Influences of expanded graphite on structural morphology and thermal performance of composite phase change materials consisting of fatty acid eutectics and electrospun PA6 nanofibrous mats

In the present work, three fatty acid eutectics of capric acid (CA)–lauric acid (LA), capric acid–palmitic acid (PA), and capric acid–stearic acid (SA) were prepared through melt-blending followed by ultrasonication and were investigated as model phase change materials (PCMs); for comparison, the individual fatty acid of CA was also studied. The DSC measurements indicated that the phase transition temperatures of fatty acid eutectics were lower than those of individual fatty acid of CA. Thereafter, the polyamide 6 (PA6) nanofibers and PA6/EG composite nanofibers with 10 wt.% expanded graphite (EG) were prepared by electrospinning; and then composite PCMs with fatty acid eutectics absorbed in and/or supported by the overlaid mats of electrospun nanofibers (e.g., PA6 and PA6/EG) were explored for storage and retrieval of thermal energy. Influences of the EG on structural morphologies, thermal energy storage properties and thermal energy storage/retrieval rates of composite PCMs were respectively characterized by scanning electron microscopy (SEM), differential scanning calorimeter (DSC) and measurement of melting/freezing times. The results indicated that the additions of EG caused the interfaces between fatty acid eutectics and PA6 nanofibrous mats to become more illegible; increased the absorption capacity of fatty acid eutectics within nanofibrous mats. The enthalpies of melting and crystallization of composite PCMs with EG were higher than those of the corresponding composite PCMs without EG, whereas there were no appreciable changes on the phase transition temperatures. The EG improved thermal energy storage/retrieval rates of composite PCMs were also confirmed by comparing the melting/freezing times of CA/PA6/EG and CA–SA/PA6/EG with those of CA/PA6 and CA–SA/PA6, respectively. The results from the SEM observation showed that composite PCMs had no or little variations in shape and surface morphology after heating/cooling processes.