Extensional Viscosity of Immiscible Polymer Multi-Nanolayer Films: Signature of the Interphase

The measurement of interfacial mechanical or rheologicalpropertiesin polymer blends is a challenging task, as well as providing a quantitativelink between these properties and the interfacial nanostructure. Here,we perform a systematic study of the extensional rheology of multilayerfilms of an immiscible polymer pair, polystyrene and poly(methyl methacrylate).We take advantage of multi-nanolayer coextrusion to increase the numberof interfaces up to thousands, consequently magnifying the interfacialresponse of the films. The transient elongational response is comparedto an additivity rule model based on the summation of the contributionof 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 strain rates, the extra stress exceeds the predictionbased on constant interfacial tension. This extra contribution isattributed to an interphase modulus on the order of 1-10 MPa,which increases with the strain rate following a power law with anexponent of 1/3. The extensional rheology of multi-nanolayer filmsis then an efficient combination to go beyond interfacial tensionand quantitatively measure the interfacial rheology of immisciblepolymer blends.