Characterizing the Viscoelastic Properties of Hydrogel Thin Films by Bulge Test

In this work, we carried out bulge test for quantifying the viscoelastic properties of poly (vinyl alcohol) (PVA) thin films with custom-developed apparatus. A viscoelastic bulge deformation (VBD) model based on the elasticity-viscoelasticity correspondence principle and spherical cap equation is established to describe the bulge deformation of polymeric thin films. The VBD model can be used to determine the time-dependent modulus by bulge test for polymeric films. Uniaxial compressive relaxation test and PRONY series fitting method are used to define the constitutive parameters of the VBD equations. We presented two types of VBD models in frequency domain under linear loading and step loading conditions. Through inverse Laplace transformation, the proposed VBD model can effectively predict the bulge deformation of PVA hydrogel thin film. Numerical simulations are also conducted to validate the VBD model under step loading conditions. This work provides a methodology to characterize the viscoelastic properties of polymeric films by bulge test.