Comparison of liquid‐phase and methanol‐swelling crosslinking processes of polyimide dense membrane forCO₂/CH₄separation

Abstract

To overcome the plasticization effect in polyimide membranes, many researchers have proposed crosslinking method. This can reduce an inter‐segmental mobility by tightening and rigidifying the polymer chains. However, it is difficult to modify the whole polymer chains throughout the membrane because the reaction can be hindered by the diffusion rate of the crosslinker. In particular, it is hard for bulky crosslinker to penetrate a dense membrane with a small d‐spacing. This study investigated the effect of crosslinking a dense Matrimid membrane with p‐phenylenediamine (p‐PDA) via two different crosslinking methods (i.e., methanol‐swelling crosslinking process [M‐SCP] and liquid‐phase crosslinking process [L‐PCP]). Most of the crosslinking reaction in M‐SCP occurs on the membrane surface due to difficulty in penetration of the bulky p‐PDA into the Matrimid dense membrane. In contrast, the L‐PCP allows uniform crosslinking across the membrane. The membranes crosslinked using L‐PCP showed excellent chemical resistance. Furthermore, the plasticization phenomenon was not observed in the membranes crosslinked using L‐PCP with p‐PDA more than 15%. Meanwhile, the membrane crosslinked using M‐SCP exhibited poor plasticization and chemical resistance properties. These results showed that the L‐PCP method can be more effective for the crosslinking of dense membrane to deliver both high plasticization and chemical resistance.

Surface Modification of Matrimid® 5218 Polyimide Membrane with Fluorine-Containing Diamines for Efficient Gas Separation

“…In contrast, in the present work, we found that all diamine-modified membranes were completely dissolved in the organic solvent (NMP), suggesting that a surface-concentrated reaction occurred when the aromatic diamines reacted with the Matrimid membrane. These results are consistent with the recent report by Kim et al that investigated the crosslinking of a Matrimid membrane by p -phenylenediamine ( p -PDA, equal to DA0 in this work) by comparing liquid-phase and methanol-swelling-induced post-treatment protocols [ 38 ]. They concluded that the Matrimid membrane modified by methanol-swelling protocol (i.e., the same method used in this work) exhibited poor solvent resistance due to the surface-limited reaction on the membrane surface compared with the liquid-phase protocol that induced crosslinking of the entire membrane, including the inner side.…”

“…The reactivity of polyimide and diamines, diamine concentration, membrane thickness, temperature, and reaction time are important factors that determine the overall reaction rate as well as the degree of crosslinking. In order to investigate the effects of diamine structures in modified Matrimid membranes, we fixed variables such as diamine concentration (10 wt.%), membrane thickness (~50 μm), temperature (25 °C), and reaction time (24 h) to values that were widely adopted in most of the diamine modification studies [ 26 , 29 , 30 , 31 , 37 , 38 , 39 ].…”

“…Here, we designated the modified membranes as OOO-MAT, where MAT stands for Matrimid and OOO stands for non-fluorinated diamine (DA0, DA1, and DA2) or fluorinated diamines (FDA1 and FDA2) used in the reaction, respectively. For all membranes, a slight decrease in the peaks of the imide group (asymmetric C=O stretching at 1775 cm −1 and C–N stretching at 1360 cm −1 ) was observed while the peaks of the amide group (C=O stretching at 1672 cm −1 and N–H bend at 1510 cm −1 ) became higher or sharper [ 37 , 38 ]. The results indicate that the diamines were successfully incorporated into the polyimide matrix by reaction mechanisms, as described in Figure 2 .…”

“…When comparing the diamine pairs sharing the same molecular structures with or without fluorine-substituted groups (i.e., DA1 vs. FDA1 and DA2 vs. FDA2), the fluorinated diamines exhibited a less pronounced reactivity than that of non-fluorinated ones. Given that the reaction occurs in the solid-state environment, the diffusion of diamines across the polyimide film is expected to be sterically constrained, which increases as the steric hindrance increases as the reaction occurs [ 31 , 38 , 39 ]. Thus, we speculated that bulkier fluorinated diamines are more likely to react at the membrane surface due to their difficulties permeating through the polymer chains, which causes limited reactivity even though the existence of methanol assists the diffusion of diamines by swelling the membrane at the initial stage.…”

A Rigid and Planar Aza-Based Ternary Anhydride for the Preparation of Cross-Linked Polyimide Membrane Displaying High CO2/CH4 Separation Performance

“…X-ray diffraction analysis (XRD) of the films was carried out on a Bruker AXS-D8 X-ray diffractometer at 40 kV, 40 mA and Cu-Kα radiation. The d -spacing was calculated using Bragg’s equation, , where n is the order of reflection, λ is the wavelength of X-ray radiation and θ is the XRD angle [ 31 ].…”