Mechanics Underpinning Phase Separation of Hydrogels

This paper reveals the underpinning role of mechanical constraints and dynamic loading in triggering volume phase transitions and phase separation of hydrogels. Using the Flory–Rehner free energy that does not predict phase separation of hydrogels under equilibrium free swelling, we show that mechanical constraints can lead to coexistence of multiple phases. We systematically obtain the states of equilibrium for hydrogels under various mechanical constraints and unravel how mechanical constraints change the convexity of the free energy and monotonicity of the stress–stretch curves, leading to phase coexistence. Using a phase-field model, we predict the pattern evolution of phase coexistence and show that many features cannot be captured by the homogeneous states of equilibrium due to large mismatch stretch between the coexisting phases. We further reveal that the system size, quenching rate, and loading rate can significantly influence the phase behavior, which provides insights for experimental studies related to morphological patterns of hydrogels.本论文揭示了力学约束与动态加载在触发水凝胶体积相变与相分离过程中的基础性支撑作用。基于平衡自由溶胀条件下无法预测水凝胶相分离的弗洛里-雷纳（Flory–Rehner）自由能，我们证明力学约束可引发多相共存现象。我们系统地获取了不同力学约束下水凝胶的平衡态，并阐明了力学约束如何改变自由能的凸性与应力-拉伸曲线的单调性，进而引发相共存。借助相场（phase-field）模型，我们预测了相共存的图案演化过程，并指出由于共存相之间存在显著的拉伸失配，诸多特征无法通过均匀平衡态予以捕捉。我们进一步发现，体系尺寸、淬火速率与加载速率会对相行为产生显著影响，该发现可为与水凝胶形貌图案相关的实验研究提供理论参考。