## 3 Results and Discussion
### Optimizing Microcapsule Performance by Adding Sodium Chloride  
Although the above result has demonstrated that paraffin is encapsulated by PUF shell, the SEM image in Figure 4A highlights that the morphology of the microcapsules was not ideal. Lumps of microcapsules with size of several tens microns are observed and small particles stacked on the rough surface, which is similar to previous reported paraffin/PUF microcapsules without the assistant of sodium chloride and crosslinker [40]. The poor morphology corresponds to the low latent heat and encapsulation efficiency.  
To improve the morphology of the microcapsule, sodium chloride is widely used [41]. The typical morphologies of paraffin/PUF microcapsules prepared with different addition amounts of sodium chloride are shown in Figure 4B-E. Once the sodium chloride was introduced, the morphology of the microcapsules became regular. When the addition of sodium chloride reached 1 wt%, the microcapsules were still bonded to each other but the tendency of separation was clearer. Apparent spherical morphology of microcapsules emerged after increasing the sodium chloride amount to 2 wt%, and the diameter ranged from 2 to 5 \(\upmu\)m. With an increased sodium chloride content of 4 wt% and 8 wt%, spherical and independent microcapsules with a smooth surface were found. Some small particles adhered on the surface of the microcapsules, and this was due to the self-polymerization of PUF [33]. If the addition of sodium chloride is further increased to 16 wt%, demulsification occurred and the obvious boundary can be observed between the water and paraffin phase.  
Figure 3: FTIR spectra of paraffin/PUF microcapsules (**a**), raw paraffin (**b**), and PUF (**c**).  
Figure 5 depicts the DSC curves of paraffin/PUF microcapsules with different amounts of sodium chloride. The melting enthalpy of the microcapsules increased when sodium chloride was increased and reached a maximum value of 67.5 J/g at 4 wt% sodium chloride content, the equivalent to a paraffin encapsulation efficiency of 36.4%. Increased addition of sodium chloride led to a significant decline in melting enthalpy. For instance, the melting enthalpy of the microcapsules prepared with 8 wt% sodium chloride dropped to 44.6 J/g, which is even close to the microcapsules obtained without the addition of sodium chloride. Thus, the addition of sodium can help encapsulate paraffin, but an excess dose of sodium chloride will a show remarkably negative effect.  
Considering the melting enthalpy and morphology data, 4 wt% was the optimal amount of sodium chloride to obtain such microcrystalline paraffin/PUF microcapsules. The shape, separation degree, and surface smoothness the of microcapsules were remarkably improved as the content of sodium chloride increased. However, salt solution is also widely adopted for demolusion because hydration of inorganic salt can reduce free water and break the oil-in-water structure [42]. Therefore, the lower level of enthalpy seen in the microcapsules prepared with 8 wt% sodium chloride should be due to the hydration effect being of sodium chloride being, which reduced the encapsulation of paraffin.  
Figure 4: SEM images of microcapsules prepared with different addition amounts of sodium chloride.  
0 (**A**), 1 wt% (**B**), 2 wt% (**C**), 4 wt% (**D**), and 8 wt% (**E**). The scale bars are all 10 \(\upmu\)m.  
Figure 5: DSC curves of heating (**A**) and cooling (**B**) processes of microcapsules prepared with different amounts of sodium chloride. The heating and cooling rates are 10 \({}^{\circ}\)C/min