Molecular dynamics simulation on the glass transition temperature and mechanical properties of polyimide/functional graphene composites
Authors: Yang Wen-Long  , Han Jun-Sheng  , Wang Yu  , Lin Jia-Qi  , He Guo-Qiang  , Sun Hong-Guo  ,   Department of Applied Science, Harbin University of Science Technology, Harbin 150080, China;,   Key Laboratory of Engineering Dielectrics and Its Application, Harbin University of Science Technology, Harbin 150080, China;,   School of Material Science and Engineering, Cultivation Base for State Key Laboratory of Prcesing for Non-ferrous Metal and Featured Materials, Guangxi University, Nanning 530004, China;,   Polymer Composites Engineering Laboratory, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China
Published: December 2020

Polyimide (PI) and the functional graphene modified with nano-composite models of hydroxyl,carboxyl and amino groups are realized by a multi-scale modeling method.The influences of the functional graphenes with different functional groups on the microstructure,mechanical and thermodynamic performances of polyimide-based composite models are investigated by the molecular dynamics simulation.The cell parameters,solubility parameters,elastic coefficients, Young's moduli,shear moduli,and the values of glass-transition temperature (Tg) of polyimide-based composite models are calculated with the COMPASS force field.Moreover,the interaction energies and hydrogen bonds of composites are analyzed to explore the internal mechanisms for improving mechanical and thermodynamic properties.The results demonstrate that the density of PI matrix is 1.312 g·cm-3 and the solubility parameter of PI matrix is 21.84 J1/2·cm-3/2, which are in accord with the actual PI parameters.The Young's moduli of the composites increase obviously with the increase of the interaction energy between the PI matrix and the functional graphenes with hydroxyl,carboxyl and amino groups at 298 K and 1 atm.The Young's moduli of PI and PI/graphene with carboxyl groups are respectively 3.174 GPa and 4.946 GPa and the shear moduli are respectively 1.139 GPa and 1.816 GPa.Comparing with pure PI/graphene composite,the average hydrogen bonds increase obviously after graphene has been functionalized.Because the interaction between the functional graphene and PI matrix increases,the movement of PI molecular chain needs more energy,and the rigidity of the composite is enhanced.The Tg of the composite also relates to the interaction energy.It is also found that the Tg of the nano-composite effectively decreases by the hybrid functional graphene.The Tg of pure PI is 663.57 K,while the Tg values of PI/graphene and PI/graphene with carboxyl groups nanocomposites are 559.30 K and 601.61 K,respectively.Moreover,the density and interaction energy of hydrogen bonds of the PGCOOH are 784.81 kcal/mol and 1.396 g/cm3,respectively,which are the largest among their counterparts of the composite systems.The elastic coefficients show that the PGCOOH is more uniform than that other composites.All of these indicate that the graphene with carboxyl group can greatly enhance the interaction between graphene and PI,improve the mechanical properties and adjust the Tg value of the PI matrix.The chemical modification of interaction energy in matrix is deemed to be of benefit to the improvement in composite performance,and the interaction energy calculation is considered to be an effective method of predicting the structures and performances of new composites.