*3.3. Thermal Conductivity*

Figure 6 shows the comparison of the thermal conductivity between block-type PUSX nanofibers and films and graft-type PUSX nanofibers and films. For the thermal conductivity analysis, there was no obvious trend of varying chemical structures such as varying silicone chain lengths, varying silicone concentrations and block or graft structures of PUSX. The thermal conductivity was influenced mostly by the shapes of the samples. Both block- and graft-type PUSX nanofibers have much lower conductivity than films, because of the pores keeping the air inside. The heat insulating property is proved here. This result can be explained by the heat-retaining property of nanofibers because of the high porosity.

**Figure 6.** Comparison of the thermal conductivity (W/mK). (**a**) Block-type PUSX nanofibers and films with various chain lengths, (**b**) Block-type PUSX nanofibers and films with various silicone concentrations, (**c**) Graft-type PUSX nanofibers and films with various chain lengths.

It is worth mentioning that in solid objects, heat is transferred via conduction. For electrospun structures, much of the volume is made of empty spaces between fibers. Usually, the conduction along the fiber as part of the whole volume is negligible. But if the fibers becomes too compact, the packing density will be increased beyond an optimum standard, heat transfer via conduction through the solid fiber becomes significant, and the overall insulation property will be decreased. The insulation properties of the electrospun nanofibers are made more controllable due to their packing density compared to the films, to meet the demands of the market. As an ideal alternative to PUSX films, PUSX nanofibers provides good heat insulating properties and lightness in weight.
