Molecular dynamics and finite element investigation of polymer interphase effects on effective stiffness of wavy aligned carbon nanotube composites

Interphase effects have been studied for their effect on composite properties for many decades, and it is well documented that an interphase can exist in polymer composites comprised of nanofibers as well. We present a first study of interphase effects on the basic elastic response of wavy aligned carbon nanotube (A-CNT) polymer nanocomposties (PNCs). Waviness is characterized by ex situ pre-fabrication imaging of A-CNT forests and used as an input to finite element analyses of the PNCs containing an interphase region in the thermoset polymer defined using molecular dynamics (MD) simulations. The interphase thickness of ~1nm is found to be independent of crosslink density and contain regions of both higher and lower mass density than the bulk polymer. Finite element analyses of wavy single and double-wall A-CNT PNCs incorporating this interphase, allow the effective stiffness based on a representative volume element to be calculated. Waviness of the A-CNTs dominates the effective axial stiffness of the PNCs, with the interphase having a relatively small effect. The interphase changes the stress and strain distribution local to the CNT ‘fiber’ and this is expected to play an important role in failure, such as CNT pullout, of the CNT-polymer system. These findings, in addition to the relatively high volume fraction (V_f) of the interphase in PNCs with high CNT V_f, suggests that the interphase may play a more important role in PNCs than in micron-scale (typical) collimated fiber composites. Future work in this area includes inelastic polymer response during nanofiber pullout.