Ministry of Education Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China.
Department of Macromolecular Chemistry I and Bavarian Polymer Institute, University of Bayreuth, Bayreuth 95440, Germany.
ACS Appl Mater Interfaces. 2020 Nov 25;12(47):53376-53384. doi: 10.1021/acsami.0c17198. Epub 2020 Nov 10.
Gradient materials exist widely in natural living organisms, affording fascinating biological and mechanical properties. However, the synthetic gradient hydrogels are usually mechanically weak or only have relatively simple gradient structures. Here, we report on tough nanocomposite hydrogels with designable gradient network structure and mechanical properties by a facile post-photoregulation strategy. Poly(1-vinylimidazole--methacrylic acid) hydrogels containing gold nanorods (AuNRs) are in a glassy state and show typical yielding and forced elastic deformation at room temperature. The gel slightly contracts its volume when the temperature is above the glass-transition temperature that results in a collapse of the chain segments and formation of denser intra- and interchain hydrogen bonds. Consequently, the mechanical properties of the gels are enhanced, when the temperature returns to room temperature. The mechanical performances of hydrogels can also be locally tuned by near-infrared light irradiation due to the photothermal effect of AuNRs. Hydrogels with arbitrary two-dimensional gradients can be facilely developed by site-specific photoirradiation. The treated and untreated regions with different stiffness and yielding stress possess construct behaviors in stretching or twisting deformations. A locally reinforced hydrogel with the kirigami structure becomes notch-insensitive and exhibits improved strength and stretchability because the treated regions ahead the cuts have better resistance to crack advancement. These tough hydrogels with programmable gradient structure and mechanics should find applications as structural elements, biological devices, etc.
梯度材料广泛存在于自然生物中，赋予了它们迷人的生物和机械性能。然而，合成梯度水凝胶通常机械强度较弱，或者只有相对简单的梯度结构。在这里，我们通过一种简单的后光调控策略，报道了具有可设计梯度网络结构和力学性能的坚韧纳米复合水凝胶。含有金纳米棒（AuNRs）的聚（1-乙烯基咪唑-甲基丙烯酸）水凝胶在室温下处于玻璃态，表现出典型的屈服和强制弹性变形。当温度高于玻璃化转变温度时，凝胶会稍微收缩体积，导致链段的坍塌和更密集的分子内和分子间氢键的形成。因此，当温度回到室温时，凝胶的力学性能会增强。由于 AuNRs 的光热效应，水凝胶的机械性能也可以通过近红外光照射进行局部调节。通过特定位置的光照射，可以轻松地开发出具有任意二维梯度的水凝胶。具有不同硬度和屈服应力的处理区和未处理区在拉伸或扭曲变形中具有构建行为。具有折纸结构的局部增强水凝胶由于处理区在切口前具有更好的抗裂纹扩展能力，因此变得对缺口不敏感，并表现出更好的强度和拉伸性。这些具有可编程梯度结构和力学性能的坚韧水凝胶有望作为结构元件、生物器件等得到应用。
