Simulation of Thermodynamic Properties of Aramid/Functionalized Carbon Nanotubes Composites Based on Molecular Dynamics

Para-aramid paper based materials are widely used in the field of electrical insulation because of their excellent dielectric and mechanical properties. However, pure para-aramid materials have poor thermal conductivity, which cannot dissipate the heat generated in the operation of electrical equipment in time. Recently, researchers have doped nanoparticles into para-aramid fiber to improve its thermal conductivity, and carbon nanotubes (CNT) have attracted much attention for the excellent thermal conductivity and mechanical properties, but there are few reports on the properties of functional carbon nanotubes doped para-aramid composite system. This paper constructs the models of pure para-aramid, non-functionalized, hydroxylated, carboxylated and aminated carbon nanotube-doped aramid composites. Materials Studio and LAMMPS were used for molecular dynamics simulation to obtain the thermal conductivity, glass transition temperature, mechanical properties and structural parameters. The properties of different material systems are analyzed and the mechanism explanation is provided from the molecular level. Firstly, NEMD simulation was carried out in LAMMPS to calculate the thermal conductivity. The thermal conductivity of arylon composites doped with carbon nanotubes was significantly improved, among which the thermal conductivity of PPTA/CNT—COOH had the greatest change, and was increased by 75.2% compared with pure PPTA molecules, PPTA/CNT—OH and PPTA/CNT—NH2 were increased by 70.1% and 63.2% respectively. Then, the glass transition temperatures (Tg) of different material models were obtained by specific volume-temperature curve method. The Tg of PPTA/CNT—COOH increased by 43.29 K, and the Tg of PPTA/CNT, PPTA/CNT—NH2 and PPTA/CNT—OH increased by 27.03 K, 35.52 K and 39.74 K, respectively. In the calculation of mechanical properties, the static constant strain method is used to press a certain direction of stress on the surface of the material, and Young's modulus E and shear modulus G are solved by the obtained stiffness matrix. The results show that the doping of carbon nanotubes can enhance the elastic modulus of composites by more than 30% and shear modulus by more than 15%, among which the mechanical properties of PPTA/CNT—COOH are enhanced most obviously, and the damage effect of temperature rise on mechanical properties is reduced to a certain extent. In the calculation of structural parameters, the FFV, MSD and the number of hydrogen bonds were calculated respectively, and the reasons for the changes in the above thermodynamic properties were explained from the perspective of intermolecular interaction and molecular chain movement. On the one hand, carbon nanotubes have excellent thermal conductivity and mechanical properties, and doping can promote the performance improvement of the composite system by enhancing the strength of the matrix network skeleton. On the other hand, hydrogen bonds were formed between the functionalized carbon nanotubes and aramid molecules, which enhanced the interaction between molecules and formed a stronger molecular network, and improved the stability of the system. Due to the difference of electronegativity of N and O and the number of grafts, there are PPTA/CNT—COOH＞PPTA/CNT—OH＞PPTA/CNT—NH2 in the number and strength of hydrogen bonds. The relationship is reflected in the gain variation of thermodynamic properties of the composite system. The results show that the thermodynamic properties of the carboxylated carbon nanotubes aramid composite system are better than those of other functionalized doping systems, and the doping of functionalized carbon nanotubes does not change the dielectric constant of the composite system significantly, hence, it can still provide excellent insulation and protection properties. © 2024 China Machine Press. All rights reserved.