## 3 Results and Discussion
Effects of the Shell Polymer Crosslinking Degree on the Expansion Properties of Polymer Microspheres  
As shown in Table 2, under the fixed monomer feeding mass ratio of VDC-MMA-AN was 50/30/20 and pentane as the blowing agent, the expansion properties of copolymer microspheres with different content of DVB crosslinking agents were significantly different. The impacts of DVB on the gel content in shell polymer, \(E_{\mathrm{r}}\) and \(E_{\mathrm{v}}\) of copolymer microspheres are shown in Figure 2. The morphologies of copolymer microspheres before and after expansion with different DVB content are shown in Figure 3, and the morphologies of copolymer microspheres can be seen in our pre-study [27]. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**VDC/MMA/AN** & **Shell Composition of** & \(E_{\mathrm{r}}\) **(\%)** & \(T_{\mathrm{g}}\) **(\({}^{\circ}\)C)** & \(T_{\mathrm{st}}\) **(\({}^{\circ}\)C)** & \(T_{\mathrm{sh}}\) **(\({}^{\circ}\)C)** & \(T_{\mathrm{r}}\) **(\({}^{\circ}\)C)** & \(E_{\mathrm{v}}\) \\ \multicolumn{1}{c}{**(Mass)**} & \multicolumn{1}{c}{**PVDC/PMMA/PAN**} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \\ \hline
50/50/0 & 46.1/53.9/0 & 1.7 & 56.3 & - & - & - & 0 \\
60/40/0 & 54.6/45.4/0 & 6.8 & 53.6 & 80 & 85 & 5 & 3 \\
70/30/0 & 61.0/39.0/0 & 27.2 & 53.5 & - & - & - & 0 \\
50/40/10 & 48.5/42.0/9.5 & 17.5 & 63.8 & 80 & 105 & 25 & 19 \\
70/20/10 & 61.5/29.3/9.2 & 17.2 & 51.4 & 55 & 65 & 10 & 7.9 \\
50/30/20 & 41.2/41.0/17.8 & 20.6 & 63.4 & 76 & 100 & 24 & 30.3 \\
60/20/20 & 48.8/31.5/19.7 & 20.4 & 61.5 & 70 & 90 & 20 & 28.9 \\
70/10/20 & 61.7/17.9/20.4 & 20.2 & 42.4 & 65 & 75 & 10 & 27 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Expansion characteristics of polymer microspheres synthesized at different monomer feeding mass ratio. Figure 1: Morphologies of vinylidene chloride-methyl methacrylate-acrylonitrile (VDC-MMA-AN) copolymer microspheres before (**a**–**f**) and after (**a**–**f**) expansion prepared at different monomer feeding mass ratio of VDC-MMA-AN: (**a**,**a**) 60/40/0, (**b**,**b**) 50/40/10, (**c**,**c**) 70/20/10, (**d**,**d**) 50/30/20, (**e**,**e**) 60/20/20, (**f**,**f**) 70/10/20. Without DVB crosslinking agent, the microspheres shrank or cracked immediately after expansion. The \(T_{\text{st}}\) is same as the \(T_{\text{sh}}\) as shown in Table 2. The morphologies of microspheres before and after expansion have almost no change as shown in Figure 3a,a\({}^{\prime}\). It was because the \(E_{\text{r}}\) of blowing agent was very low (only 12.0%) and produced relatively low gas pressure. Moreover, because of low shell polymer crosslinking degree, the shell polymer was difficult to resist the internal gas pressure produced by the blowing agent in the expansion process, and the deformation of shell polymer was very poor and easy to crack. As seen from Table 2, when the content of crosslinking agent was increased from 0 wt% to 0.4 wt%, the gel content of shell polymer increased from 5.9% to 69.6%, the \(T_{\text{st}}\) dropped from 80 \({}^{\circ}\)C to 75 \({}^{\circ}\)C, the \(T_{\text{sh}}\) increased from 80 \({}^{\circ}\)C to 105 \({}^{\circ}\)C, the \(T_{\text{r}}\) increased from 0 \({}^{\circ}\)C to 30 \({}^{\circ}\)C, and the \(E_{\text{v}}\) sharply increased from 0 to 37.1. On the one hand, the gas pressure required by the expansion process significantly increased because of the increase of the \(E_{\text{r}}\) of blowing agent. On the other hand, increasing the content of crosslinking agent resulted in the improvement in the crosslinking degree of the shell polymer, then the viscoelasticity properties of the shell polymer enhanced and improved the ability of the shell polymer to resist the gas pressure produced by the blowing agent. However, sequentially increasing the content of DVB to 0.6 wt%, reduced the \(T_{\text{st}}\), \(T_{\text{sh}}\), \(T_{\text{r}}\), and \(E_{\text{v}}\).