Selection of geometry for nonlinear rheology using large amplitude oscillatory shear: Poly (vinyl alcohol) based complex network systems

Abstract

AbstractOscillatory rheology is a common technique to characterize the linear and nonlinear mechanical response of complex materials. The present work studies two crosslinked systems of poly(vinyl alcohol) (PVA): physically crosslinked PVA‐Borax, and glutaraldehyde (GA) based chemically crosslinked PVA‐hyaluronic acid (HA) gel. Both exhibit nonlinear viscoelasticity with characteristic intracycle mechanisms during large amplitude oscillatory shear (LAOS), owing to H‐bonding (physical) and covalent (chemical) interactions respectively, unlike PVA solutions/vinyl polymer melts. Parallel plate (PP) geometry is commonly utilized for rheology of crosslinked systems due to flexible gaps, and ease to retain exact material shapes. Unlike the widely employed cone and plate (CP) geometry for LAOS analysis, the inherent deformation and flow fields are not uniform in PP, which manifests as qualitative difference in the response of the material within the two geometries. A quantitative dissimilarity in the response of PVA based crosslinked systems is demonstrated in this work. Three approaches for PP correction from literature are explored to obtain a geometry independent response of the systems. Therein, the true stresses are computed from the apparent stresses via torque balance, their harmonics, and harmonic ratios based single‐point correction. The first two are shown to have limited application, owing to their implicit assumption of geometry independence in the linear regime. The third approach is found to be effective upto medium strain amplitudes. We outline these shortcomings, and present guidelines to analyze PP LAOS results for the two PVA systems. The analysis can be generalized for the broader class of complex network systems' nonlinear rheology.