Critical Role of the Molecular Interface in Double-Layered Pebax-1657/PDMS Nanomembranes for Highly Efficient CO₂/N₂ Gas Separation

Abstract

In this work, we deposited a CO2-selective block copolymer, Pebax-1657, as a selective layer with a thickness of 2–20 nm on the oxygen plasma-activated surface of poly­(dimethylsiloxane) (PDMS) used as a gutter layer (thickness ∼400 nm). This double-layered structure was subsequently transferred onto the polyacrylonitrile (PAN) microporous support and studied for CO2/N2 separation. The effect of interfacial molecular arrangements between the selective and gutter layers on CO2 permeance and selectivity has been investigated. We have revealed that the gas permeance and selectivity do not follow the conventional theoretical predictions for the multilayer membrane (resistance in series transport model); specifically, more selective CO2/N2 separation membranes were achieved with ultrathin selective layers. Detailed characterization of the chemical structure of the outermost membrane surface suggests that nanoscale blending of the ultrathin Pebax-1657 layer with O2 plasma-activated PDMS chains on the surface takes place. This nanoblending at the interface between the selective and gutter layers played a critical role in enhancing the CO2/N2 selectivity. CO2 permeances in the developed thin-film composite membranes (TFCM) were between 1200 and 3500 gas permeance units (GPU) and the respective CO2/N2 selectivities were between 72 and 23, providing the gas separation performance suitable for CO2 capture in postcombustion processes. This interpenetrating polymer interface enhanced the overall selectivity of the membrane significantly, exceeding the separation ability of the pristine Pebax-1657 polymer.

A new strategy for membrane-based direct air capture

“…4). We believe it is promising to provide membranes for m-DAC because the layers are often made of versatile, low-cost polymers that can be fabricated with thicknesses of a few tens of nanometers without losing the performance [18], while the selectivity can be improved by surface modification [19]. Moreover, for m-DAC, it is important to study all materials that show promising bulk performance in the form of thin membranes or thin composites to determine whether the separation is influenced by the thickness and whether large-scale, mechanically stable membranes can be fabricated.…”

“…For the initial investigation of the feasibility of m-DAC, we assumed a membrane with a permeance of 40,000 GPU and selectivity of 70, for which performances have been separately achieved in different membranes [18,19]. Aspen Plus V11 (AspenTech, USA) flowsheeting software with the Peng-Robinson state equation thermodynamic model was used.…”

Poly(ether-block-amide) Copolymer Membranes in CO2 Separation Applications

“…58 Highly gas permeable polydimethylsiloxane (PDMS) and poly(1-(trimethylsilyl)-1-propyne) (PTMSP) coatings are the most commonly used gutter layers in a TFC membrane fabrication. The ideal gutter layer material should be characterized by a high permeability to gas, thus providing the perfect interface to decrease the TFC thickness down to 2−20 nm, 60 as recently achieved with Pebax MH1657 coated on oxygen plasma treated PDMS. 61 Moreover, support and selective top layers can be separately optimized to achieve a high performing composite membrane.…”

“…CO 2 permeance of the final Pebax-PEG-DME/amino-PDMS/PAN composite membrane reached 400 GPU with CO 2 /N 2 selectivity over 65. Moreover, a Pebax MH1657 layer with 2–20 nm thickness prepared on an oxygen plasma activated PDMS surface was transferred to a PAN support yielding CO 2 permeances between 1200 and 3500 GPU with respective CO 2 /N 2 selectivities between 72 and 23 Table presents the CO 2 /N 2 separation performance of different Pebax based composite membranes along with the type of support used.…”

“…Moreover, as it was mentioned in section on composite membranes, PDMS and PTMSP can be used to coat a selective layer to seal all the possible cracks and defects in order to optimize and achieve a high-performing MMM …”