## 3 Results and Discussion
### Analysis of Typical Performances  
#### 3.2.1 Crystallization Characteristics  
The crystallinity of polyethylene can directly affect its various properties, but the effect of doping microcapsules on the crystallization characteristics of polyethylene is not clear. Therefore, the crystallization characteristics of samples were analyzed in this paper. Figure 5 shows the heat flow curve of the samples. The crystallinity can be calculated by Formula (1) [43]:  
\[X_{c}=\frac{\Delta H_{m}}{(1-\omega)H_{N}}\times 100\%, \tag{1}\]  
where \(X_{c}\) is the crystallinity of the polymer, \(\Delta H_{m}\) is the melting heat, \(H_{N}\) is the melting heat when the crystallinity of polyethylene reaches 100%, and it is 293.6 J/g, \(\omega\) is the mass fraction of microcapsule in composite. The results are shown in Table 2, where \(T_{p}\) is the peak melting temperature and \(\Delta T_{p}\) is the half-height width of the melting peak. The crystallinity (\(X_{c}\)) of polyethylene decreased with the change in preparation technology. However, the microcapsule could increase the crystallinity, and the crystallinity of the composite was the highest. The melting peak (\(T_{p}\)) was negatively correlated with the flexibility of the polymer. Moreover, the polymer with higher crystallinity had a tighter internal structure and smaller free volume, so the flexibility of the molecular chain was lower [36]. Therefore, with the increase in the crystallinity, the \(T_{p}\) was increased. The half-height width of the melting peak (\(\Delta T_{p}\)) denotes the distribution of crystal grain size in polyethylene. The distribution of  
Figure 5: Heat flow curves of samples. grain size in polyethylene can be more uniform by changing the preparation process, and it can be more concentrated by the doping microcapsule. Compared with different preparation processes, the change in crystallinity was mainly related to the working stress magnitude and melting time during processing. At the action of stress, the orientation occurs along the direction of external force. The orientated structure can induce nucleation, which accelerates the nucleation rate, increases the number of crystal nuclei, and shortens the crystallization time [44]. Therefore, the original process with higher working stress has higher crystallinity. High temperatures can destroy the original crystalline structure of the polymer [36; 44]. The amount of destroyed crystalline structure is increased after prolonging the heating time, and the material tends to undergo a \"homogeneous nucleation\" crystallization process when the sample is cooled. Hence, the crystallization speed becomes slower and the distribution of grain size is more uniform. The decrease in crystal nucleus number and the destruction of the crystalline region can directly decrease the crystallinity. Moreover, the decrease in crystallization rate can reduce the grain size. In addition, the homogeneous nucleation can make the distribution of grain size more uniform, which leads to the decrease in \(\Delta T_{p}\). Therefore, as shown in Figure 6, changing the preparation process of polyethylene can reduce the number of large-size crystal grains and produce a few smaller crystal regions, thus reducing the crystallinity of polyethylene. Microcapsules can introduce many interface regions in the polyethylene matrix. The interface between the microcapsule and matrix plays a role of \"heterogeneous nucleation\" in the recrystallization process. Hence, the interface improves the crystallization rate and reduces the growth rate of spherulites, resulting in many uniform and orderly small spherulites [45; 46]. As shown in Figure 6, the microcapsules can increase the number of small size crystal regions in polyethylene and make the distribution of grain size more concentrated. However, microcapsules cannot change the size of the original crystal regions in the matrix. Therefore, the microcapsule increases the crystallization source of polyethylene matrix, resulting in the increase in crystallinity.