## 3 Results and Discussion
According to literature calculation methods and constants [31], the vapor pressure of the blowing agent at the expansion temperature was calculated, as shown in Table 4. The precondition of this calculation was to assume that all the microspheres were airtight structures and there were no blowing agents diffused through the shell layer and without considering the influence of possible residual monomers. The calculation results are shown in Figure 4. It can be seen that the vapor pressure values of isopentane, pentane, hexane, and heptane at the \(T_{\mathrm{st}}\) are 3.84 bar, 3.32 bar, 1.64 bar, and 0.74 bar respectively. Isopentane could provide high expansion pressure at a lower temperature, therefore the microspheres expanded at a lower temperature. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**VDC-MMA-AN** & **Blowing Agent** & \(E_{\mathrm{r}}\) **(\%)** & \(T_{\mathrm{st}}\) (\({}^{\circ}\)C)** & \(T_{\mathrm{sh}}\) (\({}^{\circ}\)C)** & \(T_{\mathrm{r}}\) (\({}^{\circ}\)C) & \(E_{\mathrm{v}}\) \\ \hline
70/30/0 & pentane & 27.2 & - & - & - & 0 \\
70/30/0 & hexane & 32.1 & - & - & - & 0 \\
70/30/0 & heptane & 24.2 & - & - & - & 0 \\
50/30/20 & isopentane & 19.8 & 73 & 105 & 32 & 28.6 \\
50/30/20 & pentane & 20.6 & 76 & 100 & 24 & 30.3 \\
50/30/20 & hexane & 21.0 & 85 & 100 & 15 & 29.8 \\
50/30/20 & heptane & 22.2 & 88 & 100 & 12 & 28.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Expansion characteristics of copolymer microspheres synthesized at different type of blowing agent. The morphologies of the VDC-MMA-AN copolymer microspheres before and after expansion are shown in Figure 5. The morphologies of microspheres-encapsulated hexane or heptane were not as smooth as those of pentane, and the surface of microspheres were uneven. The reason was that the microspheres with hexane and heptane as blowing agents had many folds and lots of pits in the polymerization process, which were reported in our pre-study [27]. It could be seen that the type of blowing agent determined the driving force of the expansion process. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Blowing** & **Boiling** & **Solubilitypara** & \multicolumn{4}{c}{**Vapor Pressure Constants**} \\ \cline{4-7}
**Agent** & **Point (\({}^{\circ}\)C)** & **Meter (Cal/cm\({}^{3}\))\({}^{1/2}\)** & **C1** & **C2** & **C3** & **C4** & **C5** \\ \hline isopentane & 28 & 6.7 & 72.350 & \(-5010.9\) & \(-7.8830\) & \(8.9795\times 10^{-6}\) & 2 \\ pentane & 36.1 & 7.0 & 78.741 & \(-5420.3\) & \(-8.8253\) & \(9.6171\times 10^{-6}\) & 2 \\ hexane & 68.7 & 7.3 & 104.650 & \(-6995.5\) & \(-12.7020\) & \(1.2381\times 10^{-5}\) & 2 \\ heptane & 99 & 7.4 & 87.829 & \(-6996.4\) & \(-9.8802\) & \(7.2099\times 10^{-6}\) & 2 \\ \hline \hline \end{tabular} \({}^{\star}\) Vapor pressure (in bar) = exp [C1 + (C2/T) + C3 \(\times\) ln (T) + C4 \(\times\) T\({}^{\text{CS}}\)] \(\times\) 10\({}^{-5}\); temperatures are in Kelvin.  
\end{table} 
Table 4: Boiling points and data used for calculating Vapor Pressures \({}^{\star}\) of the hydrocarbons used as blowing agents in this study [24; 32]. Figure 4: Calculated vapor pressures of different blowing agents at \(T_{\text{st}}\) for microspheres in this study (the temperatures used when calculating the vapor pressures for each blowing agent are given within the brackets). Figure 5: Morphologies of VDC-MMA-AN copolymer microspheres before (**a**–**d**) and after (**a**\({}^{\prime}\)**–**d**\({}^{\prime}\)) expansion: (**a**,**a**\({}^{\prime}\)) isopentane, (**b**,**b**\({}^{\prime}\)) pentane, (**c**,**c**\({}^{\prime}\)) hexane, (**d**,**d**\({}^{\prime}\)) heptane. The parameters of the expansion properties and the morphologies of the VDC-MMA-AN copolymer microspheres with the different content of pentane blowing agents are shown in Table 5 and Figure 6, respectively. As can be seen in Table 5, increasing the \(E_{\mathrm{r}}\) of pentane, the \(T_{\mathrm{st}}\) and \(T_{\mathrm{sh}}\) gradually decreased, respectively.