Influence of confinement on the thermophysical property of paraffin wax/Kevlar nanofibrous phase change film

Abstract

The introduction of porous matrix in shape stable phase change materials (PCMs) alters the phase transition behavior of confined molecules. The mechanism by which the matrix affects the thermophysical properties of composite PCMs is still unclear, even there are conflicting understandings of the same phenomenon. According to this, paraffin/Kevlar nanofibrous films (PF/KNF) with different PF contents were fabricated by one step self-assembly method to comprehensively evaluate the regulation of confinement effects on the thermophysical properties and crystallization kinetics of confined PCMs. Due to the strong self-assembly tendency of Kevlar nanofibers in proton environment, paraffin passively wrapped in the network of matrix, forming a sandwich layered structure. The confinement of matrix altered the crystal structure of paraffin, even leading to the amorphous states in films with lower paraffin content. It also leaded to the formation of a non-melting layer at host-guest interface, thereby reducing the latent heat of composite PCMs. The latent heat of PF/KNF-70, the film with the highest paraffin content without leakage, decreased by about 32.7 % compared to the theoretical enthalpy calculated based on the mass percentage of paraffin in composite. In addition, the percentage of non-phase transition layer decreased linearly with the increase of PF content. The results of crystalline dynamics indicate that the matrix plays a dual role in providing nucleation sites and inhibiting molecular motion. When crystallization occurs at higher temperature (48.8 °C in this study), providing nucleation sites and promoting crystallization dominated the process, while inhibiting molecular motion and reducing crystallization rate become more prominent when crystallization temperature decreases.