## 2 Crosslinking of PEI and Reaction Mechanisms
crosslinked PEI with ECH in a process called sedimentation polymerization [58; 59] for CO\({}_{2}\) adsorption. In this case, PEI and ECH were kept together at room temperature for 20 min. The mixture was injected through a column of canola oil using a syringe to form a sediment in the form of droplets. The column where the sediment was collected was kept in a water bath at 80 \({}^{\circ}\)C. The process resulted in crosslinked beads of PEI with a high surface area and thermal stability; so, the issues with solvent loss encountered with conventional amines were avoided. The crosslinking reaction mechanism and subsequent CO\({}_{2}\) capture via a proposed carbamate formation are shown in Scheme 11. Interestingly, the material was regenerated via microwave heating for a batch experiment. The sedimentation polymerization gave beads with a low polydispersity and with sizes in the range of 1200-1400 mm, as revealed by an SEM study. In the IR study, several peaks were observed from 1000 cm\({}^{-1}\) to 3330 cm\({}^{-1}\), corresponding to -OH stretching, -NH stretching, -CH stretching, -OH bending, -CH\({}_{2}\) bending, and C-N stretching. The amount of the ECH crosslink was varied to yield materials with a different degree of crosslinking, which was studied using IR spectroscopy. A decrease in the intensity of N-H and C-N stretching at 3268 cm\({}^{-1}\) and 1103 cm\({}^{-1}\), respectively, with the use of a higher amount of the crosslinker was attributed to a decrease in the amount of 1\({}^{\circ}\) and/or 2\({}^{\circ}\) amines in the crosslinked material. The thermogravimetric analysis demonstrated that the PEI HBs remained thermally stable at least up to 230 \({}^{\circ}\)C, which would be helpful for the regeneration of the crosslinked material for the batch adsorption of CO\({}_{2}\). A fast thermal degradation of the crosslinked material occurred at temperature above 250 \({}^{\circ}\)C. CO\({}_{2}\) adsorption details are discussed in Section 3.4. In summary, the chemical crosslinking of PEI involves using crosslinking agents such as ECH and glutaraldehyde to form covalent bonds between PEI chains. The type of crosslinker, its concentration, and the reaction conditions provide different properties to the succeeding material, such as swellability, mechanical strength, and adsorption capacity. For instance, an increase in the degree of crosslinking will impact the available amine concentration and hence affect the overall properties of the material. The chemical structure of the crosslinked materials varies depending on the crosslinker used, resulting  
Figure 11: Crosslinking of polyethylene imine with ECH and proposed CO\({}_{2}\) capture mechanism [58]. in materials having different functional groups that can form different interactions with different elements and molecules. Undoubtedly, the crosslinking reactions discussed above play significant roles in altering the final materials' properties. The reactions form a strong covalent bond, for example, in a nucleophilic substitution reaction and hence the reactions are irreversible. Recently, the idea of preparing crosslinked polymers using dynamic covalent crosslinkers that provide the materials unique properties, such as reversibility, reprocessability, self-healing, and the ability to respond to analytes, has found huge attention. For example, the synthesis and characterization of a hydrogel that responds to different stimuli have been reported [60,61]. In this regard, the use of crosslinker for PEI that provides a stable crosslinked material while reversibility is achieved when needed is interesting and deserves wider investigation in the future [62]. One such crosslinker for PEI is glutaraldehyde, which form an imine bond. It is known that the imine functionality is reversible and recyclable via a hydrolysis reaction [13]. Other dynamic covalent crosslinkers for PEI should be further studied in the future.