Yang Jun, Shao Changyou, Meng Lei
Beijing Key Laboratory of Lignocellulosic Chemistry , Beijing Forestry University , No. 35, Tsinghua East Road , Haidian District, Beijing 100083 , China.
Langmuir. 2019 Aug 13;35(32):10542-10550. doi: 10.1021/acs.langmuir.9b01532. Epub 2019 Jul 24.
In this work, the composite hydrogel toughening behaviors as manifested by strain rate-dependent viscoelastic properties and enhanced fracture mechanics, that is, suppressed catastrophic crack propagation with increased resistance, are systematically examined by using cellulose nanofibrils (CNFs) as fillers in the polyacrylamide (PAAm) matrix. The uniaxial deformation tests show that the tearing energy increases with crack velocity and becomes dominated by the viscoelastic energy dissipation in front of the crack tip. The creep dynamics of the composite hydrogels under a constant stress is examined, and the results indicate that the incorporation of the CNF pronouncedly suppresses the creep deformation. In addition, the microdeformation and failure mechanisms are analyzed through the observation of morphology of arrested crack tips and the damage zone by transmission electron microscopy and scanning electron microscopy. By aligning the CNF along the crack direction, it is possible to focus on the study of interfacial slip mechanics and identify the role of interfacial slip during the energy dissipation process. The results indicate that the CNFs are largely orientated parallel to the loading direction to maximize the energy dissipation, where the initiation of crack propagation is the primary fracture mechanism in composite hydrogels. The coarse feature on the composite fracture surface implies that the CNF initiates deflection of crack propagation fronts and thus increases the strain energy for continuation of the fracture. It is envisioned that with the incorporation of interdisciplinary strategies, one can rationally combine multiple approaches toward the creation of nanocomposite hydrogels with enhanced mechanical properties.
在这项工作中，通过在聚丙烯酰胺（PAAm）基质中使用纤维素纳米纤丝（CNFs）作为填料，系统地研究了复合水凝胶的增韧行为，该行为表现为应变率依赖性粘弹性特性和增强的断裂力学性能，即随着阻力增加抑制灾难性裂纹扩展。单轴变形试验表明，撕裂能随裂纹速度增加而增加，并由裂纹尖端前方的粘弹性能量耗散主导。研究了复合水凝胶在恒定应力下的蠕变动力学，结果表明，加入CNF可显著抑制蠕变变形。此外，通过透射电子显微镜和扫描电子显微镜观察止裂裂纹尖端和损伤区的形态，分析了微观变形和破坏机制。通过使CNF沿裂纹方向排列，可以专注于界面滑移力学的研究，并确定界面滑移在能量耗散过程中的作用。结果表明，CNF在很大程度上平行于加载方向取向，以最大限度地提高能量耗散，其中裂纹扩展的起始是复合水凝胶中的主要断裂机制。复合断裂表面上的粗糙特征表明，CNF引发裂纹扩展前沿的偏转，从而增加了继续断裂所需的应变能。可以设想，通过结合跨学科策略，可以合理地组合多种方法来制备具有增强力学性能的纳米复合水凝胶。
