In the present paper fully-atomistic models have been used for the simulation on on the microsecond time scale of modern polyimides (PI) and polyimide-based nanocomposites filled with graphene or single-walled carbon nanotubes. We have considered both commercial EXTEM™ and ULTEM™ polyimides and polyimides (namely, semi-crystallizable R-BAPB and amorphous R-BAPS and R-BAPO) synthesized in the Institute of Macromolecular Compounds in St. Petersburg; those may be used as materials for aerospace applications. In the developed simulation approach we established the crucial role of the computationally-expensive electrostatic interactions, which often are omitted. We concluded that the nanofiller surface curvature is indeed an important factor influencing the PI structure patterning near the carbon nanofiller surface.
The developed multi-scale approach demonstrates a highly efficient prediction ability and includes the parameterization of the electrostatic interactions by quantum-chemical calculations, the generation of the initial configuration, the annealing, the microsecond-scale equilibration and production runs. The molecular-dynamics simulations have been carried out using the Gromacs simulation package. The verification of the simulational approach has been carried out by the comparison of simulated thermal properties with corresponding experimental values.
The influence of the chemical structure of the polyimides on the microstructure of the composite matrix near the filler surface has been investigated. We studied density distribution far from the nanofiller surface and pair distribution functions for different atoms which characterize polyimide structure in the considered nanocomposites. The formation of the subsurface layers close to the nanofiller surface is found for all simulated composites. In the case of the crystallizable R–BAPB matrices the formation of an ordered structure could be considered as an initial stage of the matrix crystallization process observed experimentally. Similar structural features have not been observed for polyimides ULTEM™, EXTEM™, R-BAPS, and R-BAPO.
Carbon nanotubes induce the elongation of R–BAPB chains in the composites, whereas R–BAPS and EXTEM™ chains become more compact. It was shown that the electrostatic interactions do not influence the microstructure of composites but slow down significantly the dynamics of PI chains in composites. The analysis of the molecular-dynamic trajectories shows that PI chain segments tend to orient themselves near the graphene surface rather than on the nanotube surface. This confirms the assumption that the nanofiller surface curvature is indeed an important factor influencing the structure patterning of the polyimide near the carbon nanofiller surface, and explains why graphene provides better initiation of crystallization in experimental tests.
Acknowledgements
This study has been supported by the Russian Ministry of Education and Science within State Contract No. 14.Z50.31.0002. The simulations were carried out using the computational resources of the Institute of Macromolecular Compounds, Russian Academy of Sciences, and the Chebyshev and Lomonosov supercomputers at Moscow State University.