Polyethylene oxide rubbery organic framework (ROF) membranes with enhanced CO2 permeability and CO2/CH4 selectivity

Rubbery organic frameworks (ROFs), assembled via reversible covalent bonds under dynamic molecular control, represent a promising class of adaptive polymers for gas separation membranes. Elastomeric ROF membranes exhibit excellent mechanical stability, dynamic responsiveness, and intrinsic microporosity. Their affinity for carbon dioxide (CO₂) enables both high CO₂ permeability and enhanced selectivity compared to conventional glassy polymeric membranes. One effective strategy for improving CO₂ separation performance is the incorporation of polyethylene oxide (PEO) units into the ROF structure. Owing to the high CO₂ solubility and electrostatic interactions with PEO segments, this approach can significantly boost CO₂ selectivity over other gases such as methane (CH₄). In this study, a new class of PEO-based ROF membranes were developed and tailored by varying the length of PEO segments to optimize both mechanical strength and CO₂/CH₄ separation performance. The membranes were systematically characterized to understand the relationship between their molecular architecture, morphology, and gas transport properties. The resulting ROF membranes demonstrated CO₂ permeabilities ranging from 155 to 180 barrer and CO₂/CH₄ selectivities between 15 and 31. Notably, a synergistic enhancement in both CO₂ permeability and selectivity was observed with increasing PEO segment length. This improvement is attributed to a favorable balance of polymer chain packing, diffusivity, and CO₂ affinity within the membrane matrix. These findings highlight the potential of PEO-integrated ROFs as versatile and high-performance materials for advanced gas separation applications.

GRAPHICAL ABSTRACT

1 Introduction

In the last decade, membrane technology based on polymeric materials has received great attention in gas separation applications, offering numerous eco-friendly and cost-effective benefits over conventional separation processes such as sorption–desorption and cryogenic distillation (Brunetti et al., 2010) due to notable advantages such as simple design and scale-up and energy saving (Alexander Stern, 1994; Budd et al., 2005; Basu et al., 2010; D'Alessandro et al., 2010; Reijerkerk et al., 2010; Yave et al., 2010; Zou and Zhu, 2018; McKeown, 2020). Furthermore, membrane-based processes offer low capital and operating costs, design flexibility and consistent performance and are suitable for remote areas. Most significantly, they eliminate the need for phase changes or thermal forces, which can reduce energy consumption by up to 90% compared to conventional thermal separation methods (Corrado and Guo, 2020). However, the primary goal in developing gas separation membranes is to achieve both high permeability and adequate selectivity, which is typically limited by the well-known trade-off between these two properties (Robeson, 1991; Freeman, 1999; Robeson et al., 2009; Swaidan et al., 2015; Comesaña-Gándara et al., 2019).

Despite efforts to produce novel polymeric membranes for gas separation processes that can overcome the limitations of commercial membranes and compete with current separation technologies, only a few conventional polymeric materials are in use (Wang et al., 2016). Recently, both commercial and exploratory polymeric membranes were both reported for pair gas separation (CO₂/CH₄) (Freeman, 1999; Robeson et al., 2009; Corrado and Guo, 2020; Refaat et al., 2024; Yahia et al., 2024; Yahia et al., 2025). However, these membranes often exhibit either high permeability with low selectivity or vice versa (Staudt-Bickel and J. Koros, 1999; Pourafshari Chenar et al., 2006), significantly limiting their scalability and practical application. Most membranes are made from glassy or rubbery polymers. To overcome these challenges, a recent class of composite membranes, known as mixed matrix membranes (MMMs), were developed that combine the advantages of polymer material flexibility and high selectively porous materials such as metal organic frameworks (MOFs) and zeolites (ZIFs) (Aroon et al., 2010; Denny et al., 2016; Koros and Zhang, 2017; Chuah et al., 2018; Refaat et al., 2024; Yahia et al., 2024; Yahia et al., 2025). This has led to a synergistic approach of improving membrane selectivity and permeability to surpass upper-bound correlations (Swaidan et al., 2015; Comesaña-Gándara et al., 2019). Recently, polyethylene oxide (PEO)-based polymers have shown promising potential for separating CO₂ from CH₄, owing to the strong adsorption affinity between CO₂ and ether oxygen atoms. Such polymers were fabricated via UV irradiation as cross-linked polymer membranes, demonstrating good performance for CO₂/CH₄ separation (Patel et al., 2004; Lin et al., 2006a; Lin et al., 2006b). Nevertheless, these systems face several drawbacks, such as difficulty in large-scale fabrication, inhomogeneity, film defects, high crystallinity, limited mechanical strength, and high operating costs, restricting their industrial applicability (Xing and Ho, 2009). Different design strategies have been applied to solve these issues, such as copolymerization, crosslinking, and physical blending with other polymers (polyethylene glycol–PEG or polyimides) to generate permeable PEO-based membranes for CO₂ separation with controlled physical properties based on PEO molecular weight or unit length (Car et al., 2008; Petzetakis et al., 2015; Ioannidi et al., 2021). Moreover, membrane microstructure design and control over fractional free volume (FFV) have become critical topics in enhancing CO₂ separation performance (Wang et al., 2016). Therefore, several studies have investigated PEO-based membranes or membrane containing polar ether groups that can interact with acidic CO₂ to improve CO₂ separation performance (Kawakami et al., 1982; Li et al., 1995; Okamoto et al., 1995; Chatterjee et al., 1997; Suzuki et al., 1998; Bondar et al., 2000; Yoshino et al., 2000; Kim et al., 2001; Sanchez et al., 2002; Lin and Freeman, 2004).

“Dynameric membranes” refer to membranes constructed from dynamic polymers (dynamers), which are formed via reversible covalent bonds (Barboiu, 2013). These bonds allow the polymer network to undergo self-healing, reconfiguration or adaptive behavior in response to external stimuli (e.g., heat, solvents, and pH). In membrane science, such dynamic covalent frameworks provide tunability, defect correction, and potential recyclability, offering advantages over traditional static polymeric membranes. Rubbery organic framework (ROF) membranes are among the most well-known types of dynamers. Our group has recently pioneered their development for gas separation applications (Nasr et al., 2012; Zhang and Barboiu, 2016; Dupuis et al., 2022).

ROFs are formed through dynamic molecular control using reversible covalent bonds between core centers and flexible connectors, resulting in mechanically stable membranes with high permeability and enhanced selectivity compared to traditional polymeric membranes (Nasr et al., 2008; Roy et al., 2015; Dupuis et al., 2022; Sandru et al., 2024). Their structure is controlled at the molecular level, offering exceptional flexibility, mechanical stability, and guest-responsiveness. This innovative strategy opens new avenues for creating adaptive membranes with excellent gas transport performance (Nasr et al., 2008; Roy et al., 2015; Dupuis et al., 2022). Additionally, the wide variety of available building blocks with diverse shapes and chemical structures allows for unlimited design flexibility, giving ROFs high tunability for specific gas separations (Nasr et al., 2012; Zhang and Barboiu, 2016; Dupuis et al., 2022). Furthermore, they represent a promising class of dynamic polymers that differ fundamentally from conventional crystalline frameworks such as metal–organic frameworks (MOFs) and covalent organic frameworks (COFs). While MOFs and COFs rely on rigid and ordered architectures, ROFs are constructed via reversible covalent bonding (e.g., imine, hydrazone or boronate ester linkages) that affords an amorphous, elastomeric structure with inherent flexibility. This dynamic network enables ROFs to present a segmental chain mobility, leading to a unique balance of high gas permeability and selectivity, by avoiding the high cross-linking behaviors due to excessive segmental mobility. One of the key advantages of ROFs is their ability to form free-standing, flexible membrane films without requiring blending with other polymers, in contrast with MOFs and COFs that are commonly used as fillers in mixed-matrix membranes (MMMs) or as thin layers on supports. Moreover, the rubbery nature of ROFs contributes to good processability, thermal stability, and mechanical resilience—essential features for practical membrane fabrication and operation under variable conditions (Zhang and Barboiu, 2016; Kang et al., 2023; Sandru et al., 2024). These characteristics position ROFs as a versatile and scalable platform for gas separation applications, particularly for CO₂ capture and light gas separation where membrane flexibility, high throughput, and selective transport are critical.

Our study aims to construct economically and technically viable ROF membranes incorporating polyethylene oxide (PEO) segments for CO₂/CH₄ separation. To achieve optimal mechanical strength and gas separation performance, the membrane architecture was tailored and controlled using different solvents and PEO-based monomers during the polymerization reaction. Four PEO-based ROF membranes were fabricated via imine chemistry, involving a benzene-1,3,5-tricarbaldehyde core and PEO-diamine segments containing ethylene oxide chains of varying lengths. The membranes were structurally characterized to evaluate their chemical structure, thermal stability, and morphology. Their single gas separation performance was tested for CO₂/CH₄ separation, and the results were benchmarked against previously reported membranes (Luo et al., 2016; Azizi et al., 2017; Bandehali et al., 2020) and positioned on the Robeson upper bound plots (Robeson, 1991; Freeman, 1999; Robeson, 2008; Robeson et al., 2009; Rowe et al., 2010; Swaidan et al., 2015; Comesaña-Gándara et al., 2019).

2 Materials and methods

2.1 Materials

For synthesizing ROFs, benzene-1,3,5–tricarbaldehyde— 2,2′-(Ethylenedioxy)bis(ethylamine) or 4,7,10-trioxa-1,13-tridecanediamine—acetonitrile (MeCN), and N-methyl-2-pyrrolidone (NMP) were used. All reagents were purchased from Sigma-Aldrich and used without further purification (purity ≥98%).

2.2 Polymer synthesis and membrane fabrication

Four polymeric materials (P1–P4) were synthesized and used to fabricate the corresponding membranes (M1–M4). Each polymer was obtained via an imine-condensation reaction between benzen-1,3,5-tricarbaldehyde core centers and one of 2,2′-(Ethylenedioxy)bis(ethylamine) or 4,7,10-trioxa-1,13-tridecanediamine segments. Polymers P1 and P2 were prepared using the former, while P3 and P4 were synthesized using the latter with an aldehyde-to-diamine molar ratio of 1:1.5. MeCN was used as the solvent for P1 and P3, while NMP was used for P2 and P4. The resulting polymer solutions were transferred into 100-mL round-bottom flasks and refluxed at 70 °C with continuous stirring overnight. The chemical structures of the reaction units used to synthesize the four polymers are presented in Supplementary Figure S1.

After polymerization, membranes M1–M4 were obtained by casting the corresponding polymer solutions (P1–P4) onto Teflon plates, followed by slow solvent evaporation and drying under vacuum at 80 °C overnight. The choice of MeCN and NMP as solvents for ROF synthesis was based on their polar aprotic nature, which supports reversible imine bond formation and ensures good solubility of all monomers. Moreover, their distinct boiling points and polarity profiles influence membrane morphology: MeCN promotes faster evaporation and denser structures, while NMP allows more reaction time, probably leading to an open network formation due to slower solvent removal. These differences impact the microstructure and, ultimately, the gas separation performance of the resulting membranes.

2.3 Membrane characterization

The physicochemical properties of fabricated ROF membranes were characterized before gas permeability testing. The chemical structures were confirmed by ¹H NMR (BRUKER NMR AVANCE 300 MHz) and FTIR spectroscopy (Nicolet 710). Thermal stability was assessed via thermogravimetric analysis (TGA, Hi-Res TGA 2950, TA Instruments) and differential scanning calorimetry (DSC, Modulated DSC 2920, TA Instruments). The thickness and quality of the cast membranes were examined using scanning electron microscopy (SEM, Hitachi S-4800 field emission microscope). A digital micrometer was used to manually measure the membrane thickness at five different points. The average value and standard deviation were calculated to ensure accuracy and account for variability. The contact angle was measured using ImageJ software to provide insights into the membrane surface properties, particularly hydrophobicity.

2.4 Membrane permeability measurements

Single gas permeability measurements for CO₂ and CH₄ gases with purity (99.99%) were carried out at 30 °C using the experimental setup shown in Figure 1. The setup was composed of two identical compartments (feed and permeate) made of stainless steel, which were separated by the membrane to be tested with an effective surface area of 9.62 cm². The desired pure gas was pressurized through both compartments (feed and permeate) and a pressure difference (∼0.7 bar) was imposed after opening the permeate outlet. Two pressure transducers (Druck, PDCR 910 models 99166 and 991675, England) were used to monitor the pressure profiles in each compartment. An Elcometer^® 124 Thickness Gauge (United Kingdom) instrument was used to estimate the membrane thickness.

Figure 1

Figure 1. Experimental setup for single gas permeability measurements (Neves et al., 2012; Abdelrahim et al., 2017).

2.4.1 Permeability and CO₂/CH₄ ideal selectivity

The single gas permeabilities through the fabricated membranes were calculated from the pressure data recorded from the feed and permeate compartments (Figure 1) according to Equation 1 (Cussler, 2009):

$\frac{1}{\beta }\ln \left(\frac{{\left[P_{feed}-P_{perm}\right]}_0}{\left[P_{feed}-P_{perm}\right]}\right)=P\frac{t}{l}(1)$

—where feed (P_feed) and permeate pressure (P_perm) are in (bar), membrane permeability (P) is in (m²/s), membrane thickness (l) in (m), time (t) in (s), and the parameter characteristic of the cell geometry (β) in (m^-1) are calculated as per Equation 2 (Cussler, 2009).

$\beta =A\left(\frac{1}{V_{feed}}+\frac{1}{V_{perm}}\right)(2)$

—where membrane area (A) is in (m²) and the feed (V_feed) and permeate volume (V_perm) of compartments are in (m³). Gas permeability is represented as the slope from the data plotted as [ $\frac{1}{\beta }\ln \left(\frac{{∆P}_0}{∆P}\right)$ versus $\frac{t}{l}$ ], and ideal selectivity (${\alpha }_{A/B}$) is calculated by dividing the permeabilities of two different pure gases (A and B) as per Equation 3:

${\alpha }_{A/B}=\frac{P_A}{P_B}(3)$

Each gas permeability measurement was repeated three times for each membrane sample to ensure consistency. The recorded permeability values represent the average of three independent measurements, and the ±values indicate the standard deviation (Supplementary Figure S1).

3 Results and discussion

3.1 Proton nuclear magnetic resonance (¹H NMR)

The ¹H-NMR spectra for the synthesized polymers were recorded in deuterated chloroform (CDCl₃) at 300 MHz (Figures 2a,b). The spectra in Figure 2 indicate the presence of signals at 8.5–8 ppm, which are associated with the formed imine bond formation (PEO–NCH) within the polymer matrix. The signals around 8.0–7.5 ppm correspond to the aromatic benzene rings, while those at the aliphatic region (4.0–1.0 ppm) correspond to the methylene (–CH₂–) protons of PEGs groups (Vöge et al., 2014b; Abdelrahim et al., 2016).

Figure 2

Figure 2. ¹HNMR spectra for the synthesized polymers (a) M1/M2 and (b) M3/M4.

3.2 Fourier transformed infrared (FTIR)

Figure 3 shows the FTIR spectra of the synthesized ROF membranes, confirming the formation of imine bonds. The peaks at 1,661.3 cm^-1 correspond to C=N stretching mode for imine groups within the polymer structure, while the peak of aldehyde groups observed at 1700 cm^-1 disappear (Abdelrahim et al., 2016; Ioannidi et al., 2021). The peaks at 2,926.4 cm^-1 and 874.5 are respectively associated with C–H stretching and bending modes for aliphatic PEO (Luo et al., 2016; Vollas et al., 2018). The peaks at 2,871.6 cm^-1 and 1,097.0 cm^-1 are respectively attributed to C–H (aliphatic) stretching and bending modes (Abdelrahim et al., 2016; Ioannidi et al., 2021). The peaks at 1,447.3 cm^-1 and 1,254.5 cm^-1 are respectively linked to the C=C stretching mode of aromatic ring and C–O ether bonds (Vöge et al., 2014a; Luo et al., 2016; Ioannidi et al., 2021).

Figure 3

Figure 3. FTIR spectra for the fabricated polymeric membranes.

3.3 Thermal TGA and DSC analysis

Figure 4a shows the TGA measurements for the fabricated membranes. The polymeric samples were preheated at 100 °C for 30 min under nitrogen flow (10 °C/min) to remove the adsorbed water within the polymer matrix. The degradation curves were recorded in a nitrogen flow (10 °C/min) to a maximum heating temperature (700 °C). It was observed that there are two stages of thermal degradation for all membranes. The first degradation stage had a mass loss of ∼10 wt% for M1 and M2 at 100 °C–175 °C and a mass loss of ∼5 wt% for M3 and M4 at 100 °C–250 °C.

Figure 4

Figure 4. Thermal stability measurements for the fabricated membranes: (a) TGA and (b) DSC.

This first degradation stage is attributed to the evaporation of moisture and residual solvent present within the polymer’s backbone. The second degradation stage shows a mass loss of ∼95 wt% for M1 and M2 occurring between 175 °C and 450 °C, while for M3 and M4, a similar mass loss of ∼95 wt% occurred between 250 °C and 700 °C. This degradation stage should be ascribed to the thermal decomposition and loss of polyethylene oxide (PEO) chains within the membrane structure (Fares et al., 1994; Theodosopoulos et al., 2017; Ioannidi et al., 2021). The main decomposition products were non-cyclic ethers (i.e., ethoxy-ethane and methoxy-methane), ethylene oxide, CO₂, CO, and water (Fares et al., 1994; Theodosopoulos et al., 2017; Ioannidi et al., 2021).

The two stages in the figure demonstrate that higher temperatures are required for thermal degradation in membranes with longer PEO chains (M3 and M4) than those with shorter PEO chains (M1 and M2). This indicates that the thermal stability of M3 and M4 is higher than that of M1 and M2 owing to the presence of longer PEO chains. These enhance the degree of crystallinity within the membrane structures, as previously observed in other PEO-based membranes, where crystallinity increased with the length of the PEO backbone (Theodosopoulos et al., 2017; Ioannidi et al., 2021; Sandru et al., 2024).

Figure 4b shows the differential scanning calorimetry (DSC) measurements for the fabricated membranes. The measurements were performed under nitrogen flow (10 °C/min) up to a maximum heating temperature of 150 °C. As shown in the figure, the recorded glass transition temperatures (Tg) are approximately −45, −20, 5, and 50 °C for M1, M2, M3, and M4, respectively. This means that membranes M3 and M4 with longer PEO chains exhibit higher Tg values than M1 and M2, which have shorter PEO chains. As mentioned before, this might be related to the crystallization behavior of longer PEO chains within the polymer matrix, which enhances the potential for higher crosslinking through the membrane’s backbone. Furthermore, higher Tg values indicate restricted segmental motion and a more rigid ROF network, which tends to reduce chain packing defects and enhance size-sieving. This results in improved selectivity, especially for gas pairs with small differences in kinetic diameter, such as CO₂ and CH₄ with kinetic diameters of approximately 3.3 Å and 3.8 Å, respectively (Robeson, 2008; Robeson et al., 2009). Therefore, ROF membranes with higher PEO content are expected to achieve slightly increased Tg and corresponding improvements in CO₂/CH₄ selectivity, consistent with this mechanism.

3.4 Contact angle measurements

Figure 5 and Supplementary Figure S2 represent the contact angle values obtained for the fabricated ROF membranes. The measurements were recorded using the ImageJ contact angle plugin, a widely used image analysis tool for quantifying the wettability of surfaces by measuring the contact angle formed between a liquid droplet and a solid substrate. The analysis provides an accurate and reproducible estimation of surface hydrophilicity or hydrophobicity. In this study, water droplets were placed on the membrane surface, images were captured with a calibrated camera, and three measurements per sample were averaged to ensure reliability and minimize error due to surface heterogeneity (Giovambattista et al., 2007).

Figure 5

Figure 5. Contact angle measurements for the fabricated membranes M1-M4.

As shown in Figure 5 and Supplementary Figure S2, the reported contact angle values represent the average of the observed left- and right-angle measurements, which were nearly identical. The relatively high contact angles suggest that the fabricated membranes possess high hydrophobicity and a symmetrical surface morphology. Moreover, Figure 5 shows that the recorded contact angles for membranes with longer PEO chains (M3 and M4) are slightly higher than those for membranes with shorter PEO chains (M1 and M2). This means that M3 and M4 exhibited higher hydrophobic surface properties than M1 and M2, owing to the higher crosslinking degree within the M3 and M4 membrane backbones.

Furthermore, the figure showed that the synthesized membranes demonstrate this trend consistently. This is a surprising observation, showing the solvent effect on the polymerization and crosslinking properties of the fabricated polymeric membranes, as it results in variations in the chemical, crosslinking, and hydrophobic properties of the developed membranes. In addition, the contact angle measurements obtained agree with the data from the thermal analysis measurements: membranes synthesized in NMP solvent (M2 and M4) exhibited slightly higher thermal degradation stability and glass transition temperatures than those synthesized in MeCN solvent (M1 and M3, respectively) under similar polymerization conditions.

The organic solvent enhances the mobility of the building units, increasing the likelihood of the reactive groups approaching each other to initiate polymer bond formation (HC = N). Additionally, the NMP solvent interacts more strongly with the PEO segments, removing water from interactions with the PEO chains and therefore controlling the structural organization of the ROF materials; this affects their pore distribution and fractional pore volume within the membrane backbones. Therefore, the utilization of organic solvents with different polarities in the polymerization reaction affects the chemical structure of the fabricated ROF membranes, as previously observed in molecular simulation studies (Dupuis et al., 2022).

3.5 Scanning electron microscopy (SEM)

Figures 6, 7 present the SEM images of the fabricated polymeric membranes, displaying both surface and cross-section views, respectively. The SEM images reveal that the membranes exhibit a dense and nonporous structure. The thickness of the membranes was consistently approximately 300 ± 25 μm, indicating uniformity in membrane fabrication. Additionally, no visible pores or defects were observed on the membrane surface, confirming its compact nature. The recorded thicknesses agree with our previous findings (Abdelrahim et al., 2016; Sandru et al., 2024), and were chosen based on our prior experience with ROF membranes to balance mechanical stability, ease of handling, and reproducible casting. A uniform thickness is critical for minimizing defect formation during solvent evaporation. While permeance is inversely proportional to membrane thickness, selectivity remains largely unaffected as it depends on the ratio of gas permeabilities. This behavior is consistent with the solution–diffusion transport mechanism typical for dense and hybrid polymer membranes. Furthermore, Figure 7 shows that M1 and M2 display homogeneous monolithic profiles, whereas M3 and M4 reveal faint lamellar features near the edges, attributable to PEO-induced phase separation. These minor differences in surface texture and layer uniformity align with variations in solvent evaporation rates and polymer PEO segment mobility, which are critical for tailoring membrane properties.

Figure 6

Figure 6. SEM images for the fabricated membranes: surface views for M1 (a). M2 (b). M3 (c). M4 (d).

Figure 7

Figure 7. SEM images for the fabricated membranes: cross-section views for M1 (a). M2 (b). M3 (c). M4 (d).

3.6 Flexibility and mechanical measurements

The flexibility and manual mechanical test for the fabricated ROF membranes was performed by us previously (Sandru et al., 2024). As shown in Supplementary Figure S3, the ROF membrane exhibited good flexibility and mechanical integrity upon manual bending and folding. No visible cracks, delamination or structural damage were observed, indicating that the ROF membrane possesses sufficient mechanical robustness for handling and potential gas separation applications.

3.7 Gas permeabilities and ideal selectivities

The pure gas permeabilities through the fabricated membranes (M1–M4) and the ideal selectivities for the pure gases CO₂ and CH₄ were determined at 30 °C under a pressure difference of 0.7 bar (Supplementary Table S1). The results show that, for all membranes, CO₂ permeability is higher than CH₄ permeability. This behavior is attributed to specific parameters such as the dipolar interactions of CO₂ molecules with the imine (–HC = N) and PEO functional groups within the ROF membranes, as well as the higher polarity and quadrupole moment of CO₂ in comparison with the nonpolar CH₄ gas. Moreover, the kinetic diameter of CO₂ is smaller than that of CH₄, which facilitates its diffusion. These factors collectively influence CO₂ adsorption affinity and diffusion through the fabricated membranes (Bao et al., 2011; Basu et al., 2011; Herm et al., 2011; Nordin et al., 2015; Yahia et al., 2024; Yahia et al., 2025). Furthermore, CO₂ permeability through the M3 and M4 membranes was higher than that of M1 and M2, indicating the influence of membrane composition on gas transport properties. This might be related to the higher hydrophobic surface and higher glass transition properties of M3 and M4 compared to M1 and M2 owing to the differences in the PEO chains within the membrane’s backbones. As mentioned previously, M3 and M4 possess longer PEO chains, and this might cause an increase in the fractional free volume (FFV) and enhance CO₂ diffusion through the membrane chains. Furthermore, the membranes synthesized in NMP solvent showed higher permeability values and selectivity than those synthesized in the MeCN solvent. This behavior could be attributed to the solvent’s effect on the membrane’s chemical structure and crosslinking degree, as mentioned in Sections 3.3 and 3.4.

Figure 8 shows the relationship between the permeability (PCO₂) and ideal selectivity (CO₂/CH₄) for the fabricated membranes. Results show that the ideal selectivity for membranes fabricated with shorter PEO chains is higher than for membranes fabricated with longer PEO chains. This indicates that the presence of longer PEO chains within the membrane structure slightly increases both CO₂ permeability and ideal selectivity. It is worth noting that both CO₂ permeability and ideal selectivity increase simultaneously, which is uncommon (Robeson, 1991; Freeman, 1999; Robeson, 2008; Robeson et al., 2009; Swaidan et al., 2015; Comesaña-Gándara et al., 2019; Corrado and Guo, 2020). This indicates that replacing short-chain functional groups with longer-chain groups within the membrane structure improves membrane performance. Therefore, the incorporation of longer polyethylene oxide (PEO) chains and NMP solvent into the membrane matrix was expected to enhance CO₂ permeability over CH₄. This effect is attributed to structural changes in the membrane and enhanced affinity toward CO₂ (Dupuis et al., 2022; Sandru et al., 2024).

Figure 8

Figure 8. Gas separation performance as a relationship between CO₂ and CH₄ permeabilities and ideal selectivity (CO₂/CH₄) for the fabricated membranes (M1-M4).

Furthermore, the kinetic diameters of CO₂ and CH₄ are approximately 3.3 Å and 3.8 Å, respectively (Robeson, 2008; Robeson et al., 2009). This difference plays a critical role in their transport through the membrane. In polymeric and hybrid membranes such as ROF-based MMMs, gas separation occurs predominantly via the solution–diffusion mechanism, where permeability is a function of both diffusivity and solubility. The smaller size of CO₂ allows it to diffuse more easily through the membrane matrix, while its higher condensability and quadrupole moment enable stronger interactions with polar groups such as ether oxygens in PEO and imine functionalities (–HC = N) within the ROF framework (Fares et al., 1994; Giovambattista et al., 2007; Basu et al., 2011; Nordin et al., 2015). These interactions enhance CO₂ solubility via dipole-quadrupole and Lewis acid–base interactions, resulting in significantly higher CO₂ permeability and selectivity over CH₄. On the other hand, CH₄, being non-polar and less condensable, lacks such specific interactions and exhibits lower diffusivity due to its larger kinetic diameter. Therefore, the combination of size-exclusion effects, specific CO₂–polymer interactions, and the flexible as well as microporous nature of ROFs synergistically contribute to the observed superior CO₂/CH₄ separation performance.

Nevertheless, the most significant goal in gas membrane development is the combination of high permeability and acceptable selectivity, which is limited by the trade-off between permeability and selectivity. Furthermore, Figures 9a, b illustrate the CO₂/CH₄ separation performance of the fabricated ROF membranes (M1–M4) compared to previously reported PEO-based membranes (Supplementary Table S2—Rahman et al., 2013; Tena et al., 2013; Ghadimi et al., 2014; Qiu et al., 2016; Bengtson et al., 2017; Chen et al., 2017; Nebipasagil et al., 2017; Bandehali et al., 2020) plotted against the 1991, 2008, and 2015 Robeson upper-bound correlations (Robeson, 1991; Robeson, 2008). The CO₂/CH₄ selectivity is shown as a function of CO₂ permeability.

Figure 9

Figure 9. The upper-bound lines 1991 (Robeson, 1991), 2008 (Robeson, 2008), and 2015 (Swaidan et al., 2015) for (a) the membranes M1-M4 described in this work and for (b) other PEO polymeric membranes from data described in literature.

The reference lines represent the Robeson upper bounds, which indicate the classical trade-off between permeability and selectivity observed in most polymeric membranes. Typically, as permeability increases, selectivity decreases. In contrast to this trend, the fabricated membranes in this study (M1–M4) show a clear and steady increase in both CO₂ permeability and CO₂/CH₄ selectivity as we move from M1 to M4. Specifically, M1 and M2 lie below the 1991 Robeson upper bound, M3 lies directly on this upper bound, and M4 surpasses the 1991 limit and approaches the 2008 Robeson upper bound, thus demonstrating significant overall improvement in separation performance.

This remarkable behavior can be attributed to the tailored design of the ROF membranes through the incorporation of different PEO segment lengths and the use of organic solvents with varied polarity during the synthesis process. This strategy enhances polymer packing, dynamic mobility, and gas sorption affinity. Specifically, the increased CO₂ affinity of the ROF membranes (via PEO and imine groups), combined with the smaller kinetic diameter of CO₂ relative to CH₄, improves CO₂ transport without negatively impacting CH₄ transport, thus permitting performance beyond the 1991 Robeson limit and potentially enabling future designs to surpass both the 2008 (Robeson, 2008) and 2015 (Swaidan et al., 2015) upper bound limits. Hence, our findings strongly support the hypothesis that the strategic tuning of polymer architecture and processing conditions can overcome the conventional permeability–selectivity trade-off and unlock new performance frontiers for CO₂/CH₄ separation. This validates the promise of ROF membranes as a competitive platform for advanced gas separation applications. Furthermore, to evaluate the stability of the fabricated ROF membranes, preliminary aging tests were conducted by storing the membranes under ambient conditions for 1 month. The results showed no significant changes in gas permeability or selectivity, indicating good structural stability and resistance to physical aging over time.

As shown in Supplementary Table S3, the developed ROF membranes (M1–M4) demonstrate CO₂ permeability (155.54–179.76 barrer) superior to conventional polymeric membranes such as cellulose acetate (CA) (1.26–52.6 barrer) (Moghadassi et al., 2014; Sanaeepur et al., 2016; Mubashir et al., 2018; Mubashir et al., 2019; Jia et al., 2020) and Matrimid^® (12–20 barrer) (Dorosti et al., 2014; Kertik et al., 2017). While some CA-based membranes achieve higher CO₂/CH₄ selectivity (up to 53.98—Moghadassi et al., 2014), this is typically accompanied by low CO₂ permeability (≤5 barrer), highlighting the permeability–selectivity trade-off common in polymeric membranes. In contrast, the synthesized membranes (M1–M4) exhibit a balanced performance, with selectivity values (15.45–31.4) comparable to or exceeding those of commercial membranes, such as CA: 4.44–53.98 (Moghadassi et al., 2014; Sanaeepur et al., 2016; Mubashir et al., 2018; Mubashir et al., 2019; Jia et al., 2020) and polyimide (P84): 67–93 (Guo et al., 2018; Sheng et al., 2020), while maintaining significantly higher gas permeation rates. Furthermore, the solvent choice (NMP or MeCN) appears to influence membrane performance, with NMP-processed membranes (M2, M4) showing marginally higher permeability and selectivity than their MeCN counterparts (M1, M3). This trend aligns with studies on conventional polymers, where solvent selection impacts membrane morphology and gas transport properties (Moghadassi et al., 2014; Sanaeepur et al., 2016; Mubashir et al., 2018; Mubashir et al., 2019; Jia et al., 2020). Notably, the performance of M3 and M4 approaches the upper bound for polymeric membranes, suggesting their potential for industrial CO₂/CH₄ separation applications where both high permeability and selectivity are critical.

Although single-gas permeation tests were conducted in the current study due to the limitations of our manual setup (operating up to 3 bar), these tests remain widely accepted for preliminary screening of membrane materials, offering valuable insights into intrinsic permeability and selectivity (Baker and Low, 2014). The applied conditions (30 °C, up to 3 bar with pressure difference ∼0.7 bar) are also relevant to practical applications such as biogas upgrading (Sridhar et al., 2007). To complement these results, our group recently investigated structurally related ROF-based membranes under mixed-gas and elevated-pressure conditions (3–10 bar) using an advanced gas permeation setup. The results of that research (Sandru et al., 2024) revealed stable CO₂/CH₄ selectivity trends under realistic conditions and confirmed the robustness of ROF membranes up to 10 bar. These findings reinforce the current results and support the suitability of ROF membranes for applications such as biogas and landfill gas upgrading. Future research will focus on evaluating the long-term and high-pressure performance of the present ROF membranes under industrially relevant mixed-gas conditions.

4 Conclusion

In this study, we successfully constructed a new series of dynamic ROF membranes based on polyethylene oxide (PEO) segments for CO₂/CH₄ separation. By incorporating different lengths of PEO chains and adjusting the polymerization solvent (MeCN or NMP), we were able to fine-tune the membranes’ architecture, control their crosslinking density, and influence the resulting microstructure. Spectroscopic and thermal analyses (¹H NMR, FTIR, TGA, and DSC) confirmed the successful synthesis of the ROF membranes and demonstrated their good thermal stability and partially hydrophobic surfaces.

As a result of these structural modifications, all fabricated membranes displayed balanced and tunable CO₂ transport properties. Increasing the length of the PEO segment led to enhanced dynamic mobility and pore accessibility within the membranes, promoting selective CO₂ permeation. This was further supported by the strong affinity between CO₂ and the polar functional groups present in the polymer backbone. Combined with the smaller kinetic diameter and higher quadrupole moment of CO₂, these structural features enabled a notable improvement in CO₂ permeability and CO₂/CH₄ selectivity.

The CO₂/CH₄ separation performance based on the Robeson upper-bound correlations showed that the membrane with the longest PEO segment achieved the most promising results, exceeding the 1991 Robeson limit and approaching the 2008 limit. Overall, this study highlights the effectiveness of tailoring the membrane architecture through segment length and solvent choice as a straightforward and versatile strategy for improving CO₂/CH₄ separation performance in PEO-based ROF membranes. These findings can serve as useful guideline for designing future ROF membranes with enhanced gas separation properties.

Data availability statement

The raw data supporting the conclusions of this article will be made available by the authors without undue reservation.

Author contributions

MY: Writing – original draft, Data curation, Investigation. LN: Writing – review and editing, Data curation, Methodology, Supervision, Investigation. LG: Writing – review and editing, Project administration, Supervision. JC: Formal Analysis, Conceptualization, Supervision, Methodology, Writing – review and editing. MB: Validation, Supervision, Conceptualization, Writing – review and editing, Investigation, Funding acquisition.

Funding

The author(s) declare that financial support was received for the research and/or publication of this article. EUDIME Program (European Doctoral in Membrane Engineering) FCT/MCTES (LA/P/0008/2020 DOI 10.54499/LA/P/0008/2020, UIDP/50006/2020 DOI 10.54499/UIDP/50006/2020 and UIDB/50006/2020 DOI 10.54499/UIDB/50006/2020) “Evolution” funded by European Union—Nextgeneration EU and Romanian Government under the National Recovery and Resilience Plan for Romania, contract no.760033/23.05.2023 cod PNRR-C9-I8-CF16, through the Romanian Ministry of Research, Innovation and Digitalization, within Component 9, Investment I8.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Generative AI statement

The author(s) declare that no Generative AI was used in the creation of this manuscript.

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Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/frmst.2025.1653220/full#supplementary-material

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