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·搜一搜.cn/自修复材料 A highly stretchable autonomous self-healing elastomer

A new, extremely stretchable polymer film created by Stanford researchers can repair itself when punctured, a feature that is important in a material that has potential applications in artificial muscle. (Image credit: Bao Research Group)

《Nature Chemistry》报道南京大学在自修复材料方面的合作研究成果

发布时间：[2016-04-19] 作者：[南京大学科学技术处] 来源：[新闻中心]

我校化学化工学院、配位化学国家重点实验室、人工微结构科学与技术协同创新中心李承辉副教授在高弹性自修复材料方面取得重要进展，相关成果以“A highly stretchable autonomous self-healing elastomer”为题，发表在国际著名期刊《Nature Chemistry》上（2016年4月18日在线发表，DOI: 10.1038/NCHEM.2492）。该项研究工作由多方努力合作完成，李承辉副教授为论文第一作者Cheng-Hui Li,，通讯作者是我校校友、斯坦福大学化学工程系鲍哲南教授Zhenan Bao；我校化学化工学院左景林教授Jing-Lin Zuo、郑鹏教授Peng Zheng,、游效曾院士Xiao-Zeng You，物理学院曹毅教授Yi Cao、孙阳同学Yang Sun等也参与了该项研究，其他合作者有：Chao Wang, Christoph Keplinger, , Lihua Jin, , Franziska Lissel, Christian Linder。

自修复材料是近十几年来兴起的一种新型智能材料。它可以实现材料对自身裂纹的检测并自发完成对材料的修复，从而可以预防材料由于产生裂纹而存在的潜在破坏，在航天、军工、桥梁、建筑以及工程等领域中有着重要应用。传统材料为了追求高强度及柔韧性，通常都是由不可逆的共价键构筑而成的。这种共价键在断裂之后无法再重建，因此传统材料受损后很难修复。目前，用于自修复材料的可逆化学键主要有两类。一类是可逆动态共价键，例如共轭双烯与烯烃或炔烃之间可逆的Diels-Alder反应，双硫键可逆的氧化还原反应等。另一类是非共价键相互作用，例如氢键、静电吸引、疏水作用、π-π堆积等分子间弱相互作用。这两种途径各有优缺点。

李承辉副教授与斯坦福大学鲍哲南教授合作，利用配位键设计合成了一种高弹性的自修复材料。在超分子化学中，配位作用通常被认为是最强的超分子间相互作用之一。通过选择合适的配体和金属，可以得到中等强度的配位键，其强度小于共价键但比非共价弱相互作用要强得多。这种配位键将具有物理（热）可逆性或者化学可逆性，因而能够用于构筑自修复材料。同时，由于配位键强度适中，将能够克服可逆共价键和非共价弱相互作用的缺点。他们在聚甲基硅氧烷高分子中引入了一种名为2,6-吡啶二甲酰胺的配体，该配体可提供多个配位点与金属配位，得到多个不同强度的配位键。强配位键与弱配位键位置相邻，受到拉伸作用时，弱配位键断开使能量得到耗散，而强配位键仍然得以保持使材料不致断裂，因此材料具有非常好的拉伸性，最高可拉伸至原长度的45倍，改变金属与配体的比例后甚至拉伸到100倍也不会断裂。另一方面，由于强配位键与弱配位键的结合导致配位结构具有高度动态性，受到破坏后能够快速自发形成，因此材料受损后在室温下无需任何外界刺激即可完全修复，甚至低温条件下（-20 °C）也能自发修复。

值得一提的是，该材料还可用于制造人工肌肉。科学家们一般尝试通过气动驱动器、形状记忆合金等多种方法制造人工肌肉后，近年来人们开始关注一种基于电活性聚合物的人工肌肉。电活性聚合物是指能够在电流或电压作用下产生物理形变的聚合物，因而能将电能转化为机械能。基于电活性聚合物的人工肌肉具有应变高、柔软性好、质轻、无噪声等特点，被公认为最具发展潜力。利用本研究中的自修复材料所制备出的人工肌肉器件，能够随着电压变化不断膨胀与收缩，因此其动作可以通过外部电压来控制。而且，该人工肌肉受损后可自动修复，修复后的器件仍然可承受与修复前同样高的电压。该研究为人工肌肉走向智能化又迈出了重要的一步。

（化学化工学院 科学技术处）

http://news.nju.edu.cn/show_article_1_41774

Stanford researchers' stretchy material has muscular future

Stanford University News-2016年4月18日

A new, extremely stretchable polymer film created by Stanford ... to the study, “A highly stretchable autonomous self-healing elastomer,” include ...

http://www.nature.com/nchem/journal/vaop/ncurrent/fig_tab/nchem.2492_F4.html

Stanford researchers create super stretchy, self-healing material that could lead to artificial muscleAPRIL 18, 2016
Stanford researchers create super stretchy, self-healing material that could lead to artificial muscle
Researchers show how jolting this material with an electrical field causes it to twitch or pulse in a muscle-like fashion. This polymer can also stretch to 100 times its original length, and even repair itself if punctured.
BY CARRIE KIRBY AND TOM ABATE

If there’s such a thing as an experiment that goes too well, a recent effort in the lab of Stanford chemical engineering Professor Zhenan Bao might fit the bill.

One of her team members, Cheng-Hui Li,  wanted to test the stretchiness of a rubberlike type of plastic known as an elastomer that he had just synthesized. Such materials can normally be stretched two or three times their original length and spring back to original size. One common stress test involves stretching an elastomer beyond this point until it snaps.

polymer film being punctured by a pointed item
A new, extremely stretchable polymer film created by Stanford researchers can repair itself when punctured, a feature that is important in a material that has potential applications in artificial muscle. (Image credit: Bao Research Group)

But Li, a visiting scholar from China, hit a snag: The clamping machine typically used to measure elasticity could only stretch about 45 inches. To find the breaking point of their one-inch sample, Li and another lab member had to hold opposing ends in their hands, standing further and further apart, eventually stretching a 1-inch polymer film to more than 100 inches.

Bao was stunned.

“I said, ‘How can that be possible? Are you sure?'” she recalled.

Today in Nature Chemistry, the researchers explain how they made this super-stretchy substance. They also showed that they could make this new elastomer twitch by exposing it to an electric field, causing it to expand and contract, making it potentially useful as an artificial muscle.

A flexible fishnet

Artificial muscles currently have applications in some consumer technology and robotics, but they have shortcomings compared to a real bicep, Bao said. Small holes or defects in the materials currently used to make artificial muscle can rob them of their resilience. Nor are they able to self-repair if punctured or scratched.

But this new material, in addition to being extraordinarily stretchy, has remarkable self-healing characteristics. Damaged polymers typically require a solvent or heat treatment to restore their properties, but the new material showed a remarkable ability to heal itself at room temperature, even if the damaged pieces are aged for days. Indeed, researchers found that it could self-repair at temperatures as low as negative 4 degrees Fahrenheit (-20 C), or about as cold as a commercial walk-in freezer.

The team attributes the extreme stretching and self-healing ability of their new material to some critical improvements to a type of chemical bonding process known as crosslinking. This process, which involves connecting linear chains of linked molecules in a sort of fishnet pattern, has previously yielded a tenfold stretch in polymers.

First they designed special organic molecules to attach to the short polymer strands in their crosslink to create a series of structure called ligands. These ligands joined together to form longer polymer chains – spring-like coils with inherent stretchiness.

Then they added to the material metal ions, which have a chemical affinity for the ligands. When this combined material is strained, the knots loosen and allow the ligands to separate. But when relaxed, the affinity between the metal ions and the ligands pulls the fishnet taut. The result is a strong, stretchable and self-repairing elastomer.

“Basically the polymers become linked together like a big net through the metal ions and the ligands,” Bao explained. “Each metal ion binds to at least two ligands, so if one ligand breaks away on one side, the metal ion may still be connected to a ligand on the other side. And when the stress is released, the ion can readily reconnect with another ligand if it is close enough.”

Advancing artificial muscle and skin

The team found that they could tune the polymer to be stretchier or heal faster by varying the amount or type of metal ion included. The version that exceeded the measuring machine’s limits, for example, was created by decreasing the ratio of iron atoms to the polymers and organic molecules in the material.

The researchers also showed that this new polymer with the metal additives would twitch in response to an electric field. They have to do more work to increase the degree to which the material expands and contracts and control it more precisely. But this observation opens the door to promising applications. (View video.)

In addition to its long-term potential for use as artificial muscle, this research dovetails with Bao’s efforts to create artificial skin that might be used to restore some sensory capabilities to people with prosthetic limbs. In previous studies her team has created flexible but fragile polymers, studded with pressure sensors to detect the difference between a handshake and a butterfly landing. This new, durable material could form part of the physical structure of a fully developed artificial skin.

“Artificial skin is not just made of one material,” said Franziska Lissel, a postdoctoral fellow in Bao’s lab and member of the research team. “We want to create a very complex system.”

Even before artificial muscle and artificial skin become practical, this work in the development of strong, flexible, electronically active polymers could spawn a new generation of wearable electronics, or medical implants that would last a long time without being repaired or replaced.

This latest discovery is the result of two years of collaboration, overseen by Bao, involving visiting scholar Cheng-Hui Li, a Chinese organo-metallic chemist who designed the metal ligand bonding scheme; polymer chemist Chao Wang, now an assistant professor of chemistry at the University of California, Riverside, who had made previous iterations of self-healing elastomers; and artificial muscle expert Christoph Keplinger, now an assistant professor of mechanical engineering at the University of Colorado, Boulder. Other contributors to the study, “A highly stretchable autonomous self-healing elastomer,” include Jing-Lin Zuo, Lihua Jin, Yang Sun, Peng Zheng, Yi Cao, Christian Linder and Xiao-Zeng You.

The work at Stanford was supported by the Air Force Office of Scientific Research, Samsung Electronics and the Major State Basic Research Development Program of China.

Media Contacts
Tom Abate, Stanford Engineering: tabate@stanford.edu, (650) 736-2245