Methyl methacrylate (MMA, Junsei) was pre-treated using an inhibitor remover (Sigma Aldrich) to remove polymerization inhibitors. Branched PEI (M_n = 600, 1800, and 10,000) and tert-butyl hydroperoxide (TBHP, 70 wt% in water) were obtained from Sigma Aldrich and used without further purification. Hydrochloric acid (HCl, Sigma Aldrich) was diluted with tertiary distilled water to a concentration of 2 M and used for adjusting the pH of PEI.

PMMA/PEI core-shell nanoparticles were prepared in double-jacketed glass reactor equipped with a mechanical stirrer and tree inlets. First, PEI 10K (0.84 g, M_n = 10,000) was dissolved in distilled water and the solution was adjusted to a pH of 7 using 2 M HCl. MMA monomers (3.36 g) were added to the PEI 10K solution. Then, this solution was purged with N₂ gas to remove water and oxygen. After adding the initiator, TBHP (0.25 mL, 10 mM), to the solution, the reaction mixture underwent polymerization by heating for 6 h in an oil bath at 80˚C under a nitrogen atmosphere. The conditions for the synthesis of soap-free PMMA/PEI core-shell nanoparticles such as MMA/PEI weight ratios and PEI molecular weights are presented in Table 1. The nanoparticles prepared using PEI 0.6K (M_n = 600) and PEI 1.8K (M_n = 1800) were polymerized using the same procedure with emulsion composition, MMA/PEI (w/w) ratios, of 1:1 or 4:1. After completion of polymerization, the unreacted starting materials were removed by centrifugation in methanol. The product was then dried at 30˚C in a vacuum oven.

To confirm grafting of PEI and PMMA, we analyzed the core-shell particles using a Fourier-transform infrared spectrometer (FT-IR, MAGNA 550 series II, Nicolet) with a KBr disk. Scanning electron microscopy (SEM, JSM-6701F, JEOL) was used to investigate the surface relief, size, and shape of the core-shell particles.

The SEM samples were prepared by collecting particles directly on carbon tape, or by dissolving the particles in solvent, followed by pipetting 2 - 3 drops on a glass slide, drying, and coating with platinum.

The core-shell structure of the nanoparticles was analyzed using a transmission electron microscope (TEM, JEM-2100F, JEOL). The TEM samples were prepared by dissolving the particles in a solvent, and 1 - 2 drops were transferred onto a carbon-coated copper grid before drying.

A 2% solution of phosphotungstic acid (PTA) was used to stain the samples before analysis. The size and behavior of the prepared nanoparticles in solution were analyzed using a particle size analyzer (PSA, ELS-Z, Otsuka) with tertiary distilled water or methanol as the solvent.

CO₂ adsorption capacities of PMMA/PEI grafting core-shell nanoparticles was measured by thermogravimetric analysis (TGA) with the difference of weight under a CO₂ atmosphere at 75˚C for 2 hours.

Figure 1 shows a schematic flowchart of reactions that produces grafting of PMMA/PEI for the synthesis of core- shell nanoparticles. The reactions between the redox initiator, TBHP, and PEI produce free radicals, which induce

polymerization of the MMA monomers. Micelle-type nuclei are formed through the grafting reactions between PEI and MMA, and these nuclei grow spontaneously through homopolymerization of MMA and self-assembly, resulting in nanoparticles with a core-shell structure. Soap-free emulsion polymerization has the advantage that it can get cleaner latex than conventional emulsion polymerization. The mechanism for the formation of micelle depends on the oligomer radical with activated surface, which is only slightly soluble in the aqueous medium, to decompose the hydrophilic initiator. The mechanism for the formation of amphiphilic core-shell nanoparticles using a redox initiator has been described in numerous studies. The mechanism is as follows: When an amine group of PEI reacts with TBHP to generate a redox pair, a single electron is transferred from the nitrogen atom of the amine group to TBHP, forming a nitrogen radical cation and a tert-butoxy radical. Following this, the amino radical on PEI is produced as nitrogen loses a single proton, initiating MMA polymerization, while the grafting reaction between PEI and PMMA generates amphiphilic macro-radicals. The tert-butoxy radical can initiate homopolymerization of MMA, while removing a hydrogen atom from the amine group of PEI to generate macro-radicals. These amphiphilic PEI-graft-PMMA macro-radicals self-assemble to form macromolecular micelles consisting of hydrophobic MMA surrounded by hydrophilic PEI. This facilitates homopolymerization of MMA in the interior, which results in the nanoparticle with a PMMA/PEI core-shell structure.

Figure 2 shows FT-IR spectra of PMMA/PEI core-shell nanoparticles prepared in emulsions of various MMA/ PEI ratios. As shown in Figure 2, the carbonyl (C=O) stretching and ether (-OCH₃) peaks were observed at 1731 and 1150 cm⁻¹, respectively, corresponding to the molecular structure of PMMA. In addition, the peaks at 3440 (N-H stretching), 2950 (CH₂ asymmetric stretching), 2842 (CH₂ symmetric stretching), and 1629 cm⁻¹ (vibration) correspond to PEI, demonstrating successful grafting of MMA and PEI.

Table 2 shows a comparison of the PMMA/PEI nanoparticle size measured by PSA (in aqueous medium and methanol) and SEM analysis. As shown in Table 2, the average size of the PMMA/PEI nanoparticles measured using PSA is larger than those measured using SEM by about 30 - 60 nm. The increase in particle size can be ascribed to solvent induced swelling of PEI in the shell, which has a good solubility in both water and methanol, resulting in bigger particles than that expected in a dry environment. In addition, PMMA is known to swell in low molecular weight alcohols, such as methanol, ethanol, n-propanol, iso-propanol, and iso-butane. Thus, we conclude that the size of nanoparticles varies depending on the nature of the solvent and the method of measurement.

Figure 3 and Figure 4 show the SEM and TEM images of PMMA/PEI grafting core-shell nanoparticles prepared with different ratios of MMA and PEI 10K. As shown in Figures 3(a)-(c), the weight ratios of MMA/PEI are 1, 4, and 6, respectively. The average size of core-shell nanoparticles gradually increases in accordance with

MMA/PEI ratios. As shown in Figure 4, the nanoparticles prepared with various MMA/PEI ratios can be confirmed with core-shell structure. The average size of the PMMA/PEI core-shell nanoparticles is shown in Table 2. In the soap-free emulsion polymerization, the size of the nanoparticles is determined by the stability of solution and growth rate of nuclei. In other words, as the ratio of MMA/PEI gradually increased, the amount of PEI relatively decreased. Therefore, the number of nuclei produced in the early stages became lower.

To investigate the effect of PEI molecular weight on PMMA/PEI core-shell nanoparticles, we synthesized the nanoparticles by varying the PEI molecular weights, as shown in Table 3. The amounts of TBHP, PEI, and water used were identical, regardless of the MMA/PEI ratio. As shown in Figure 5, due to the effect of steric hindrance between the high molecular weight PEI and PMMA, a decrease in the size of PMMA/PEI core-shell nanoparticles is observed with an increase in molecular weight of PEI. Likewise, as shown in Figure 6, the nanoparticles prepared with various PEI molecular weights can be confirmed with core-shell structure. Low molecular weight PEI represents shorter molecules than high molecular weight PEI. So, given the same MMA/PEI ratio, low molecular weight PEI molecules in the shell are positioned farther away from PEI molecules in the nearby nanoparticles, and so the aggregation between PEI molecules decreases.

Stability of micelles can be affected by the composition of hydrophilic and hydrophobic regions of the macromolecule, cohesion of the hydrophobic core, and interactions between hydrophilic macromolecular chains in the aqueous solution. In the case of high molecular weight PEI in aqueous solution, the van der Waals forces between PEI molecules, hydrogen bonding with water, and electrostatic interactions can cause the chains to extend, which increases the PEI surface density and sequestering the hydrophobic core to form stable micelles. On the other hand, low molecular weight PEI has relatively low surface density that is insufficient to cover the surface of the micelle and so, as the core is exposed to the aqueous solution, the micelle becomes unstable.

As mentioned above, the use of lower molecular weight PEI results in unstable micelles, generating inhomogeneous nanoparticles. Therefore, we expect that for a low molecular weight PEI, a higher amount of PEI would be required to obtain more stable micelles and homogeneous nanoparticles.

Figure 7 shows the CO₂ adsorption capacity of PMMA/PEI grafting core-shell nanoparticles prepared using PEI 10k by soap-free emulsion polymerization. The values of CO₂ adsorption capacity are presented in Table 4. Particles prepared by soap-free emulsion polymerization exhibit poor CO₂ adsorption capacity. We can confirm this value to be less than 0.07 mg per g of absorbent. In Figure 7, we compared the CO₂ adsorption capacity of PMMA/PEI grafting core-shell nanoparticles prepared using PEI 10k in accordance to time. The CO₂ adsorption capacities of the MP10k-11 and MP1.8k-41 samples were approximately 0.4 mg per g of adsorbent. This can be described as a diminutive CO₂ adsorption performance. As mentioned above, PMMA/PEI grafting core-shell nanoparticles have a low CO₂ adsorption capacity. The hydrochloric acid, which is used in the preparation of soap-free emulsion at pH = 7, converted all the -NH₂ bonds required for CO₂ adsorption to quaternary ammonium salts. Therefore, the CO₂ adsorption capacity was found to be relatively low.

We, further, synthesized PMMA/PEI grafting core-shell nanoparticles with -NH₂ functional groups in emulsions of pH = 11. As a result, CO₂ adsorption capacity of the nanoparticles increased to 0.69 mg. Though the CO₂ adsorption capacity of the PMMA/PEI core-shell nanoparticles is quite low, it could possible get be further improved if the quaternary ammonium salts in the PEI shell surface was eliminated by pH control using hydrochloric acid.