A nonlinear shear and elongation rheological study of interfacial failure in compatible bilayer systems
Résumé
This work aims to examine whether or not interfacial failure can occur in a compatible polymer bilayer system under large shear and
elongation deformations, as well as to probe the sensitivity of nonlinear transient rheology to the presence of interface/interphase at
neighboring layers. For this, stress relaxation after a step strain and fast startup in simple shear and uniaxial extension experiments have
been performed on healed and coextruded poly(methyl methacrylate)/poly(vinylidene fluoride) bilayers with the presence of a robust
diffuse interphase as evaluated by energy dispersive X ray. For unhealed bilayers, interfacial failure occurred in shear flows at
intermediate deformations, while for healed bilayers the interphase greatly delayed the onset of interfacial failure to larger deformation
steps and to a higher deformation rate in the startup shear. Extensional rheology demonstrated that the presence of an interphase in the
bilayers greatly enhanced the transient extensional viscosity as well as the tensile relaxation modulus E(t) of the structure, even
though the entanglement density was relatively low. Moreover, models are presented to predict the nonlinear relaxation behavior of
bilayers and to estimate the relaxation behavior of the interphase. Fitting of the tube model to the shear relaxation indicates a dilated tube
diameter in the interphase, confirming its weak entanglement intensity and its readiness to flow-induced disentanglement under large
external deformations. Finally, the physics of the interfacial failure was assessed based on some recent molecular dynamic
theories.