Dual-cross-link hydrogels, which are composed of poly(vinyl alcohol) chains cross-linked by covalent bonds and transient physical bonds via borate ions, have received considerable interest due to the high extensibility as well as the self-healing properties. The dual-cross-link gels exhibit pronounced stress relaxation after the imposition of step strain, reflecting the breaking dynamics of temporary chains which sustain the initial stress. Multiaxial stress relaxation experiments using equibiaxial, planar, and uniaxial stretching with various degrees of strain unambiguously validate the time-strain separability in relaxation stress for a general type of deformation. The time effect is satisfactorily described by an existing model, while the strain effect is well approximated by the neo-Hookean model. Furthermore, the magnitude of total stress relaxation to the initial stress is constant, independently of the type and degree of imposed strain. These simple features provide an important basis for formulating the constitutive models of the dual-cross-link gels and for molecular design of hydrogels with transient physical cross-links.

中文翻译：

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双交联聚乙烯醇水凝胶的多轴应力松弛

双交联水凝胶由通过共价键和通过硼酸离子的瞬时物理键交联的聚乙烯醇链组成，由于其高延展性和自修复特性而受到广泛关注。双交联凝胶在施加阶跃应变后表现出明显的应力松弛，反映了维持初始应力的临时链的断裂动力学。使用等双轴、平面和单轴拉伸的多轴应力松弛实验明确地验证了一般变形类型的松弛应力中的时间应变可分离性。现有模型可以令人满意地描述时间效应，而新胡克模型可以很好地近似应变效应。此外，总应力松弛到初始应力的大小是恒定的，与施加应变的类型和程度无关。这些简单的特征为制定双交联凝胶的本构模型和具有瞬时物理交联的水凝胶的分子设计提供了重要基础。