Behnam Ghalei

Controlled thermal oxidative crosslinking of polymers of intrinsic microporosity towards tunable molecular sieve membranes

Organic open frameworks with well-defined micropore (pore dimensions below 2 nm) structure are attractive next-generation materials for gas sorption, storage, catalysis and molecular level separations. Polymers of intrinsic microporosity (PIMs) represent a paradigm shift in conceptualizing molecular sieves from conventional ordered frameworks to disordered frameworks with heterogeneous distributions of microporosity. PIMs contain interconnected regions of micropores with high gas permeability but with a level of heterogeneity that compromises their molecular selectivity. Here we report controllable thermal oxidative crosslinking of PIMs by heat treatment in the presence of trace amounts of oxygen. The resulting covalently crosslinked networks are thermally and chemically stable, mechanically flexible and have remarkable selectivity at permeability that is three orders of magnitude higher than commercial polymeric membranes. This study demonstrates that controlled thermochemical reactions can delicately tune the topological structure of channels and pores within microporous polymers and their molecular sieving properties.

Plasticization resistant crosslinked polyurethane gas separation membranes

Polyurethanes (PUs) with good film formation ability and high gas separation properties are promising materials for gas separation membranes. However, low mechanical properties and high CO2 plasticization limit the industrial application of these membranes. Here, we synthesized a crosslinkable PU structure using a 1 : 3 : 2 molar ratio of Pluronic L61, isophorone diisocyanate (IPDI) and 3,5-diaminobenzoic acid (DABA). In order to improve both mechanical properties and plasticization resistance, a series of crosslinking agents with different chain lengths and functionalities were used to crosslink the PU via an esterification-based reaction. Pure (H2, CO2, N2, CH4, and C2H6) and mixed (CO2/N2 and CO2/CH4) gas permeability experiments were performed on the crosslinked PU (XPU) membranes. The XPU membranes showed enhanced mechanical properties and chemical stability and improved plasticization resistance to an extent about three times higher than the non-crosslinked PU and commercial membranes (PEBAX® 2533). Mechanical tests indicated an improvement of over 600% in Youngs modulus and 200% in hardness for XPUs compared to the pristine PU. The resulting crosslinked membranes with high CO2 separation performance (CO2/N2 ∼ 30) and superior thermal and mechanical properties are attractive candidates for industrial separation processes.

Structural and transport properties of polydimethylsiloxane based polyurethane/silica particles mixed matrix membranes for gas separation

Mixed matrix membranes of synthesized polyurethane (PU) based on toluene diisocyanate (TDI), polydimethylsiloxane (PDMS) and polytetramethylene glycol (PTMG) with polyvinyl alcohol based polar silica particles were prepared by solution casting technique. The homogeneity and thermal properties of the prepared PDMS-PU/silica membranes were characterized using scanning electron microscope (SEM), differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). The SEM micrographs confirmed the distribution of silica particles in the polymer matrix without agglomerations. Gas permeation properties of membranes with different silica contents were studied for pure CO2, CH4, O2, He and N2 gases. The obtained results indicated the permeability of the condensable and polar CO2 gas was enhanced whereas permeability of other gases decreased upon increasing the silica content of the mixed matrix membranes. The permeability of CO2 and its selectivity over N2 was increased from 68.4 Barrer and 22 in pure PDMS-PU to 96.7 Barrer and 64.4 in the mixed matrix membranes containing 10 wt% of the silica particles.

Surface functionalization of high free-volume polymers as a route to efficient hydrogen separation membranes

There is a sparcity of polymeric membranes with sufficient selectivity for efficient hydrogen separation from syn-gas products, primarily carbon dioxide. Despite hydrogens significantly smaller kinetic diameter, low selectivity arises as other gases are generally more condensable within typical polymeric membranes. Here we report an in situ-controllable, surface polymerization of polydopamine (PDA) and polyaniline (PANI) on high free-volume glassy polymer films, specifically the well studied polymer of intrinsic microporosity (PIM-1) and poly(1-trimethylsilyl-1-propyne) (PTMSP). The resulting nanolayer composite membranes demonstrate a remarkable hydrogen selectivity against N2, CH4 and CO2 (H2/CO2 ∼ 50). The PDA or PANI layers principally serve to increase the diffusive selectivity towards hydrogen whilst the high free volume supports of PIM-1 or PTMSP provide a highly permeable interface for defect-free growth of the selective layer. Whilst both PANI and PDA are effective, these selective layers were found to grow by heterogeneous or homogeneous modes respectively.

Characterization and Gas Permeation Properties of Synthesized Polyurethane-Polydimethylsiloxane / Polyamide 12-b-Polytetramethylene Glycol Blend Membranes

Blend membranes of synthesized polyurethane (PU) based on toluene diisocyanate (TDI), polydimethylsiloxane (PDMS) and polytetramethylene glycol (PTMG) with polyamide 12-b- polytetramethylene glycol (PA12-b-PTMG) were prepared by a solution casting technique. The heterogeneous microstructures of the blend membranes (PU /PA12-b-PTMG) were characterized by Fourier transform infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). Gas transport properties were determined for O2, N2, CH4, and CO2 gases and the obtained permeabilities were correlated with polymer properties and morphology of the membranes. Comparison of the results with that of the pure PU membrane indicates that the blend membranes had higher permeability to CO2, but lower permeability to O2, N2 and CH4 gases, and, therefore, had higher values of CO2/N2 and CO2/CH4 ideal gas pair selectivities. The blend membrane with 20 % (wt) PA12-b-PTMG showed the highest CO2 permeability (≈105 Barrer) compared to the PU and other blend membranes. In the blend membranes with 5–20 % (wt) PA12-b-PTMG contents an enhancement of CO2/CH4 (≈10) and CO2/N 2 (≈52) selectivities was observed. The experimental permeabilities of the blend membranes were compared with the calculated permeabilities based on a modified additive logarithmic model.

Pentiptycene-Based Polyurethane with Enhanced Mechanical Properties and CO2-Plasticization Resistance for Thin Film Gas Separation Membranes

The development of thin film composite (TFC) membranes offers an opportunity to achieve the permeability/selectivity requirements for optimum CO2 separation performance. However, the durability and performance of thin film gas separation membranes are mostly challenged by weak mechanical properties and high CO2 plasticization. Here, we designed new polyurethane (PU) structures with bulky aromatic chain extenders that afford preferred mechanical properties for ultra-thin-film formation. An improvement of about 1500% in Youngs modulus and 600% in hardness was observed for pentiptycene-based PUs compared to the typical PU membranes. Single (CO2, H2, CH4, and N2) and mixed (CO2/N2 and CO2/CH4) gas permeability tests were performed on the PU membranes. The resulting TFC membranes showed a high CO2 permeance up to 1400 GPU (10-6 cm3(STP) cm-2 s-1 cmHg-1) and the CO2/N2 and CO2/H2 selectivities of about 22 and 2.1, respectively. The enhanced mechanical properties of pentiptycene-based PUs result in high-performance thin membranes with the similar selectivity of the bulk polymer. The thin film membranes prepared from pentiptycene-based PUs also showed a twofold enhanced plasticization resistance compared to non-pentiptycene-containing PU membranes.

A facile synthesis of contorted spirobisindane-diamine and its microporous polyimides for gas separation

Microporous polyimides (PIM-PIs, KAUST-PIs) and polymers containing Trogers base (TB) derivatives with improved permeability and selectivity have great importance for separation of environmental gas pairs. Despite the tremendous progress in this field, facile synthesis of microporous polymers at the industrial scale via designing new monomers is still lacking. In this study, a new potential approach for large scale synthesis of spirobisindane diamine (DAS) (3) has been reported from commercially available 5,5′,6,6′-tetrahydroxy-3,3,3′,3′-tetramethyl-1,1′-spirobisindane (TTSBI) and 3,4-difluoronitrobenzene. A series of DAS diamine based microporous polyimides were also synthesized. The resulting polymer membranes showed high mechanical and thermal properties with tunable gas separation performance.

Effect of Polyvinyl Alcohol Modified Silica Particles on the Physical and Gas Separation Properties of the Polyurethane Mixed Matrix Membranes

Silicon based particles were prepared using tetraethoxysilane (TEOS) as a silica monomer, and low concentration of polyethylene oxide-polypropylene oxide block copolymer (pluronic) with polyvinyl alcohol (PVA) as templating agents. The synthesized particles showed higher polarity compare with conventional silica particles. PU/silica mixed matrix membranes (MMMs) were prepared by solution casting technique. The membranes were characterized using Fourier transform infrared spectroscopy (FT-IR), scanning electron microscope (SEM) and differential scanning calorimetry (DSC). FT-IR result confirmed the existence of PVA in the final structure of the synthesized silica network. The SEM micrographs indicated an appropriate distribution of silica particles in the polymer matrix. Gas transport properties of membranes were studied for pure CO2, CH4, O2 and N2 gases at 10 bar and 25 ∘C. The results showed that the permeabilities of CH4 and CO2 enhanced whereas that of other gases decreased with increasing the modified silica contents. In the membrane with 10 wt.% silica content, an enhancement of CO2/CH4 (α ≈ 7.7) and CO2/N2 (α ≈ 91.4) selectivities was observed.

Tuning the Gas Selectivity of Tröger's Base Polyimide Membranes by Using Carboxylic Acid and Tertiary Base Interactions

Polyimide-based materials provide attractive chemistries for the development of gas-separation membranes. Modification of inter- and intra-chain interactions is a route to improve the separation performance. In this work, copolyimides with Trögers base (TB) monomers are designed and synthesized. In particular, a series of copolyimides is synthesized with different contents of carboxylic acid groups (0-50 wt %) to alter the inter- and intra-chain interactions and enhance the basicity of the TB-polyimides. A detailed thermal and structural analysis is provided for the new copolyimides. Gas permeation data reveal a tunable trend in separation performance with increasing carboxylic acid group content. Importantly, this is one of the few examples of copolyimide membranes materials that show enhanced plasticization resistance to high-pressure gas feeds through physical cross-linking.

Sol-Gel Synthesized Nanostructured Silica Particles for Application in Gas Transport Properties of PU-PDMS Based Mixed-Matrix Membranes

New Silicon based nanostructures particles were successfully synthesized by tetraethoxysilane (TEOS) as silica precursor, mixture of two copolymers of PPG-b-PPE-b-PPG and PEO-b-PPG-b-PEO (Plutonic) as templates. PU-PDMS and PU-PDMS/silica composite membranes were prepared by solution casting technique. Synthesized mesoporus silica particles and hybrid membranes were characterized using Nitrogen adsorption-desorption (BET), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). As confirmed by BET analysis, the mean pore diameters of SPB1,2 Silicon particles are around 9 nm. The SEM micrographs confirmed the nanoscale distribution of silica particles in the polymer matrix. Gas permeation of PU-PDMS/silica hybrid membranes with silica contents of 5, 10 and 20 wt.% was studied for N2, CO2 and He single gases at8 bar. The obtained results suggest a significant increase in permeability of all gases upon increasing the silica particles content. The possible reasons for such behavior were stated and discussed.