a promising sorbent material for carbon capture from flue gas and natural gas purification. Moreover, the PMOP material displayed superior absorption capacities for organic solvents and dyes. For example, the maximum adsorption capacities for methylene blue and Congo red were 394 and 932 mg g^–1, respectively, promoting the potential of the PMOP as an excellent sorbent for environmental remediation and water treatment

Resolving the Difference in Electric Potential within a Charged Macromolecule

The difference of the electric potential between the middle and end of polystyrenesulfonate (PSS^–) chain is discovered experimentally. Using a pH-responsive fluorophore attached to these two locations on the PSS^– chain, the local pH value was determined by single molecule fluorescence technique: photon counting histogram (PCH). By the observation of a very high accumulation of proton (2–3 orders of magnitude in concentration) at the vicinity of the PSS^– as a result of the electrostatic attraction between the charged chain and protons, the electric potential of the PSS^– chain is determined. A higher extent of counterion adsorption is discovered at the middle of the PSS^– chain than the chain end. The entropy effect of the counterion adsorption is also discoveredupon the dilution of protons, previously adsorbed counterions are detached from the chain

Finely Tuning the Free Volume Architecture in Iptycene-Containing Polyimides for Highly Selective and Fast Hydrogen Transport

Iptycene-based polyimides have attracted extensive attention recently in the membrane gas separation field due to their unique structural hierarchy and chemical characteristics that enable construction of well-defined yet tailorable free volume architecture for fast and selective molecular transport. We report here a new series of iptycene-based polyimides that are exquisitely tuned in the monomer structure to afford preferred microcavity architecture for hydrogen transport. In particular, a triptycene-containing dianhydride (TPDAn) was prepared to react with two iptycene-containing diamines (i.e., TPDAm and PPDAm) or 2,2′-bis­(3-amino-4-hydroxy­phenyl)­hexa­fluoropropane (6FAP) to produce entirely or partially iptycene-based polyimides. The incorporation of iptycene units effectively disrupted chain packing, which resulted in ultrafine microporosity in the membranes with a desired bimodal size distribution with maxima at ∼3 and ∼7 Å, respectively. Depending on the combination of diamine and dianhydride, the microporosity was feasibly tuned and optimized to meet the needs of challenging H₂ separations, especially for H₂/N₂ and H₂/CH₄ gas pairs. Particularly, a H₂ permeability of 27 barrers and H₂/N₂ and H₂/CH₄ selectivities of 142 and 300, respectively, were obtained for TPDAn-6FAP

Direct Thermal Oxidative Cross-Linking in Air toward Hierarchically Microporous Polymer Membranes for Advanced Molecular Sieving

In situ thermal oxidative cross-linking provides an efficient strategy for manipulating microporosity in polymeric gas separation membranes. Herein, we report a rational macromolecular design combining microporous polymers with thermal oxidative cross-linking to fabricate highly permselective membranes for advanced energy-efficient gas separations. We demonstrate that direct thermal treatment in air induces both oxidative chain scission and thermal oxidative cross-linking, leading to a hierarchically microporous architecture enabling simultaneous enhancement of permeability and selectivity. Consequently, the TOC-PI-Trip-TB-450-30min membrane containing both the triptycene and Tröger’s base moieties upon thermal treatment at 450 °C for 30 min in air exhibits H2 and CO2 permeabilities of 1138 and 640 Barrer, respectively, and H2/N2, H2/CH4, and CO2/CH4 selectivities of 76, 121, and 68, respectively, exceeding or approaching the state-of-the-art upper bounds. This study also confirmed that the microcavity characteristics and gas permeation performance of the thermal-oxidatively cross-linked membranes are highly tunable by regulating the polymer structure, oxidative temperature, and reaction time

Highly CO₂‑Selective Gas Separation Membranes Based on Segmented Copolymers of Poly(Ethylene oxide) Reinforced with Pentiptycene-Containing Polyimide Hard Segments

Poly­(ethylene oxide) (PEO)-containing polymer membranes are attractive for CO₂-related gas separations due to their high selectivity toward CO₂. However, the development of PEO-rich membranes is frequently challenged by weak mechanical properties and a high crystallization tendency of PEO that hinders gas transport. Here we report a new series of highly CO₂-selective, amorphous PEO-containing segmented copolymers prepared from commercial Jeffamine polyetheramines and pentiptycene-based polyimide. The copolymers are much more mechanically robust than the nonpentiptycene containing counterparts due to the molecular reinforcement mechanism of supramolecular chain threading and interlocking interactions induced by the pentiptycene structures, which also effectively suppresses PEO crystallization leading to a completely amorphous structure even at 60% PEO weight content. Membrane transport properties are sensitively affected by both PEO weight content and PEO chain length. A nonlinear correlation between CO₂ permeability with PEO weight content was observed due to the competition between solubility and diffusivity contributions, whereby the copolymers change from being size-selective to solubility-selective when PEO content reaches 40%. CO₂ selectivities over H₂ and N₂ increase monotonically with both PEO content and chain length, indicating strong CO₂-philicity of the copolymers. The copolymer film with the longest PEO sequence (PEO2000) and highest PEO weight content (60%) showed a measured CO₂ pure gas permeability of 39 Barrer, and ideal CO₂/H₂ and CO₂/N₂ selectivities of 4.1 and 46, respectively, at 35 °C and 3 atm, making them attractive for hydrogen purification and carbon capture

Investigate the Glass Transition Temperature of Hyperbranched Copolymers with Segmented Monomer Sequence

Hyperbranched copolymers with segmented structures were synthesized using a chain-growth copper-catalyzed azide–alkyne cycloaddition (CuAAC) polymerization via sequential monomer addition in one pot. Three AB₂-type monomers that contained one alkynyl group (A), two azido groups (B), and one dangling group, either benzyl or oligo­(ethylene oxide) (EO_<i>x</i>, <i>x</i> = 3 and 7.5), were used in these CuAAC reactions. Varying the addition sequences and feed ratios of the monomers produced a variety of hyperbranched copolymers with tunable compositions, molecular weights, segmented structures, and consequently glass transition temperature (<i>T</i>_g). It was found that the <i>T</i>_g of hyperbranched copolymers was little affected by the polymer molecular weights when <i>M</i>_n ≥ 5000. However, the values of <i>T</i>_g were significantly determined by the compositions of the terminal groups and the outermost segment of the hyperbranched copolymers. The last added AB₂ monomer in the polymerization formed an outermost “shell” and shielded the contribution of inner segments to the glass transition of the copolymers, reflecting a chain sequence effect of hyperbranched polymers on the thermal properties

Highly Selective and Permeable Microporous Polymer Membranes for Hydrogen Purification and CO₂ Removal from Natural Gas

This paper reports a new macromolecular design that incorporates hierarchical triptycene unit into thermally rearranged polybenzoxazole (TR-PBO) structures for highly selective and permeable gas separation membranes with great potential for H₂ purification and CO₂ removal from natural gas. We demonstrate that triptycene moieties not only effectively disrupt chain packing enabling microporous structure for fast mass transport, but also introduce ultrafine microporosity via the unique internal free volume intrinsic to triptycene unit that allows for superior molecular sieving capability in resulting PBO membranes. Consequently, these triptycene-based polybenzoxazole (TPBO) membranes display among the highest gas selectivities for H₂ separations (i.e., α­(H₂/N₂) = 96; α­(H₂/CH₄) = 203) and CO₂ removal from natural gas (i.e., α­(CO₂/CH₄) = 68) among existing glassy polymeric membranes. It is also demonstrated that microporous structure and gas transport properties of TPBO films are highly tailorable by adjusting the triptycene content and the <i>ortho</i>-functionality of the precursors. The highly diverse tunability, along with the excellent resistance toward membrane plasticization and physical aging, render the TPBO membranes with extremely versatile separation capability applicable for a wide range of important industrial processes to get clean or low carbon fuels and reduce carbon footprint