  In multi-component soft matter, interface properties often play a key role in
determining the properties of the overall system. The identification of the
internal dynamic structures in non-equilibrium situations requires the
interface rheology to be characterized. We have developed a method to quantify
the rheological contribution of soft interfaces and evaluate the dynamic
modulus of the interface. This method reveals that the dynamic shear responses
of interfaces in bilayer systems comprising polypropylene and three different
polyethylenes can be classified as having hardening and softening effects on
the overall system: a interface between linear long polymers becomes more
elastic than the component polymers, while large polydispersity or
long-chain-branching of one component make the interface more viscous. We find
that the chain lengths and architectures of the component polymers, rather than
equilibrium immiscibility, play an essential role in determining the interface
rheological properties.
