This difference in
surface roughness can result from the following reasons. After
fracture, a layer of soft phase rearranges to cover the surface of the neat polyurethane because the soft phase has a lower surface
energy and a low Tg [28]. This soft-phase surface layer
creates a smooth surface to minimize the surface area. In the
nanocomposite, the soft phase cannot efficiently cover the surface
because of poor phase separation and, most importantly, it
is constrained by the interfacial interactions between the polymer
and the nanoparticles (as deduced from the increase
in Tg). Surface modification of the ZnO nanoparticles and
FTIR measurements [8] confirmed that the interfacial reaction between the nanoparticles and polyurethane is a key reason for
the disruption of phase separation in the nanocomposite system.
The disruption of the phase separation can be controlled
by modifying the surface of ZnO nanoparticles as shown by
our previous work [8].