Preparation, structure, properties and thermal behavior of rigid-rod polyimide/montmorillonite nanocomposites

Polyimide/montmorillonite nanocomposites were prepared from solution of poly(amic acid) precursors and the dodecyl-montmorillonite (DMONT) using N-methyl-2-pyrrolidone as a solvent. The cured films of the rigid-rod polyimide/DMONT hybrids as characterized by FTIR, TEM and WAXD were exfoliated nanocomposites at low DMONT content (<1 wt.%) and partially exfoliated nanocomposites at high DMONT content (containing aggregates of DMONT). The nanocomposite showed optimum properties at 3 wt.% DMONT with low water absorption, improved mechanical strength and modulus, and high dielectric strength. The clay content significantly influences thermal behavior of the nanocomposite films, such as thermal expansion coefficient (CTE), glass transition and yielding temperatures of the rigid-rod and flexible polyimide nanocomposites. It was found that the rigid polyimide matrix gave a superior thermal behavior; i.e. lower CTE, higher glass transition and yielding temperatures than the flexible ones. Effect of DMONT content was significant at high temperature operation where a side reaction of dodecylamine and a poly(amic acid) should be aware to cause a reduction in degree of imidisation and glass transition temperature and the formation of silicate layer aggregates. High DMONT content nanocomposites (aggregates were formed) showed high yielding temperatures indicating the suppression of polymer flow by the rigidity of the clay. The flexible polyimide nanocomposites showed higher thermal deformation than the rigid ones. Moreover, the rigid polyimide nanocomposite with 9 wt.% DMONT showed a thermally reversible behavior indicating that the silicate layers efficiently suppressed the deformation of the rigid polyimide.