Wrinkle Formation in Multilayer Polymer-Based Composite Materials

In this study, we present a nano-scaled wrinkle formation on a multilayered polymer-based material coated with hard skin with heterogenous constitutive relation. This composite material consists of two layers: a thick hydrogel layer (600 µm) as the substrate and a thin hard skin of polytetrafluoroethylene (50 nm) on top. The formation of wrinkles initiated by application of sufficient external strain to the system followed by strain release leading to the generation of internal stress. Wrinkling is the response of the system to strain relief and minimizes the free energy of the system. Wrinkles characterization is of utmost importance in autoregulatory composite materials as the roughness of the surface is a sensor for humidity variation of the hydrogel substrate. Time-dependent finite element analysis is performed to characterize wrinkle formation. Different viscoelastic models (Generalize Maxwell (GM), Generalize Kelvin-Voigt (GKV) and Standard Linear Solid (SLS)) are used to explore the characteristics of the wrinkles (i.e, wavelength and amplitude). The effect of hydrogel’s relaxation time and strain rate on the characteristics of wrinkles is investigated. The increase in relaxation time leads to decrease in wavelength and amplitude for GM and SLS models. While for GKV model the same wavelength pattern is observed, amplitude did not show huge dependence on the relaxation time. Strain rate did not play major role in the characteristics of the wavelengths and amplitude in GM and SLS models both for high (10 sec) and low relaxation times (2000 sec). However, in GKV of low relaxation times (10 sec), the wavelength and amplitude increased in high strain rate.