# pH-Responsive Hollow Polymeric Microspheres from Irradiated Cyclic Ether Aqueous Solution
## 3 Results and Discussions  
### Characterization of Microspheres and Possible Formation Mechanism  
Radiation-induced synthesis can be carried out in aqueous systems, which minimizes the use of organic solvents and the need for separation and purification of the final product. As can be seen, by only one-step \(\gamma\)-irradiation, well-defined hollow microspheres with the average diameter of ca. 2.6 \(\upmu\)m and narrow size distribution were prepared successfully from DCH18C6 aqueous solution (Figure 1). In addition, no other organic or inorganic templates were used to obtain the hollow structure via this approach. The relationship between the yield of hollow microspheres and irradiation time was then determined by a gravimetric method. As can be seen from Figure 2a, the yield of hollow microspheres increased linearly and dramatically with increasing irradiation time, and DCH18C6 was completely converted to hollow microspheres after 167 h of irradiation. Further prolonging the irradiation time did not influence the morphology, and only slightly reduced the average diameters of the prepared hollow microspheres (Figure S1). The chemical structure of the hollow microspheres was determined by micro-FTIR, 1H NMR, and XPS (Figures S2-S4). It was found that the polymer consists of a hydrophobic main chain and hydrophilic side chains containing oligo (ethylene glycol) and functional groups (OH, CHO, and COOH groups), which is similar to the structure of the amphiphilic polymer reported in our previous work [22], but the hollow microspheres had much higher average molecular weight (Mn = 2.69 \(\times\) 10\({}^{6}\)) than that of amphiphilic polymers (Mn = 1050). Thus, we suggest that the hollow microspheres were prepared from self-organization of amphiphilic polymer micelles obtained by irradiating DCH18C6 aqueous solution (Scheme 1). When the DCH18C6 was exposed to gamma irradiation in a dilute aqueous solution, most of the radiation energy was absorbed by water. Then, short-lived  
Figure 1: The SEM image (**a**), size distribution (**b**), and CSLM optical micrograph (**c**) of the hollow microspheres obtained by irradiating 0.01 mol L\({}^{-1}\) DCH18C6 for 167 h, respectively, and TEM image of the ultra-thin section of the hollow microspheres (**d**). reactive species, including \(\cdot\)OH radicals, H-atoms, and hydrated electrons, were formed. The C-O bonds of cyclic ethers could be cleaved during irradiation in water due to the reaction with \(\cdot\)OH radicals from the radiolysis of water, and intermediate species containing end C=C groups were produced consequently [23]. These intermediate species would polymerize to form amphiphilic polymers with hydrophobic backbones, hydrophilic side chains containing oligo (ethylene glycol), and functional groups (OH, CHO, and COOH groups), and the resultant amphiphilic polyDCH18C6 (named as am-DCH18C6) existed in the water as core-shell micelles. Then, with the increase of irradiation time, the am-DCH18C6 micelles could be further self-organized to form the hollow microspheres, and crosslinking of polymer micelles perhaps occurred simultaneously [18]. It was found that, in the presence of N\({}_{2}\)O, the hydrated electrons from radiolysis of water could be changed to OH radicals. The yield of hollow microspheres increased from 51.9 \(\pm\) 0.7% to 85.5 \(\pm\) 0.7% at irradiation time of 111 h, indicating that the \(\cdot\)OH radicals play a key role in the formation of hollow microspheres. Moreover, the yield of hollow microspheres increased with decreasing dose rate. It is attributed to the fact that the lower dose rate favors an intermolecular recombination due to a lower probability for the presence of two or more radical sites on a single polymer chain in the steady state [24].