Polydimethylsiloxane (PDMS) is widely used as an interlayer material due to its high permeability. However, its hydrophobic nature derived from the repeating ‒Si(CH₃)₂‒O‒ structure leads to poor interfacial compatibility with the hydrophilic Pebax separation layer. Therefore, surface modification is required to fabricate a PDMS interlayer with both high hydrophilicity and stable interfacial adhesion, so as to optimize the CO₂/N₂ separation performance of Pebax composite membranes.

Among existing hydrophilic modification methods for PDMS, plasma treatment has attracted extensive attention owing to its simple operation and remarkable effects. Conventional He or air plasma can introduce functional groups such as carboxyl, hydroxyl, and amino groups, but the modified layer often suffers from hydrophobic recovery. In comparison, CO₂ plasma possesses unique advantages in chemical mechanisms and synergistic effects. Carbon atoms from CO₂ may participate in surface reconstruction, forming more stable oxygen-containing carbon composite structures and introducing hydrophilic groups (especially carboxyl groups ‒COOH) more efficiently. This synergistic effect further optimizes the gas separation performance.

Effects of CO₂ Plasma Treatment on the Structure and Properties of PDMS Membranes

Water Contact Angle (WCA)

The water contact angles of PDMS membranes modified by CO₂ plasma under different treatment times are shown in Figure 1(a). The results indicate that WCA decreases linearly from the initial value of 115.3° for pristine PDMS, reaching 55.6° after 60 seconds of treatment.

Figure 1(b) illustrates the WCA variation of PDMS membranes treated at a constant time of 30 s with different plasma power levels. A slow decline in WCA can be observed, with a smaller slope than that in Figure 1(a). The WCA is 80° at a treatment power of 125 W. It can be concluded that, compared with plasma power, treatment time exerts a more significant influence on the surface hydrophilicity of PDMS membranes.

When the plasma treatment time is 15 s and 20 s, the WCA values of PDMS surfaces are 95.6° and 90.8°, respectively. During coating with 0.5 wt% and 1.0 wt% Pebax solutions, the coating solution distributes in a fishnet-like morphology and cannot uniformly cover the PDMS surface. When the plasma treatment time exceeds 30 s (i.e., WCA < 80°), a dense and uniform Pebax layer can be successfully deposited on the plasma-treated PDMS surface.

The improved hydrophilicity of PDMS after plasma treatment provides a favorable interface for sufficient spreading of the hydrophilic Pebax coating solution, enabling the formation of a defect-free ultrathin separation layer.

Figure 1 Water contact angles of PDMS membranes treated by CO₂ plasma:
(a) Power = 110 W, treatment time = 0–60 s;
(b) Treatment time = 30 s, power = 0–125 W

CO₂ plasma modification effectively enhances the surface hydrophilicity of PDMS base membranes, reflected in an immediate reduction of surface contact angle. However, the hydrophilicity of the modified membrane surface is not permanent, and the WCA tends to recover to some extent with storage time, as shown in Figures 2(a) and 2(b). The WCA increases sharply at the initial stage and gradually stabilizes after approximately 7 days.

This phenomenon is mainly attributed to the reorientation and rearrangement of polar groups (such as ‒COOH and Si‒OH) introduced by plasma modification, as well as the migration of hydrophobic moieties of PDMS polymer chains. The high surface energy state after modification drives polar groups to migrate toward the bulk phase rather than segmental motion. Meanwhile, the low surface energy characteristic of the PDMS bulk promotes the migration of hydrophobic Si‒CH₃ groups toward the surface, resulting in the attenuation of hydrophilicity over time. Therefore, to maintain a satisfactory modification effect (WCA < 80°), Pebax coating should be performed as soon as possible after plasma treatment.

Figure 2 Evolution of water contact angles of CO₂-plasma-treated PDMS membranes over time: (a) Power = 110 W, treatment time = 0–60 s; (b) Treatment time = 30 s, power = 0–125 W