State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, China.
University of Chinese Academy of Sciences, Beijing, 100049, China.
Small. 2016 Aug;12(32):4386-92. doi: 10.1002/smll.201601893. Epub 2016 Jul 4.
The poor mechanical strength of hydrogels has largely limited their wide applications, and improving hydrogels' mechanical strength is a hot and important topic in the hydrogel research field. Although many successful strategies have been proposed to improve hydrogels' mechanical strength during the past decades, a hydrogel with a tensile stress surpassing dozens of mega Pascal is desirable, yet still a big challenge. To address this issue, the Fe(3+) -mediated physical crosslinking formed under stretch conditions was employed in a chemically crosslinked poly (acrylamide-co-acrylic acid) network to achieve a dual-crosslinked hydrogel. The expected molecular orientation occurs under stretch and allows the maximumu chelating interaction between pendant carboxylic anions and Fe(3+) and molecules conformation being frozen, leading to the mechanical strength improving dramatically. As a result, an unprecedentedly high mechanical strength, but anisotropic dual-crosslinked hydrogel was obtained. By optimizing the experimental parameters, the nominal tensile stress along pre-stretching direction can reach as high as ≈40 MPa with elastic modulus of ≈40 MPa at large strain (>200%). In addition, the molecular orientation also leads to big difference of mechanical performance between parallel and perpendicular direction.
水凝胶的机械强度较差在很大程度上限制了其广泛应用，提高水凝胶的机械强度是水凝胶研究领域的一个热点和重要课题。尽管在过去几十年中已经提出了许多成功的策略来提高水凝胶的机械强度，但人们还是希望得到一种拉伸应力超过数十兆帕的水凝胶，这仍然是一个巨大的挑战。为了解决这个问题，在化学交联的聚丙烯酰胺-co-丙烯酸钠网络中采用了在拉伸条件下形成的 Fe(3+)介导的物理交联，以实现双交联水凝胶。拉伸下预期的分子取向允许侧挂羧酸根阴离子和 Fe(3+)之间的最大螯合相互作用，并且分子构象被冻结，从而使机械强度显著提高。结果，得到了一种具有前所未有的高机械强度但各向异性的双交联水凝胶。通过优化实验参数，可以在沿预拉伸方向获得高达约 40 MPa 的名义拉伸应力，在大应变（>200%）时弹性模量约为 40 MPa。此外，分子取向也导致了机械性能在平行和垂直方向上的巨大差异。
