Extended Chemical CrossLinking of a Thermoplastic Polyimide: Macroscopic and Microscopic Property Development

Polyimides are well established as gas separation membranes due to their intrinsically low free volume and correspondingly high H₂ selectivity relative to other gases such as CO₂. Prior studies have established that H₂/CO₂ selectivity can be improved by crosslinking polyimides with diamines differing in spacer length. In this work, we follow the evolution of macroscopic and microscopic properties of a commercial polyimide over long crosslinking times (t_x) with 1,3‐diaminopropane. According to spectroscopic analysis, the crosslinking reaction saturates after ≈24 h, whereas tensile, nanoindentation and stress relaxation tests reveal that the material stiffens, and possesses a long relaxation time that increases with increasing t_x. Although differential scanning calorimetry shows that the glass transition temperature decreases systematically with increasing t_x, permeation studies indicate that the permeabilities of H₂ and CO₂ decrease, while the H₂/CO₂ selectivity increases markedly, with increasing t_x. At long t_x, the polyimide becomes impermeable to CO₂, suggesting that it could be used as a barrier material.

     

Azide‐based Cross‐Linking of Polymers of Intrinsic Microporosity (PIMs) for Condensable Gas Separation

Cross‐linked polymers of intrinsic microporosity (PIM)s for gas separation membranes, were prepared by a nitrene reaction from a representative PIM in the presence of two different diazide cross‐linkers. The reaction temperature was optimized using TGA. The homogenous membranes were cast from THF solutions of different ratios of PIM to azides. The resulting cross‐linked structures of the PIMs membranes were formed at 175 °C after 7.5 h and confirmed by TGA, XPS, FT‐IR spectroscopy and gel content analysis. These resulting cross‐linked polymeric membranes showed excellent gas separation performance and can be used for O₂/N₂ and CO₂/N₂ gas pairs, as well as for condensable gases, such as CO₂/CH₄, propylene/propane separation. Most importantly, and differently from typical gas separation membranes derived from glassy polymers, the crosslinked PIMs showed no obvious CO₂ plasticization up to 20 atm pressure of pure CO₂ and CO₂/CH₄ mixtures.

     

Superior Nanoporous Polyimides via Supercritical CO2 Drying of Jungle‐Gym‐Type Polyimide Gels

We report novel nanoporous polyimides formed from jungle‐gym‐type rigid polyimide gels by supercritical CO₂ drying. By virtue of supercritical CO₂ drying to avoid the collapse of nanostructure, porosity above 90 vol.‐% was achieved. We found a rich variety of nanoporous structures in the range of 50–800 nm such as crisp fragments, minute network, and highly‐connected beads. These characteristic structures were formed by the competitive progress of liquid‐liquid phase separation and crystallization induced due to the two chemical reactions of end‐crosslinking and thermal imidization during gelation.

     

High Performance Polyimide with High Internal Free Volume Elements

This paper reports a new polyimide design with high internal free volume elements for fast mass transport simultaneously with high selectivity. Here, we show that the polymer design using a three‐dimensional rigid molecular structure having internal void space can lead to the formation of high fractional free volume with proper cavity size to separate small gas molecules with high selectivities as high permeabilities. These findings could strongly impact emerging gas separation applications using polymeric membranes such as natural gas purification and biogas purification to get clean energy resources.

     

PEG‐Imbedded PEO Membrane Developed by a Novel Highly Efficient Strategy Toward Superior Gas Transport Performance

Low‐molecular‐weight poly(ethylene glycol) (PEG) is deliberately incorporated into synthesized swellable poly(ethylene oxide) (PEO) membranes via a facile post‐treatment strategy. The membranes exhibit both larger fractional free volume (FFV) and a higher content of CO₂‐philic building units, resulting in significant increments in both CO₂ permeability and CO₂/H₂ selectivity. The separation performance correlates nicely with the microstructure of the membranes. This study may provide useful insights in the formation and mass transport behavior of highly efficient polymeric membranes applicable to clean energy purification and CO₂ capture, and possibly bridge the material‐induced technology gap between academia and industry.

     

Microwave‐Assisted Aqueous Polyimidization Using High‐Throughput Techniques

High‐throughput methodology, as used in pharmaceutical drug discovery, was successfully applied to high performance polyimide development. Using a Biotage® microwave reactor fitted with a flask handling robot and a high‐throughput approach, we have optimized a copolyimide material with high T_g, yet maintaining solution processability. Additionally, the development time for this material has been reduced from months to just weeks. The polyimide was produced using combinations of the monomers 4,4′‐ODA, 6‐FDA and PMDA. The optimal combination was found to be 2:1:1 of 4,4′‐ODA/6‐FDA/PMDA with a molecular weight (MW) of 31 500 and T_g of 347 °C. The resulting polymer can be solution‐cast into strong, flexible membranes for solution and gas phase separations.

     

Preparation of the Ultra‐Low Dielectric Constant Polyimide Fiber Membranes Enabled by Electrospinning

In this article, electrospinning technique has been demonstrated for the synthesis of ultra‐low dielectric constant polyimide fiber membranes. Poly(amic acid) fiber membranes have been prepared as precursor. After the treatment of thermal imidization, ultra‐low dielectric constant polyimide fibers membranes can be obtained. The morphologies and structures of precursors and products are characterized by scanning electron microscopy (SEM), Fourier transmission infrared (FTIR) spectra, and a radio frequency (RF) impedance/capacitance material analyzer. The DK of the as‐prepared polyimide membrane ranges from 1.53 to 1.56, which could be applied in the electronic packaging industry.

     

Multi‐Morphological Complex Aggregates Formed from Amphiphilic Poly(ethylene oxide)‐block‐Polydimethylsiloxane‐block‐Poly(ethylene oxide) Triblock Copolymers

Summary: Amphiphilic triblock copolymers (PEO_x‐b‐PDMS_y‐b‐PEO_x) with different block lengths were synthesized and multi‐morphological complex crew‐cut, star‐like, and short‐chain aggregates were prepared by self‐assembly of the given copolymers. The morphologies and dimensions of the aggregates can be well controlled by variation of the preparation conditions. TEM, SEM, FFR‐TEM, and LLS studies show the resulting morphologies range from LCMs, unilamellar or multilayer vesicles, LCVs, porous spheres to nanorods.

TEM images of the vesicles formed from PEO‐b‐PDMS‐b‐PEO.     

Molecularly Engineered 6FDA‐Based Polyimide Membranes for Sour Natural Gas Separation

Glassy polyimide membranes are attractive for industrial applications in sour natural gas purification. Unfortunately, the lack of fundamental understanding of relationships between polyimide chemical structures and their gas transport properties in the presence of H₂S constrains the design and engineering of advanced membranes for such challenging applications. Herein, 6FDA‐based polyimide membranes with engineered structures were synthesized to tune their CO₂/CH₄ and H₂S/CH₄ separation performances and plasticization properties. Under ternary mixed sour gas feeds, controlling polymer chain packing and plasticization tendency of such polyimide membranes via tuning the chemical structures were found to offer better combined H₂S and CO₂ removal efficiency compared to conventional polymers. Fundamental insights into structure–property relationships of 6FDA‐based polyimide membranes observed in this study offer guidance for next generation membranes for sour natural gas separation.     

Synthesis of Hydrogels from Polyallylamine with Carbon Dioxide as Gellant: Development of Reversible CO2 Absorbent

Hydrogels were successfully synthesized utilizing CO₂ as a gellant. A cross‐linking reaction of polyallylamine (PAA) with CO₂ in the presence of 1,8‐Diazabicyclo[5,4,0]‐undec‐7‐ene (DBU) provided hydrogels bearing urea cross‐linking points and residual amino groups in the side chains. The obtained hydrogels absorbed CO₂ at 25 °C and gave a maximum absorption four times larger than that of PAA aqueous solution and 2.8 times larger than that of the most commonly used absorbent, monoethanolamine. The PAA hydrogels desorbed the absorbed CO₂ completely under a N₂ atmosphere at 120 °C, and could be repeatedly recycled without loss of efficiency, indicating their potential application as recyclable CO₂ absorption materials.

     

Quantifying Polymer Swelling Employing a Linear Variable Differential Transformer: CO2 Effects on SBS Triblock Copolymer

A linear variable differential transformer (LVDT) was employed to evaluate CO₂‐polymer plasticization. Preliminary results on polystyrene‐block‐polybutadiene‐block‐polystyrene (SBS) elastomer are presented. At 22 °C under CO₂ pressure, SBS undergoes compression due to hydrostatic pressure. However, sample expansion occurs upon depressurization. At 45 °C, SBS undergoes swelling of 0.7% due to CO₂ plasticization, while no post‐pressurization expansion is observed. The contrasting result is explained by change in PS domain mobility and discontinuity in the density‐pressure relationship.

Linear displacement of SBS as a function of time at 56 and 134 bar CO₂.     

In situ Growth of Pd Nanoparticles in Crosslinked Polymer Matrices

Polymer nanocomposites continue to receive considerable attention as multifunctional hybrid materials, with most nanocomposites fabricated by physical dispersion of surface‐functionalized nanoscale objects. In this study, we explore the viability of growing Pd‐containing nanoparticles from Na₂PdCl₄ in two different polymers: hypercrosslinked polystyrene (HPS) and an aromatic polyimide (PIm). In HPS, single Pd‐containing nanoparticles possessing a relatively narrow size distribution (ca. 1–4 nm) form upon reduction of the divalent PdCl ions. Single nanoparticles with a broad size distribution ranging from ≈2 to 16 nm develop in PIm, which simultaneously undergoes chemical crosslinking during ion reduction. Such hybrid materials hold promise in molecular catalysis and gas separation.

     

Molecularly engineering polymeric membranes for H2/CO2 separation at 100–300 °C

Over the last two decades, polymers with superior H₂/CO₂ separation properties at 100–300 °C have gathered significant interest for H₂ purification and CO₂ capture. This timely review presents various strategies adopted to molecularly engineer polymers for this application. We first elucidate the Robeson's upper bound at elevated temperatures for H₂/CO₂ separation and the advantages of high-temperature operation (such as improved solubility selectivity and absence of CO₂ plasticization), compared with conventional membrane gas separations at ~35 °C. Second, we describe commercially relevant membranes for the separation and highlight materials with free volumes tuned to discriminate H₂ and CO₂, including functional polymers (such as polybenzimidazole) and engineered polymers by cross-linking, blending, thermal treatment, thermal rearrangement, and carbonization. Third, we succinctly discuss mixed matrix materials containing size-sieving or H₂-sorptive nanofillers with attractive H₂/CO₂ separation properties.     

Thermally Stabilized Blue Luminescent Poly(p‐phenylene)s Covered with Polyhedral Oligomeric Silsesquioxanes

A novel series of poly(p‐phenylene)s (PPPs) with polyhedral oligomeric silsesquioxanes (POSSs) on their side chains was prepared. The obtained POSS‐modified PPPs are as follows: 25POSS‐PPP ( 2b , containing 25 mol‐% of POSS units in all side chains), 50POSS‐PPP ( 2c , containing 50 mol‐% of POSS units in all side chains), 100POSS‐PPP ( 2d , containing 100 mol‐% of POSS units in all side chains), and 0POSS‐PPP ( 2a , as a blank polymer). Films polymer 2d showed the same absorption and photoluminescence (PL) spectra as those in CHCl₃ solution, indicating that bulky POSS units strongly suppressed intermolecular aggregation of the PPP backbone. Polymer 2d showed the same PL spectra even after thermal annealing at 150 °C for 6 h. This enhancement of PL stability is due to the significant effect of the bulky POSS units.

     

Copolymers of Intrinsic Microporosity Based on 2,2′,3,3′‐Tetrahydroxy‐1,1′‐dinaphthyl

A series of new copolymers with high molecular weight and low polydispersity, prepared from tetrahydroxydinaphthyl, tetrahydroxyspirobisindane, and tetrafluoroterephthalonitrile monomers, prevent efficient space packing of the stiff polymer chains and consequently show intrinsic microporosity. One copolymer, DNPIM‐33, has an excellent combination of properties with good film‐forming characteristics and gas transport performance, and exhibits higher selectivity than the corresponding spirobisindane‐based homopolymer PIM‐1 for gas pairs, such as O₂/N₂, with a corresponding small decrease in permeability. This work demonstrates that significant improvements in properties may be obtained through development of copolymers with intrinsic microporosity (CoPIMs) that extends the spectrum of high‐molecular‐weight ladder structures of poly(dibenzodioxane)s.

     

Chain Transfer and Efficiency of End‐Group Introduction in Free Radical Polymerization of Methyl Methacrylate in the Presence of Poly(methyl methacrylate) Macromonomer

Summary: Experimental and modeling studies of addition–fragmentation chain transfer (AFCT) during radical polymerization of methyl methacrylate in the presence of poly(methyl methacrylate) macromonomer with 2‐carbomethoxy‐2‐propenyl ω‐ends (PMMA‐CO₂Me) at 60 °C are reported. The results revealed that AFCT involving PMMA‐CO₂Me formed in situ during methyl methacrylate polymerization has a negligible effect on the molecular weight distribution.

     

Ethylene Polymerization Catalyzed by Titanium(IV) Complexes of a Triaryloxoamine Ligand [TiX{(OArCH2)3N}]

Summary: Titanium complexes containing a triaryloxoamine ligand, [TiX{(O‐2,4‐R₂C₆H₂‐6‐CH₂)₃N}] (R = Me, tBu; X = OiPr, O‐2,6‐iPr₂C₆H₃), exhibited notable catalytic activity for ethylene polymerization in the presence of MAO, especially at temperatures between 100 and 120 °C. Their activity increased upon the addition of a small amount of AlMe₃.

     

Carbon Nanotubes as a Ligand in Cp2ZrCl2‐Based Ethylene Polymerization

Summary: We report a simple method for tuning catalytic property of a metallocene‐based catalyst, Cp₂ZrCl₂, for ethylene polymerization by the direct adsorption of Cp₂ZrCl₂ onto multi‐walled carbon nanotubes (MWCNTs). The direct interactions between MWCNTs and the Cp rings of Cp₂ZrCl₂ controlled the polymerization behaviors, and we could generate polyethylene with an extremely high molecular weight ( = 1 000 000) at 30 °C and under 1 atm of ethylene gas.

Preparation of Cp₂ZrCl₂‐MWCNT.