Linear and nonlinear viscoelastic properties of hydrogels significantly contribute to functionality, long-term performance, and stability of the hydrogels. With respect to the nonlinear viscoelastic response of chemically crosslinked hydrogels, the vast majority of publications have reported the type III response (weak strain overshoot). Herein, to measure the true mechanical response of hydrogels subjected to large shear deformations, we developed a technique by chemically bonding and sandwiching two surfaces of a hydrogel to treated glass slides attached to the oscillating rheometer's metal plates. Employing this method, for the first time, we were able to completely alleviate errors attributed to the wall slip in the rheological measurements of soft materials, enabling the accurate evaluation of nonlinear behavior of hydrogels. The results show that these hydrogels follow a type II (strain hardening) response. It is argued that the observed type III response of hydrogels, widely reported in the literature, originates from the wall-slip condition, rather than the inherent viscoelasticity of the hydrogels. This insight has important implications for the future development of hydrogel-based or other soft materials.

中文翻译：

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大剪切变形下共价交联水凝胶的粘弹性行为：消除壁滑的一种方法

水凝胶的线性和非线性粘弹性质极大地影响了水凝胶的功能性，长期性能和稳定性。关于化学交联水凝胶的非线性粘弹性响应，绝大多数出版物都报告了III型响应（弱应变超调）。本文中，为了测量经受大剪切变形的水凝胶的真实机械响应，我们开发了一种技术，该技术通过将水凝胶的两个表面化学键合并夹在与流变仪的金属板相连的已处理玻璃载玻片上。首次使用这种方法，我们能够完全缓解软材料流变学测量中壁滑引起的误差，从而能够准确评估水凝胶的非线性行为。结果表明，这些水凝胶遵循II型（应变硬化）响应。据认为，在文献中广泛报道的观察到的水凝胶的III型反应起因于壁滑条件，而不是水凝胶固有的粘弹性。这种见解对水凝胶基或其他软质材料的未来发展具有重要意义。