Direct Observation of Damage Zone around Crack Tips in Double-Network Gels
Direct Observation of Damage Zone around Crack Tips in Double-Network Gels
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DOI：
10.1021/ma900622s
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发表时间：
2009-06-23
期刊：
影响因子：
5.5
通讯作者：
Gong, Jian Ping
中科院分区：
文献类型：
--
作者：
Yu, Qiu Ming;Tanaka, Yoshimi;Gong, Jian Ping
大多数来自天然或合成来源的水凝胶具有低机械强度。2003年，我们通过引入双网络结构（DN）成功开发出具有极高机械强度的新型水凝胶，该结构由紧密交联的聚电解质凝胶和松散交联的中性网络组成。2尽管DN凝胶含有90%左右的水，但其断裂能G在102 ~ 103 J/m2之间，比聚合物浓度与DN凝胶相似的普通聚丙烯酰胺（PAAm）凝胶（10 J/m2）或聚2-丙烯酰胺-2-甲基丙烷酸（PAMPS）凝胶（10-1 J/m2）的断裂能G大100 ~ 1000倍。3-5由于DN凝胶异常高的机械强度和高含水量，它们有望有各种应用，特别是在再生医学领域（用于制造人工组织和软骨等）。6-8 DN凝胶的基本增韧机理是研究人员非常感兴趣的问题。已经进行了一些实验和理论研究来解释这一机制。4,5,9 -14拉伸试验中观察到的颈缩变形和循环加载试验中观察到的与速率无关的迟滞表明，DN凝胶在遭受宏观断裂之前可以积累内部损伤；损伤积累后，DN凝胶变得更加柔软。我们认为，在微观层面上，屈服是由脆性的第一个网络的部分断裂和破碎以及第二个网络的聚合物链在碎片之间的连接造成的。Brown10和Tanaka12提出了类似的模型，可以定性地解释异常高的断裂能，假设DN凝胶在裂纹尖端周围局部损伤，并且损伤积累的能量耗散增强了有效断裂能。根据他们的模型，损伤区域的大小可以达到几百μm量级，而断裂能可以达到几百J/m2量级。通过AFM测量，我们成功地检测到断裂面下方存在软化区域，这支持了局部损伤积累的假设。然而，为了证实模型的假设，必须对损伤区进行直接观察，特别是从裂纹表面估计其厚度。本文首次对DN凝胶的裂纹尖端进行了直接观察，确定了损伤区的存在及其厚度。并将实验结果与模型结果进行了比较。
Most hydrogels derived from natural or synthetic sources have low mechanical strength. 1 In 2003, we succeeded in developing novel hydrogels with extremely high mechanical strength by introducing a double-network structure (DN), consisting of a densely cross-linked polyelectrolyte gel and a loosely cross-linked neutral network. 2 Even though DN gels contain about 90% water, the fracture energy G ranges from 102 to 103 J/m2, which is 100-1000 times larger than that of normal polyacrylamide (PAAm) gels (10 J/m2) or poly (2-acrylamido-2-methylpropanesulfonic acid)(PAMPS) gels (10-1 J/m2) whose polymer concentrations are similar to those of DN gels. 3-5 Because of the anomalously high mechanical strength and high water content of DN gels, they are expected to have various applications, especially in the field of regenerative medicine (for fabricating artificial tissues and cartilage, etc.). 6-8 The fundamental toughening mechanism of DN gels is of great interest to researchers. Several experimental and theoretical studies have been performed to explain this mechanism. 4, 5, 9-14 Necking deformation9 that was observed through tensile tests and rate-independent hysteresis13 observed through cyclic loading tests have indicated that DN gels can accumulate internal damage before the suffering macroscopic fracture; after damage accumulation, the DN gels become much softer. We assume that on the microscopic level yielding is caused by the partial breakage and fragmentation of the brittle first network and interconnection among the fragments by the polymer chains of second network. 9 Brown10 and Tanaka12 have proposed similar models that can qualitatively explain the anomalously high fracture energy, assuming that the DN gel is locally damaged around the crack tip and that the energy dissipated for damage accumulation enhances the effective fracture energy. According to their models, the size of the damage zone could reach the order of several hundred μm, while the fracture energy could reach the order of several hundred J/m2. 9 Using AFM measurements, 15 we successfully detected the existence of softened regions just below the fracture surfaces, which supports the assumption of localized damage accumulation. However, direct observation of the damage zone, especially, the estimation of its thickness from the crack surface, should be performed in order to confirm the assumptions of the models. In this paper, we report the direct observation of the crack tips of DN gels for the first time to ascertain the existence of the damage zone and to determine its thickness. We also compare experimental results with the results from the models.