## 3 Results and Discussion  
### Performance of Microcapsules without Sodium Chloride and Crosslinker  
Paraffin/PUF microcapsules without the addition of sodium and crosslinker were first prepared and characterized as the control. As shown in Figure 1, the microcapsules exhibited a melting point of 57.7 \({}^{\circ}\)C and melting enthalpy of 42.8 J/g. By comparing to melting enthalpy of raw microcrystalline paraffin (185.2 J/g), the encapsulation efficiency of paraffin was only 23.1%. However, the microcapsules showed higher crystallization temperature than raw paraffin during cooling process, 54.6 \({}^{\circ}\)C vs. 50.8 \({}^{\circ}\)C, which benefited the fast release of latent heat. The improvement of crystallization behavior might originate from two aspects. On one hand, PUF possibly plays the role of nucleating agent for paraffin. On the other hand, the interface between PUF and paraffin with curve geometry could improve the primary nucleation of paraffin [35; 36; 37].  
To directly visualize the successful encapsulation of paraffin by PUF, both the as-prepared product and raw paraffin were placed on a hot stage prefixed at temperature of 70 \({}^{\circ}\)C. Figure 2 shows that the raw paraffin immediately melted as soon as it was put on the hot-stage. However, the appearance of the as-prepared microcapsules stayed unchanged, and no liquid leakage occurred even it has been heated for over 20 min. Thus, the paraffin was well accommodated by the compact PUF shell.  
Figure 1: Differential scanning calorimetry (DSC) heating and cooling curves of paraffin (solid line) and microcapsules prepared without sodium chloride and crosslinker (dash line).  
Figure 2: Photographs of the evolution of raw paraffin and paraffin/PUF microcapsules on hot-stage fixed at 70 \({}^{\circ}\)C.  
The FTIR spectra of paraffin, PUF, and the microcapsules are shown in Figure 3. Spectra (a) and (b) show strong multiple absorption bands between 3000 and 2800 cm\({}^{-1}\), which are associated with the aliphatic C\(-\)H stretching vibrations, while those of PUF are relatively weak in spectra (c). The absorption band at 1464 cm\({}^{-1}\) is assigned to the C\(-\)H bending vibrations and the band at 722 cm\({}^{-1}\) is attributed to the groups of CH\({}_{2}\) inplane vibrations. The characteristic peaks mentioned above belong to the core material, paraffin. The broad band at 3350 cm\({}^{-1}\) is assigned to N\(-\)H stretching vibrations and it shifts because the hydrogen of amide participates in polymerization, which is unfavorable for the stretching vibration [38] The absorption bands at 1637 and 1543 cm\({}^{-1}\), which can be observed in spectra (a) and (c), are attributed to the C=O and C\(-\)N stretching vibrations of amide, respectively. Bands between 1377 and 1024 cm\({}^{-1}\), belonging to amide III are caused by the multiple reaction of active hydrogen of amide [39] These absorption bands represent the PUF shell materials. Through the above analysis, characteristic bands of paraffin and PUF can be found in spectra of microcapsules and almost no extra bands appear, which infers that no chemical reaction happens between the core and shell. The paraffin is well encapsulated by PUF shell physically.