  Micromechanical modeling and mechanical properties of polyurethane (PU)
hybrid nanocomposite foams with multi-walled carbon nanotubes (MWCNTs) and
graphene nanoplatelets (GNPs) were investigated by mean of tensile strength,
hardness, impact strength and modified Halpin-Tsai equation. Three types of
graphene, with varied flake sizes and specific surface areas (SSA), were
utilized to study the effect of graphene types on the synergistic effect of
MWCNT/GNP hybrid nanofillers. The results indicate a remarkable synergetic
effect between MWCNTs and GNP-1.5 (1:1) with a flake size of 1.5 {\mu}m and a
higher SSA (750 m^2/g), which tensile strength of PU was improved by 43% as
compared to 19% for PU/MWCNTs and 17% for PU/GNP-1.5 at 0.25 wt% nanofiller
loadings. The synergy was successfully predicted using unit cell modeling, in
which the calculated data agrees with the experimental results.
