Influence of in situ reactive interphase with graft copolymer on shear and extensional rheology in a model bilayered polymer system

Influence of an in situ reactive interphase composed of graft copolymer on the melt shear and extensional rheology was systematically studied with a model bilayer of polyamide-6 (PA6)/maleic anhydride grafted poly(vinylidene fluoride) (PVDF-g-MAH). Firstly, small-amplitude oscillatory shear was used in situ to probe the development of reactive interphase from the interfacial reactions by tracking the changes of viscoelastic responses. Secondly, shear (start-up shear, shear stress relaxation) and extensional rheology was comparatively performed on the healed bilayers to evaluate the effects of reactive interphase. Interestingly, shear stress relaxation and especially extensional rheology were pretty sensitive to the presence of reactive interphase, whereas start-up shear showed negligible sensitivity. Specifically, the reactive interphase retarded the stress relaxation of healed bilayers subjected to step strains. Particularly, extensional rheology provided a more direct and quantitative view of the contribution of reactive interphase to melt rheology in both linear and nonlinear regimes. The remarkable increase in transient extensional viscosity and enhanced strain hardening resulting from the interfacial reaction in bilayer were demonstrated in terms of the interfacial stress. Besides, effect of reaction extent on extensional rheology was further examined, where it was found that interfacial stress increased with reaction time. The observed changes in rheology were attributed to the in situ formed interphase with graft copolymers and the resulting increased entanglements between neighboring layers. This study highlights the remarkable sensitivity of shear stress relaxation and especially extensional rheology to a reactive interphase.