Electrospun ultrafine composite fibers consisting of lauric acid and polyamide 6 as form-stable phase change materials for storage and retrieval of solar thermal energy

Abstract

The ultrafine composite fibers consisting of lauric acid (LA) and polyamide 6 (PA6) with varied LA/PA6 mass ratios of 80/100, 100/100, 120/100, and 150/100 were prepared via the technique of electrospinning as form-stable phase change materials (PCMs); morphological structures of the fibers were characterized, and their thermal stability and thermal energy storage properties were investigated. SEM results indicated that electrospun LA/PA6 composite fibers possessed the ribbon-shaped morphology with fiber diameters slightly larger than those of the neat PA6 fibers. The study revealed that the LA primarily existed as phase separated domains and the domains were randomly dispersed in as-electrospun ultrafine composite fibers. To reveal the evolution of morphological structures and the resulting properties for storage and retrieval of thermal energy, the heat treatment at 60°C for the electrospun composite fibers with LA/PA6 mass ratio of 100/100 was carried out; upon the heat treatment, the complete encapsulation of LA domains inside PA6 matrices would occur, and the heat-treated fibers might even possess the partial core/sheath structure with LA being the core and PA6 being the sheath. TGA results indicated that the degradation of electrospun LA/PA6 ultrafine composite fibers had two steps, and the charred residue at 700°C of the composite fibers was lower than that of the neat PA6 fibers. DSC measurements suggested that the amount of LA in the fibers played an important role on the values of heat enthalpies for the composite fibers; while it had no appreciable effect on the temperatures of phase transitions. After the heat treatment, the values of heat enthalpies for the composite fibers decreased slightly and this was due to the complete encapsulation of LA and/or the confinement effect of PA6 matrix, both of which constrained the movement of LA molecules. It is envisioned that the electrospun ultrafine composite fibers with PCMs (e.g., fatty acids) encapsulated in the supporting polymeric matrices (e.g., PA6) would be innovative form-stable PCMs for storage and retrieval of solar thermal energy.