Zhu Hongli, Zhu Shuze, Jia Zheng, Parvinian Sepideh, Li Yuanyuan, Vaaland Oeyvind, Hu Liangbing, Li Teng
Department of Materials Science and Engineering, University of Maryland, College Park, MD 20742;
Department of Mechanical Engineering, University of Maryland, College Park, MD 20742.
Proc Natl Acad Sci U S A. 2015 Jul 21;112(29):8971-6. doi: 10.1073/pnas.1502870112. Epub 2015 Jul 6.
The quest for both strength and toughness is perpetual in advanced material design; unfortunately, these two mechanical properties are generally mutually exclusive. So far there exists only limited success of attaining both strength and toughness, which often needs material-specific, complicated, or expensive synthesis processes and thus can hardly be applicable to other materials. A general mechanism to address the conflict between strength and toughness still remains elusive. Here we report a first-of-its-kind study of the dependence of strength and toughness of cellulose nanopaper on the size of the constituent cellulose fibers. Surprisingly, we find that both the strength and toughness of cellulose nanopaper increase simultaneously (40 and 130 times, respectively) as the size of the constituent cellulose fibers decreases (from a mean diameter of 27 μm to 11 nm), revealing an anomalous but highly desirable scaling law of the mechanical properties of cellulose nanopaper: the smaller, the stronger and the tougher. Further fundamental mechanistic studies reveal that reduced intrinsic defect size and facile (re)formation of strong hydrogen bonding among cellulose molecular chains is the underlying key to this new scaling law of mechanical properties. These mechanistic findings are generally applicable to other material building blocks, and therefore open up abundant opportunities to use the fundamental bottom-up strategy to design a new class of functional materials that are both strong and tough.
在先进材料设计中，对强度和韧性的追求是永恒的；不幸的是，这两种机械性能通常是相互排斥的。到目前为止，同时实现强度和韧性的成功案例有限，这通常需要特定材料、复杂或昂贵的合成过程，因此很难应用于其他材料。解决强度和韧性之间冲突的通用机制仍然难以捉摸。在此，我们报告了一项关于纤维素纳米纸的强度和韧性对其组成纤维素纤维尺寸依赖性的开创性研究。令人惊讶的是，我们发现随着组成纤维素纤维尺寸的减小（从平均直径27μm降至11nm），纤维素纳米纸的强度和韧性同时增加（分别提高了40倍和130倍），揭示了纤维素纳米纸机械性能的一种反常但非常理想的标度律：尺寸越小，强度和韧性越高。进一步的基础机理研究表明，固有缺陷尺寸的减小以及纤维素分子链之间强氢键的易于（重新）形成是这种新的机械性能标度律的潜在关键。这些机理发现通常适用于其他材料构建单元，因此为利用自下而上的基础策略设计一类新型的既强又韧的功能材料开辟了大量机会。
