## 3 Results and Discussion
### Improve the Microcapsule Performance by Crosslinker  
With the optimum amount of sodium chloride, microcapsules with latent heat of 67.5 J/g and good morphology were obtained. But it is still essential to improve thermal enthalpy. Therefore, a crosslinker was further applied to the optimum system above to improve capsulation efficiency, which also helped shorten the curing time. The crosslinker chosen was composed of resorcinol and ammonium chloride with a mass ratio of 1:1. Figure 8 shows the SEM images of microcapsules prepared with different amounts of crosslinker. The microcapsule spheres dispersed better when 0.25 wt% crosslinker was incorporated. Compared with Figure 4D, the smoothness of the surface of the microcapsules slightly decreased while the diameter hardly changed. With the addition of 0.35 wt% crosslinker, the microcapsules started to connect to each other and the morphology became worse. When the crosslinker content reached 0.5 wt%, complete spheres begin to disappear, and independent spheres could not be observed. Only large size paraffin/PUF particles with irregular shapes and rather rough surfaces were obtained when 1 wt% crosslinker was introduced (Figure 8D).  
The DSC curves of samples are presented in Figure 9. The melting enthalpy increased to 110.0 J/g with the addition of 0.25 wt% crosslinker, which was an improvement of 63% when compared to microcapsules prepared without crosslinker. The enthalpy value further rose with more crosslinker and achieved 137.0 J/g at crosslinker addition amount of 1 wt%, which is a rather high value when compared to the literature [25; 26].  
The two components, ammonium chloride and resorcinol, in as-used crosslinker played different roles in the polymerization process of the shell material. The main effect of ammonium chloride was to create an acidic condition in order to shorten the curing time. Ammonium chloride reacted with prepolymers as shown in formula (h) of Scheme 2 and the pH value was reduced. Figure 10 directly shows the measured pH value change of reaction system with different additions of crosslinker. The pH value gradually declined as the amount of crosslinker increased, the decline rate leveled off and reached 2.7. Normally, reduction of pH value can accelerate the curing process, which benefits the encapsulation of paraffin. However, excessive incorporation of crosslinker significantly reduces the pH value and leads to high reaction rate which goes against the formation of separated and spherical  
Figure 8: SEM images of microcapsules prepared with different amount of crosslinker. 0.25 wt% (**A**), 0.35 wt% (**B**), 0.5 wt% (**C**), 1 wt% (**D**). The scale bars are all 10 \(\upmu\)m.  
microcapsules. As shown in formula (i), resorcinol can directly participate in the polymerization process and increase connection between branches, which improves the crosslinking degree and capsulation efficiency. As one of the monomers, a small amount of resorcinol can generate positive effect but excess usage will affect the performance of PUF [46]. Thus, the crosslinker should be added after the prepolymers fully deposit on the surface of droplets, and the balanced addition amount of crosslinker for semi-crystalline paraffin/PUF microcapsules with well morphologies and high latent heat is 0.25 wt% here.  
Figure 10: Dependence of reaction system pH value on the addition of crosslinker.  
Figure 9: DSC curves of heating (**A**) and cooling (**B**) process of microcapsules prepared with further addition of different amount of crosslinker.