Molecular-dynamics simulation of polyimide matrix pre-crystallization near the surface of a single-walled carbon nanotube

Polyimide-based composite materials with a single-walled carbon nanotube as filler were studied by means of extensive fully-atomistic molecular-dynamics simulations. Polyimides (PI) were considered based on 1,3-bis-(3',4-dicarboxyphenoxy)-benzene (dianhydride R) and various types of diamines: 4,4'-bis-(4 ''-aminophenoxy)- diphenylsulfone (diamine BAPS) and 4,4'-bis-(4 ''-aminophenoxy)-diphenyl (diamine BAPB). The influence of the chemical structure of the polyimides on the microstructure of the composite matrix near the filler surface and away from it was investigated. The formation of subsurface layers close to the nanotube surface was found for all composites considered. In the case of R-BAPB-based composites, the formation of an organized structure was shown that could be the initial stage of the matrix crystallization process observed experimentally. Similar structural features were not observed in the R-BAPS composites. Carbon nanotubes induce the elongation of R-BAPB chains in composites whereas R-BAPS chains become more compact similar to what is observed for EXTEM (TM) polyimide. It was shown that electrostatic interactions do not influence the microstructure of composites but slow down significantly the dynamics of PI chains in composites.