## 3 Results and Discussion  
Effects of the Shell Polymer Composition and \(T_{g}\) on the Expansion Properties of Polymer Microspheres  
The \(T_{g}\) of shell polymer, \(T_{\mathrm{st}}\), \(T_{\mathrm{sh}}\), \(T_{\mathrm{r}}\), and \(E_{\mathrm{v}}\) of copolymer microspheres synthesized by different monomer composition under the condition of pentane as blowing agent and DVB crosslinking agent was 0.2 wt% are shown in Table 1. The microscopic morphologies of the microspheres before and after expansion are shown in Figure 1. Because the reactivity ratios of monomers are different, r\({}_{12}\) = 0.32, r\({}_{21}\) = 0.92, r\({}_{13}\) = 0.36, r\({}_{31}\) = 2.38, r\({}_{23}\) = 0.138, r\({}_{32}\) = 1.322, 1-VDC, 2-AN, 3-MMA [30]. The shell polymer compositions of the vinylidene chloride copolymer microspheres are quite different from that of the actual feeding monomer. The VDC-MMA copolymer microspheres present extremely poor expansion properties. Although there were a few number of the VDC-MMA copolymer microspheres with the feeding mass ratio of 60/40 exhibiting expandable behavior. However, when that microspheres expanded, they easily cracked and collapsed in a narrow \(T_{\mathrm{r}}\), as shown in Figure 1a,a'. It is mainly due to the increase of PVDC segment units in the shell polymer, which decreases the \(T_{\mathrm{g}}\) of shell polymer of the copolymer microsphere to 53.6 \({}^{\circ}\)C. While the boiling point of pentane is 36.1 \({}^{\circ}\)C. When the expansion temperature exceeded over the boiling point of pentane and \(T_{\mathrm{g}}\) of shell polymer, pentane produced high internal gas pressure inside the microspheres, then the microspheres began to soften and expand at 80 \({}^{\circ}\)C. Whereas the microspheres had not yet expanded, they were brittlely cracked and collapsed by the high gas pressure because of the low \(T_{\mathrm{g}}\) of shell polymer. In particular, when the temperature was raised over the \(T_{\mathrm{g}}\) of shell polymer, a higher gas pressure produced by a high blowing agent \(E_{\mathrm{r}}\) (27.2%) in the VDC-MMA copolymer microspheres with the feeding mass ratio of 70/30 resulted in the microspheres bursting and collapsing immediately, and the expansion behavior was almost invisible. As can be seen from Table 1 and Figure 1, all of the VDC-MMA-AN copolymer microspheres exhibited thermally expandable behavior. Especially, the copolymer microspheres with the monomer feeding mass ratios of VDC-MMA-AN were 50/40/10, 50/30/20, and 60/20/20, respectively. 80% of these copolymer microspheres expanded and exhibited outstanding expansion properties, in which the \(T_{\mathrm{r}}\) were wide and the expanded microspheres remained in perfect spherical shape, as shown in Figure 1b',d',e'. However, although the core-shell VDC-MMA-AN copolymer microspheres encapsulated a high blowing agent \(E_{\mathrm{r}}\) (17.2% and 20.2%) with the monomer feeding mass ratio of 70/20/10 and 70/10/20, respectively [27], having the relatively narrow \(T_{\mathrm{r}}\), and some of microspheres cracked or shrank under a slightly higher temperature, therefore they did not keep the uniformity of morphologies and presented a poor expansion properties. Because the chain segment unit of PVDC in the shell polymer increased, the \(T_{\mathrm{g}}\) of shell polymer decreased, the shell polymer was insufficient to resist the internal gas pressure under the high \(E_{\mathrm{r}}\) of blowing agent. Obviously, the monomer composition not only affected the formation of core-shell structure and the \(E_{\mathrm{r}}\) of blowing agent in polymerization process [27], but also impacted the \(T_{\mathrm{g}}\) of shell polymer of copolymer microspheres, and then it further affected the expansion properties of copolymer microspheres.