Thermo-Viscoelastic residual stress behavior of fluorinated polyimide based on fluid Instability-Driven shear exfoliated graphenic nanosheet

Abstract

Attempts to reduce the size of electronic devices result in residual stresses in the polymeric optoelectronic materials used therein, leading to detrimental effects such as microscopic defect formation and macroscopic dimensional changes. In this study, fluorinated polyimide (FPI) reinforced with reduced graphene oxide (RGO) was fabricated by exploiting the instability of the Taylor − Couette flow. This method affords exceptional ultralow residual stresses. High fluid-wall shear stress in Taylor vortex flow (TVF) with axisymmetric instability accelerates the reduction of graphene oxide (GO) even at a low temperature of 85 °C. Using ascorbic acid as a green reducing agent, a highly exfoliated RGO@TVF nanohybrid was obtained, which activated the interaction at the polymer-filler interface. During high-temperature cycling, real-time residual stress–temperature profiles generated at the interface between the FPI/RGO@TVF nanohybrid films and Si wafer substrate were analyzed. The results demonstrated that the residual stress strongly depends on the processing temperature and microstructure and that the stress eventually converges to a zero-stress value (<7 MPa) even at a low RGO@TVF loading of 3 wt%. In addition, the incorporation of RGO@TVF into the FPI resulted in nanohybrid films with a high storage modulus (2126 MPa) and glass transition temperature (330 °C). The results show that the highly exfoliated RGO@TVF provides more contact points between the polyimide chains and RGO@TVF sheets, which increases the polymer-filler compatibility, thereby improving the thermal and dimensional stability.