임무 끝나면 '사르르'..녹는 군용 소재 나온다 VIDEO: Disappearing act: Device vanishes on command after military missions

Disappearing act: Device vanishes on command after military missions (video)

AMERICAN CHEMICAL SOCIETY

SAN DIEGO, Aug. 26, 2019 -- A polymer that self-destructs? While once a fictional idea, new polymers now exist that are rugged enough to ferry packages or sensors into hostile territory and vaporize immediately upon a military mission's completion. The material has been made into a rigid-winged glider and a nylon-like parachute fabric for airborne delivery across distances of a hundred miles or more. It could also be used someday in building materials or environmental sensors.

(폴 콜 미국 조지아공대 화학 및 분자생물공학부 교수 연구팀은 햇빛을 받으면 원하는 시간 내로 녹아내리는 폴리머를 개발했다. 연구팀이 개발한 폴리머(왼쪽)는 오전 6시의 아침 햇살을 받으며 30분 만에(오른쪽) 상당 부분이 녹아내렸다. 미국화학회 유튜브 캡처과학)

임무 끝나면 '사르르'..녹는 군용 소재 나온다      전쟁 도중 적진에 떨어진 아군의 무인 비행체는 아군에게 위협적인 정보를 적에게 제공하게 될 수도 있다. 하지만 무인 비행체가 떨어진 지 한 시간 만에 녹아 내린다면 이러한 위험은 사라질 것이다. 미국 연구팀이 미 국방부의 지원을 받아 햇빛을 받으면 원하는 시간 내로 녹아내리는 소재를 개발했다. 폴 콜 미국 조지아공대 화학 및 분자생물공학부 교수 연구팀은 햇빛에 노출되면 녹아내리는 폴리머를 개발했다는 연구결과를 이달 26일 미국 샌디에이고에서 열리는 ‘미국화학회(ACS) 연례 학술대회’에서 발표했다. 연구팀이 개발한 폴리머는 무인 비행체용 소재로 활용할 만큼 단단하고 낙하산용 나일론 직물처럼 활용할 수도 있는 것으로 알려졌다. 플라스틱과 같은 고분자 중합체를 뜻하는 폴리머가 녹아 없어지려면 ‘천장 온도’를 넘어서야 한다. 폴리스티렌을 비롯한 대부분의 플라스틱은 이 온도가 상온보다 훨씬 높아 안정적이다. 폴리스티렌은 천장 온도가 섭씨 310도다. 이 온도 이상으로 올라가도 고분자가 아예 사라지는 건 시간이 오래 걸린다. 고분자는 단량체가 연결되며 고분자를 만드는데 이것이 일일이 끊어져야 분해가 돼 사라지기 때문이다. 연구팀은 빛이라는 ‘버튼’ 하나로 녹아 없어지는 폴리머를 개발했다. 연구팀은 천장 온도를 상온 이하로 낮춘 폴리머에 태양빛을 받으면 촉매로 작용해 폴리머가 빠르게 분해되도록 돕는 감광제를 첨가했다. 감광제는 빛의 파장에 따라 반응하도록 개발해 가시광선 혹은 자외선을 받으면 분해되도록 했다. 콜 교수는 “자외선에만 반응하도록 만들면 실내에서는 녹지 않지만 햇빛에 노출되면 사라지게 된다”고 설명했다. 연구팀은 1시간이나 2시간처럼 특정 시간에 맞춰 사라질 수 있도록 폴리머의 성분을 조절하는 데도 성공했다. 이 기술은 미국 국방부의 지원을 받아 개발됐지만 군사적 용도가 아니라도 잠재력이 있다고 연구팀은 보고 있다. 연구팀은 이 소재를 활용해 건축 자재를 임시로 붙였다가 빛을 쪼이면 뗄 수 있는 접착제를 개발했다. 콜 교수는 “제임스 본드 같은 물질이 이미 군사 장치에 통합되고 있다”며 “환경 모니터링을 위한 센서로 쓰면 데이터 수집을 마친 후 센서가 증발하도록 만들 수도 있을 것”이라고 말했다. [조승한 기자 shinjsh@donga.com] 동아사이언스

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The researchers will present their results today at the American Chemical Society (ACS) Fall 2019 National Meeting & Exposition. ACS, the world's largest scientific society, is holding the meeting here through Thursday. It features more than 9,500 presentations on a wide range of science topics.

A brand-new video on the research is available at http://www.acs.org/HLS_Disappearing_Glider.

"This is not the kind of thing that slowly degrades over a year, like the biodegradable plastics that consumers might be familiar with," says Paul Kohl, Ph.D., whose team developed the material. "This polymer disappears in an instant when you push a button to trigger an internal mechanism or the sun hits it." The disappearing polymers were developed for the Department of Defense, which is interested in deploying electronic sensors and delivery vehicles that leave no trace of their existence after use, thus avoiding discovery and alleviating the need for device recovery.

The key to making a polymer disappear, or break apart, is "ceiling temperature." Below the ceiling temperature, a polymer configuration is favored, but above that temperature, the polymer will break apart into its component monomers. Common polymers, like polystyrene, have a ceiling temperature above ambient temperature and are very stable. And even when they are warmed above their ceiling temperature, some of these materials can take a long time to decompose. For example, thousands of chemical bonds link all of the monomers together in polystyrene, and all of these bonds must be broken for the materials to decompose. But with low ceiling-temperature polymers, such as the cyclic ones Kohl is using, only one bond needs to break, and then all of the other bonds come apart, so the depolymerization happens quickly. The process can be initiated by a temperature spike from an outside or embedded source, or by a light-sensitive catalyst.

For many years, researchers have attempted to make these polymers, but were unsuccessful because of the materials' instability at room temperature. Kohl's research group at the Georgia Institute of Technology discovered that they could overcome this issue if they were careful to remove all impurities formed during the synthesis. In addition, they found a number of aldehydes, including phthalaldehyde, that readily form cyclic polymers. Once they had optimized this polymer's synthesis, they focused on ways to make it disappear.

To do this, the researchers incorporated into the polymer a photosensitive additive, which absorbs light and catalyzes depolymerization. "Initially, we made it photosensitive to just ultraviolet light so we could make the parts in a well-lit room with fluorescent lighting, and it was just fine; it was stable," Kohl says. But when the polymer was placed outside, exposure to sunlight vaporized it (or reverted it back to a liquid, in some cases). A vehicle deployed at night would, therefore, disappear with the sunrise.

IMAGE: A POLYMER (LEFT) DEPOLYMERIZES AND DISAPPEARS AFTER BEING EXPOSED TO SUNLIGHT FOR 10 MIN (RIGHT).

Kohl's group has since discovered new additives that can trigger depolymerization at different wavelengths of visible light, so the polymer can decompose indoors. "We have polymers designed for applications in which you come in the room, you turn the light on, and the thing disappears," Kohl says.

The group has also determined how to stall depolymerization. "We have a way to delay the depolymerization for a specific amount of time - one hour, two hours, three hours," he says. "You would keep it in the dark until you were going to use it, but then you would deploy it during the day, and you would have three hours before it decomposes." The team has considered chemical methods to start the decomposition process, as well. In addition, they are testing various copolymers that can be added to phthalaldehyde to change the material's properties without altering its ability to vanish.

Kohl says that this "James Bond"-like material is already being incorporated in military devices by other researchers. But he also sees the potential of the materials for non-military applications. For example, the researchers have made a disappearing epoxy for a temporary adhesive that could be used in building materials. They also imagine the material could be used as sensors for environmental monitoring. Once the sensors are finished collecting data, there is no risk of littering the environment since they can be triggered to vaporize. The material can also be used for delivery vehicles in remote areas where recovery is difficult.

https://www.eurekalert.org/pub_releases/2019-08/acs-dad072319.php

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