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A D V A N C E D M A T E R I A L S & P R O C E S S E S | S E P T E M B E R 2 0 1 8 9 these applications to be explored more fully. In the two-step process, the sci- entists first grow small particle seeds to which they slowly add more of the building blocks under precisely con- trolled conditions. The slow addition causes the building blocks to add to the seeds instead of creating new seeds. The result is bigger, high quali- ty particles made up of large, hexago- nal sheets instead of many aggregated crystals. Particles are also perfectly uni- form throughout the entire structure. The team’s findings show that energy can move through these materials for degree of hydrogen bonding is the key to self-healing and could be exploited to make other self-healing materials. These new polymers provide an advan- tageous platform for building coatings, functional films, membranes, and de- vices with longer lifetimes. ornl.gov. Covalent organic frameworks hold prom- ise for water purification, energy storage, and electronic devices. much longer distances than the siz- es available through older methods. northwestern.edu . STRETCHY POLYMERS HEAL THEMSELVES Researchers at Oak Ridge Na- tional Laboratory, Tenn., developed ultra-stretchy polymers with impres- sive self-healing abilities that could lead to more durable consumer prod- ucts. The new polymers are among the world’s most pliable and can elongate about 1000-5600% before breaking. Af- ter breaking, they can be healed with complete elastic restoration by merely touching adjacent pieces. By tailoring the properties of segments and how they link to the polymer, scientists tuned the tensile strength, toughness, and elastic recovery. In addition, they used stretchy polymeric strips to cre- ate a permeable membrane that selec- tively separated two gases. After that membrane broke, it self-healed to once again separate the gases. Tailoring the Rubbery segments in a ribbon-shaped polymer membrane make it stretchy. Hydrogen-bonding molecules (yellow and green) allow the material to self-heal after a cut and recover its ability to separate gases such as CO 2 (red and black) from nitrogen (blue). Courtesy of Pengfei Cao and Bingrui Li/ORNL.