Young's modulus of multinanolayer polymer films: the role of the interfaces

The number of free surfaces in a polymer ultra-thin film has a strong influence on its physical and mechanical properties. In this study, Young's modulus as well as the glass transition temperature of multinanolayer coextruded polymer films, hence with no free surfaces, have been measured. We observe that contrary to the case of freestanding or supported ultra-thin films, there is no depression of the glass transition temperature or the Young modulus, but an increase for the latter as the layer thickness decreases, whatever the polymer pair (with various compatibilities). It is proposed that this increase is associated with the rise of interphase volume fraction in the films. An interphase modulus of about 25 GPa can be extracted from the empirical model, about 10 times higher than the typical modulus value of a glassy polymer. This value does not appear to depend on the nature of the polymers, meaning that the presence of entanglements at the interfaces is not a key factor to explain the increase in Young's modulus of multinanolayer films.