Fundamental studies have been devoted to probe and modelize the interfacial phenomena at multilayered systems based on two model compatible polymers of PVDF and PMMA with varying molar masses. Linear and nonlinear rheology have been demonstrated to be sensitive to the presence of diffuse interphase triggered at polymer/polymer interface. Firstly, the interdiffusion kinetics as well as the development of the interphase have been investigated using SAOS measurements. Results were analyzed according to Doi-Edwards theory. The PMMA/PVDF mixtures have been examined to be a couple of weak thermorheological complexity, owning close monomeric friction coefficients. Based on this physics, a new rheological model was developed to quantify the interdiffusion coefficients. Rheological and geometrical properties of the interphase have been quantified through this model and validated by SEM-EDX and TEM. Secondly, step strain, startup shear and in uni-axial extension have been carried experiments out on the multilayer structures to investigate the sensitivity of nonlinear rheology to the well-defined diffuse interphase. An original model was proposed to fit the stress relaxation behavior of multilayer and to estimate the relaxation behavior of the interphase. Lack of entanglement at the interface and weak entanglement intensity at the diffuse interphase make them to be subsequently readily to suffer from interfacial yielding after continuous large deformations. Besides, elongational properties of the multilayer structures have been shown to be a function of composition as controlled by layer number and interphase properties. Finally, the diffuse interphase development coupled to flow in practical coextrusion process has been considered. The compromising result between negative effect of chain orientation and favorable effect of flow has been discussed. Presence of the diffuse interphase was demonstrated to significantly weaken the viscous and elastic instabilities.