Molecular Dynamics Study of Optically Controlled Phase Change Materials

The optically controlled phase change technology makes the phase change material produce an obvious energy barrier between a solid and a liquid, which can effectively prevent spontaneous heat loss. Based on the molecular dynamics method, the microscopic model of the optically controlled composite system (capric acid/4-(phenyldiazenyl)phenyl decanoate) was established and the simulation of the AZO trans–cis isomerization process was based on the modification of the dihedral angle parameters of AZO in the condensed phase. A theoretical method for predicting the temperature of optically controlled phase transition was proposed, and the reason for the temperature difference caused by isomerization was analyzed. At a given molar concentration (30 mol %), the cis and trans phase transition temperature, which shows a temperature difference of 4.49 K, results from the destruction of molecular symmetry and the differences in the aggregation and nucleation ability of the dopant before and after photoisomerization. Experiments verify the reliability of the method. In addition, the analysis of the thermal conductivity of the composite systems was conducted based on the nonequilibrium molecular dynamics method. No matter whether the azophenyl group is a cis structure or a trans structure, doping slightly weakens the thermal conductivity of the raw material due to the destruction of the ordered lattice structure. Aggregation and nucleation of trans-azophenyl groups lead to a higher degree of order of the fatty acid chain than that of the corresponding cis system, resulting in higher thermal conductivity, which can be explained quantitatively by the order parameter. This study provides theoretical support for the thermophysical properties and formation reasons of optically controlled phase change materials containing the azophenyl group and provides ideas for the improvement, development, and application of such materials in the future.