Pushing Rubbery Polymer Membranes To Be Economic for CO2 Separation: Embedment with Ti3C2Tx MXene Nanosheets.

Sustainable and energy-efficient molecular separation requires membranes with high gas permeability and selectivity. This work reports excellent CO2 separation performance of self-standing and thin-film mixed matrix membranes (MMMs) fabricated by embedding 2D Ti3C2Tx MXene nanosheets in Pebax-1657. The CO2/N2 and CO2/H2 separation performances of the free-standing membranes are above Robeson's upper bounds, and the performances of the thin-film composite (TFC) membranes are in the target area for cost-efficient CO2 capture. Characterization and molecular dynamics simulation results suggest that the superior performances of the Pebax-Ti3C2Tx membranes are due to the formation of hydrogen bonds between Ti3C2Tx and Pebax chains, leading to the creation of the well-formed galleries of Ti3C2Tx nanosheets in the hard segments of the Pebax. The interfacial interactions and selective Ti3C2Tx nanochannels enable fast and selective CO2 transport. Enhancement of the transport properties of Pebax-2533 and polyurethane when embedded with Ti3C2Tx further supports these findings. The ease of fabrication and high separation performance of the new TFC membranes point to their great potential for energy-efficient CO2 separation with the low cost of $29/ton separated CO2.

Thermal Degradation of Polystyrene under Extreme Nanoconfinement.

Extreme nanoconfinement has been shown to significantly affect the properties of materials. Here we demonstrate that extreme nanoconfinement can significantly improve the thermal stability of polystyrene (PS) and reduce its flammability. Capillary rise infiltration (CaRI) is used to infiltrate PS into films of randomly packed silica nanoparticles (NPs) to produce highly confined states. We demonstrate that as the NP size is decreased, increasing the degree of confinement, the isothermal degradation time is dramatically increased, by up to a factor of 30 at 543 K for PS confined in ∼3 nm pores. The activation energy of PS degradation is also increased, by 50 kJ/mol in the most confined state (∼3 nm pores). We demonstrate that the degradation proceeds through the film surface and from the center of large holes toward NP surfaces, indirect evidence that the process is diffusion limited. The surface-driven process dramatically reduces char formation even in large NP packings that show no significant changes in their dynamics and glass transition temperature (Tg) compared to the bulk.

A New Pentiptycene-Based Dianhydride and Its High-Free-Volume Polymer for Carbon Dioxide Removal.

In addition to possessing excellent chemical, mechanical, and thermal stability, polyimides and polyetherimides have excellent solubility in many solvents, which renders them suitable for membrane preparation. Two new monomers [a pentiptycene-based dianhydride (PPDAn) and a pentiptycene imide-containing diamine (PPImDA)] and a pentiptycene-based polyimide [PPImDA-4,4'-hexafluoroisopropylidene diphthalic anhydride (PPImDA-6FDA)] have been synthesized and characterized by FTIR and 1 H NMR spectroscopy, gel-permeation chromatography, mass spectrometry, X-ray photoelectron spectroscopy, thermogravimetric analysis, differential scanning calorimetry, BET surface area, and X-ray diffraction. High-molecular-weight PPImDA-6FDA has remarkable thermal stability and excellent solubility in common organic solvents. It also has an extraordinarily high fractional free volume (0.233) owing to the presence of -C(CF3 )2 - units, the rigid diamine, and the pentiptycene moiety in the polymer structure. It has high CO2 permeability (812 Barrer) owing to poor chain packing, which is caused by the fact that its rigid groups veil the influence of the ethereal oxygen groups in its backbone. It has the highest CO2 permeability among all reported pentiptycene-containing polymers (about six times higher than that of the most permeable one) without sacrificing selectivity. The high free volume, good microporosity, high solubility in many solvents, and remarkable thermal stability of PPImDA-6FDA point to the great potential of this polymer for CO2 removal.

Improving the Transport and Antifouling Properties of Poly(vinyl chloride) Hollow-Fiber Ultrafiltration Membranes by Incorporating Silica Nanoparticles.

Poly(vinyl chloride) (PVC)/SiO2 nanocomposite hollow-fiber membranes with different nano-SiO2 particle loadings (0-5 wt %) were fabricated using the dry-jet wet-spinning technique. Effects of SiO2 nanoparticles on the morphology of the prepared hollow-fiber membranes were investigated using scanning electron microscopy. Transport and antifouling properties of the fabricated membranes were evaluated by conducting pure-water permeation, solute rejection, and fouling resistance experiments. These studies indicated that incorporating silica nanoparticles into the PVC matrix during phase inversion lowers the hydraulic resistance through the membrane and narrows the selective membrane pores. Moreover, the nanocomposite membranes showed better antifouling properties compared to the pristine membrane during the ultrafiltration of a milk solution because of improved hydrophilicity and uniform dispersion of the nanoparticles. This work indicates that embedding silica nanoparticles into the PVC matrix is a promising method for producing cost-effective hollow-fiber ultrafiltration membranes with superior transport and antifouling properties.

Exploiting Synergetic Effects of Graphene Oxide and a Silver-Based Metal-Organic Framework To Enhance Antifouling and Anti-Biofouling Properties of Thin-Film Nanocomposite Membranes.

Thin-film composite (TFC) membranes still suffer from fouling and biofouling. In this work, by incorporating a graphene oxide (GO)-silver-based metal-organic framework (Ag-MOF) into the TFC selective layer, we synthesized a thin-film nanocomposite (TFN) membrane that has notably improved anti-biofouling and antifouling properties. The TFN membrane has a more negative surface charge, higher hydrophilicity, and higher water permeability compared with the TFC membrane. Fluorescence imaging revealed that the GO-Ag-MOF TFN membrane kills Escherichia (E.) coli more than the Ag-MOF TFN, GO TFN, and pristine TFC membranes by 16, 30, and 92%, respectively. Forward osmosis experiments with E. coli and sodium alginate suspensions showed that the GO-Ag-MOF TFN membrane by far has the lowest water flux reduction among the four membranes, proving the exceptional anti-biofouling and antifouling properties of the GO-Ag-MOF TFN membrane.