## 3 Results and Discussion
### Formation Mechanism of Walnut-like Superparamagnetic Microspheres  
To investigate the formation mechanism of the walnut-like morphology, we monitored the polymerization reaction by varying the net absorbed dose. Additionally, we adjusted certain components and their concentrations to assess their influence on the final product. Details of these changes are provided in Table 1. The results indicated that several factors, including the absorbed dose and monomer type, significantly impacted the final morphology of the microspheres, as discussed in the following paragraphs. In this study, the degree of polymerization of DVB was directly related to the absorbed dose, which, in turn, was proportional to the irradiation time. We hypothesized that higher doses would result in a higher degree of polymerization. As shown in Figure 3, walnut-like microspheres began to form after 22 h of irradiation (\(\sim\)66 kGy absorbed dose). When the irradiation time did not exceed 16 h, hollow microspheres with smooth surfaces were observed (Figure 3a,b). At lower absorbed doses, the polymerization degree of the monomers was limited due to the shorter irradiation times and fewer active radicals being generated. This resulted in more DVB monomers remaining inside the microspheres, with thinner PDVB walls. These hollow microspheres, having thinner walls (Figure 4a,b), were more likely to collapse during subsequent washing and drying steps. Conversely, longer irradiation times led to a higher degree of polymerization and a lower amount of unreacted monomer, which resulted in solid microspheres (Figures 3c and 4c). This prevented the collapse of PDVB microspheres. The unique walnut-like roughness observed on the microsphere surfaces (Figures 1a,b and 4c) can be attributed to late-stage polymerization, where DVB monomers were excluded from the crosslinked PDVB network. This exclusion caused the network to shrink, forming a corrugated surface. To further validate this, we replaced DVB with styrene and conducted the same experiment as for S-4 (Table 1). The results in Figure 5 show that S-5, made solely from styrene monomer, produced hollow microspheres with a single hole and smooth surfaces. No walnut-like morphology was observed. In contrast, S-4, prepared with a DVB monomer, exhibited a distinct walnut-like surface (Figure 1a,b) and a condensed internal structure, as shown by the TEM image of the cross-section of the walnut-like microspheres (Figure 1d). This difference can be attributed to the better solubility of PS in styrene compared to PDVB in DVB, which prevents phase separation during polymerization. In contrast, phase separation and the contraction of the crosslinked network between PDVB and DVB, combined with the low degree of following due to the high degree of polymerization, led to the formation of a walnut-like morphology. Hypothesis for Walnut-like MPCPs Formation  
Based on our experimental results, we proposed a hypothesis to explain the formation of walnut-like MPCPs and created an illustration of it, as shown in Figure 6. In our study, a mini-emulsion served as the soft template, and \(\gamma\)-radiation was used to initiate the polymerization and crosslinking of DVB. Therefore, the reaction likely begins at the oil-water interface and propagates inward toward the center of the oil droplets. At shorter irradiation times (e.g., S-1 and S-2), a larger amount of unreacted DVB remains, resulting in  
Figure 4: TEM images of magnetic polymer particles at different radiation times: (**a**) S-1 (6 h), (**b**) S-2 (16 h), and (**c**) S-3 (22 h). Figure 5: (**a**) Scanning electron microscopy (SEM) image; (**b**) transmission electron microscopy (TEM) image of sample S-5, as defined in Table 1.  
Figure 3: SEM images of magnetic polymer particles at different radiation times: (**a**) S-1 (6 h), (**b**) S-2 (16 h), and (**c**) S-3 (22 h). the formation of hollow MPCPs with thin PDVB shells. These thin-walled structures are prone to collapse during the subsequent washing and drying processes.