Large amplitude oscillatory shear flow: Microstructural assessment of polymeric systems

Since the first publications of large amplitude oscillatory shear (LAOS) responses of polymeric materials in the 1960s, different approaches have been developed to express, analyze, and interpret nonlinear viscoelastic behavior of materials. LAOS is currently recognized as one of the most powerful rheological techniques to characterize nonlinear viscoelasticity, as the amplitude and frequency of the employed deformation can be controlled, allowing researchers to tune the strength of flow and time scale independently. Additionally, in contrast to small amplitude oscillatory shear (SAOS) flow, LAOS is not limited to a perturbation about equilibrium. Such flexibility makes LAOS an attractive rheological protocol to mimic numerous industrial applications. Recently, there has been significant progress in the LAOS characterization of polymeric materials via different analysis techniques, including Fourier transform rheology, stress decomposition, Chebyshev coefficients, sequence of physical processes, and intrinsic nonlinearity. These advances have been achieved through utilization of modern commercial rheometers with high torque resolution and strong computational power, making the exploitation of these techniques possible in the recent decade. This increased accessibility has seen the number of LAOS publications dramatically increase in the past few years. The current surge in interest in the use of LAOS tests necessitates the search for appropriate measurement techniques to probe the nonlinear response of complex fluids.  Although this area has been significantly investigated by many researchers who measured and reported LAOS data from a wide range of rheologically-complex materials, there is still one question that needs to be answered: which method of LAOS analysis is best for a specific polymeric system? In fact, the high volume of publications in the past decade provides us a unique opportunity for such a query to validate the efforts on the LAOS developments and compare the utility and sensitivity of these protocols. In this review, we highlight the history and fundamentals of each technique in chronological order. We present parameters and equations of each method that are indispensable for LAOS interpretation. We also present experimental work in LAOS characterization of polymeric materials, classified into four categories: (1) polymer solutions and melts, (2) polymer nanocomposites, (3) polymer blends, and (4) hydrogels. Indeed, the raison d’être of this review paper is that the nonlinear rheological properties of the mentioned polymeric systems are very sensitive to their microstructural features. In each section, the challenges and perspectives will be discussed to enhance understanding of the performance level of each method. This review offers inspiration for the interpretation of LAOS data to help rheologists and newcomers, and to contribute to current and future developments in the field.