A Molecular Dynamics Study on Mechanical and Tribological Properties of Polyimide Modified with Graphene: Size and...

Graphene, with excellent mechanical and self-lubricating properties for polymer modification, can be single- or multi-layered (3-10 layers). In this study, molecular dynamics simulations have been employed to qualitatively explore the relative trends and internal modification mechanism of polyimide (PI) modification by single-layer graphene and three-layer graphene with different sizes. Small-sized single-layer graphene (SSLG), small-sized multi-layer graphene (SMLG), large-sized single-layer graphene (LSLG), and large-sized multi-layer graphene (LMLG) were introduced into the PI matrix at an identical mass fraction with initially uniform dispersion during model construction. The tensile mechanical and frictional behaviors of graphene-modified PI were systematically examined. The results indicate that graphene addition effectively improves both the mechanical and tribological properties of PI. At a fixed filler mass fraction, SSLG exhibits the strongest interaction with PI, with a binding energy of 396.8 kJ/mol. The fractional free volume of SSLG-reinforced PI reaches 15.3%, which is considerably lower than the value calculated for pure PI (20.3%). The average elastic modulus of the SSLG-modified PI is 70.4% higher than that of pure PI, an increase which exceeds that of the SMLG-modified PI (45.2%), LSLG-modified PI (26.5%), and LMLG-modified PI (14.0%). In terms of tribological properties, the SMLG-modified PI exhibits optimal friction with an average friction coefficient of 0.105, which is 48.3% lower than that of pure PI and lower than the values for the SSLG (0.138), LSLG (0.156), and LMLG (0.182) systems. This work mainly draws qualitative structure-property rules and provides key theoretical fundamentals and design principles for tailoring the mechanical and tribological performance of high-performance graphene-reinforced polyimide composites.