  The measurement of interfacial mechanical or rheological properties in
polymer blends is a challenging task, as well as providing a quantitative link
between these properties and the interfacial nanostructure. Here, we perform a
systematic study of the extensional rheology of multilayer films of an
immsicible polymer pair, polystyrene and poly(methyl methacrylate). We take
advantage of multinanolayer coextrusion to increase the number of interfaces up
to thousands, consequently magnifying the interfacial response of the films.
The transient elongational response is compared to an addivity rule model based
on the summation of the contribution of each polymer as well as the interfacial
one. At low strain rates, the model reproduces the transient extensional
viscosity up to strain-thinning, while at larger ones, the extra stress exceeds
the prediction based on constant interfacial tension. This extra-contribution
is attributed to an interphase modulus on the order of 1-10 MPa, which
increases with strain rate following a power-law with an exponent 1/3.
Extensional rheology of multinanolayer films is then an efficient combination
to go beyond interfacial tension and measure quantitatively the interfacial
rheology of immiscible polymer blends.
