Title:Binary-cooperative patterned-crisscrossing membranes with gas separation

First author and affiliation: Bo Wang (Professor), Tianjin University of Technology

Corresponding authors and affiliation: Bo Wang (Professor) and Tie Wang (Professor), Tianjin University of Technology

Link:https://doi.org/10.1021/jacs.6c00692

1.Research Background.

Gas separation technology plays a critical role in addressing the dual challenges of energy transition and climate change. Membrane separation technology, owing to its relatively low energy consumption, operational simplicity, and environmental friendliness, has emerged as an alternative to traditional energy-consuming separation processes. Polymer membranes, due to their superior processability and low manufacturing costs, have consistently dominated the industrial gas separation market. Crosslinking is an effective approach to enhancing the gas separation performance of polymer membranes; however, several issues remain. First, condensable gas molecules tend to swell the polymer, increasing free volume and reducing separation efficiency. Second, high-pressure gas molecules exert external stress on the polymer, which can lead to the formation of microscopic defects, thereby weakening separation performance. Low-crosslinked polymers possess high chain segment mobility, allowing rapid diffusion of gas molecules and effectively alleviating stress concentration caused by gas accumulation on the membrane surface, but they suffer from low gas selectivity. Therefore, maintaining high gas permselectivity and structural stability of polymer membranes under high-pressure conditions remains a challenging problem that urgently needsto be addressed.

2.Full Text Summary.

A research team led by Tie Wang and Bo Wang from Tianjin University of Technology published a paper in theJournal of the American Chemical Societytitled“Binary-cooperative patterned-crisscrossing membranes with gas separation”.Inspired by the hard-soft alternating structural motifs found in natural mechanical materials, the team proposed a design strategy for “binary-cooperative patterned-crisscrossing membranes”.Using a zone-thermal-crosslinking strategy, the crosslinking density distribution of the polymer membranecan beprecisely controlled. By employing porous plates with five different pore sizes (1, 3, 5, 7, and 9 mm), they fabricated five types of patterned-crisscrossing polymer (PCCP) membranes featuring alternating high- and low-crosslinking regions with distinct crosslinking characteristics, named PCCP-1, PCCP-2, PCCP-3, PCCP-4, and PCCP-5. This approach simultaneously improved the gas permselectivity and structural stability of the polymer membrane under high pressure.

3.Graphic and Textual Guide.

Conventional polymer membranes are typically prepared using a single crosslinking method. As shown in Figure 1, the high-crosslinked state exhibits excellent gas separation performance; however, there is still limited room for improving separation performance under high pressure. Moreover, under high pressure, the accumulation of a large number of gas molecules on the membrane surface leads to stress concentration, which easily results in surface defects (Figure 1a). In contrast, in the low-crosslinked state, gas molecules can rapidly permeate, avoiding surface molecular accumulation, but the swelling behavior under high pressure also causes the membrane to lose gas selectivity (Figure 1b). Therefore, we propose the construction of a patterned-crisscrossing polymer (PCCP) structure within the polymer membrane (Figure 1c). Finite element simulations confirm that the external stress attached to the high-crosslinked regions in the PCCP structure can be released and absorbed by the low-crosslinked regions, enabling the PCCP membrane to exhibit excellent gas permeability and selectivity.

Fig. 1.Gas transport behavior and mechanical simulation of (a) high-crosslinked membrane, (b) low-crosslinked membrane, and (c) PCCP membrane.

We used a zone-thermal-crosslinking strategy to construct a series of polydimethylsiloxane (PDMS) PCCP-x membranes (where x=1-5 corresponds to pore sizes of 1, 3, 5, 7, and 9 mm, respectively), all featuring spatially arranged high-and low-crosslinked regions. As shown in Figure 2a, a polysulfone (PSf)substratewas preheated on a porous plate until thermal equilibrium was reached. Then, a PDMS prepolymer solution was cast onto the substrate and crosslinked to form the final PCCP membrane. The difference in heat transfer coefficients between the pore and plate regions of the porous plate induced the formation of distinct crosslinked regions. The maximum temperature difference between the pore and plate regions of the PCCP membrane was 35 °C. AFM revealed differences in surface roughness between the pore and plate regions, and XRD together with molecular weight between crosslinks (M_c) analysis also confirmed the difference in crosslinking states between the plate and pore regions.

Fig. 2.Fabrication process and structural characterization of PCCP membranes.

The mechanical behavior of PCCP membranes was analyzed using nanomechanical imaging and tensile testing. Figure 3 shows that the modulus imaging of the pore and plate regions reveals significant mechanical differences, particularly for the PCCP-3 to PCCP-5 membranes. Nanoindentation tests provided three key parameters: indentation depth, adhesion, and Young’s modulus, which reflect the material’s stiffness, chain segment mobility, and resistance to deformation, respectively. The pore regions (low-crosslinked regions) exhibited greater adhesion and indentation depth but lower Young’s modulus compared to the plate regions, indicating higher chain mobility and flexibility. Finite element simulations demonstrated that the low-crosslinked regions effectively release the tensile stress applied to the high-crosslinked regions, with the PCCP-3 membrane exhibiting the best stress release effect. Uniaxial tensile tests also confirmed that the PCCP-3 membrane achieves an elongation at break of 135% and a fracture stress of 14.2 MPa, achieving a balance between toughness and strength.

Fig. 3.Analysis of mechanical properties of PCCP membranes.

The PCCP membranes exhibit excellent highpressure gas separation performance. The CO₂/CH₄selectivity of PCCP3 reaches 42, much higher than that of the single-crosslinked membranes HC-70 and LC-30, both of which haveCO₂/CH₄selectivitybelow 10. After highpressure exposure, PCCP-3 maintains its structural stability; the modulus of the low-crosslinked region even shows a slight increase. Young’s modulus calculations and molecular simulations together confirm its structural stability and high gas diffusivity. Moreover, the gas separation performance of the PCCP membranes surpasses that of many advanced membrane materials. The excellent performance is attributed to the synergistic mechanism of stress dispersion and swelling suppression. In the PCCP membrane, the swelling stress (F₁) generated by gas molecules and the compressive stress (F₂) induced by external high pressure are antagonistic.F₁usually dominates, leading to a decline in sieving capability. In the PCCP membrane, the roles ofF₁andF₂can be dynamically tuned with the size of the low-crosslinked regions: when the spacing is large (11.5 mm, PCCP-1), the lowcrosslinked region cannot effectively shareF₂, swelling dominates, and the performance resembles that of the single highcrosslinked state HC-70. When the spacing decreases to 7.5 mm (PCCP-3),F₂is partially absorbed by the low-crosslinked region, avoiding stress concentration and cracking of the high-crosslinked network. Meanwhile, the absorbed swelling stress is converted into an inward compressive stress that acts on the surrounding low-crosslinked region, suppressing its swelling. This achieves maximum synergy between stress dispersion and swelling suppression, resulting in optimal CO₂/CH₄selectivity. When the spacing is too small (3.5 mm, PCCP-5), the proportion of the low-crosslinked region becomes too high, the continuous high-crosslinked sieving network is disrupted, gas transport is dominated by swelling, leading to high permeability but low selectivity, similar to the single low-crosslinked membrane LC-30.

Fig. 4.Gas separation performance of PCCP.

4. Summary and Outlook.

In this study,azone-thermal-crosslinking strategy wasadoptedto construct polymer membranes with a patterned crisscrossing architecture. The patterned structure not only combines the structural advantages of high-and low-crosslinked regions but also generates a synergistic effect of stress dispersion and swelling suppression, effectively maintaining the structural stability of the membrane under high pressure. This work provides a design concept for next-generation membrane materials capable of maintaining robust selective separation in complex industrial environments. It breaks away from the conventional thinking of material selection and demonstrates that the spatial arrangement of functional domains—i.e., the size effect—is an important variable for designing high-performance membrane materials. Moreover, this design can be extended to other functional polymer systems, such as polar/non-polar, rigid/flexible, or magnetic/non-magnetic combinations, significantly broadening the application fields of membrane materials.