Adam Feinberg had no sooner made a bright yellow thin sheet of plastic when he shred it. Then, he filled a mold with the plastic bits and stuck it in a hot oven.
“I opened up the mold and there was this beautiful yellow ‘I,’ ” he recalled of the shape he had chosen in honor of the University of Illinois at Urbana-Champaign, where he is a chemist. His new plastic passed the first test — it was moldable with heat like regular plastic. But there was another important step left in rethinking the world of durable plastics.
Feinberg placed the “I” under a white light, and five minutes later, only half of it remained. The plastic did not simply melt. Its building blocks, the synthetic polymers within, had reverted to their molecular units. “It was a phenomenal feeling,” he said.
Most synthetic polymers were not designed to disintegrate. They were meant to last as long as possible once they began replacing metals and glass in long-lasting things like automobiles and airplanes.
But synthetic polymers became so popular and adaptable that decades later, they are at the root of the global burden of billions of tons of plastic waste. The latest villains in environmental campaigns are disposable plastic products formed from synthetic polymers — straws, cigarette filters, coffee cup lids, etc.
The environmental effects of plastic buildup and the declining popularity of plastics have helped to spur chemists on a quest to make new materials with two conflicting requirements: They must be durable, but degradable on command. In short, scientists are in search of polymers or plastics with a built-in self-destruct mechanism. “It’s two diametrically opposed criteria that we’re trying to juggle,” Feinberg said. It is easier to mold a robust plastic without destroying it, he said, but at the same time, it should not last forever.
“The real trick is to make them stable when you’re using them, and unstable when you don’t want to use them,” said Marc Hillmyer, who leads the Center for Sustainable Polymers at the University of Minnesota.
While not a silver bullet for plastic waste, self-destructing plastics could also enable new applications in drug delivery, self-healing materials and even some electronics.
The starting point requires picking polymers that are inherently unstable. Given a choice, their units would rather stay as small molecules. Then scientists force those molecules to link up into long chains, and then trap the resulting polymers. Dismantling them is sometimes called unzipping them, because once the polymers encounter a trigger that removes those traps, their units fall off until the polymers have completely switched back to small molecules. In theory, these next-generation polymers would lead to chemical recycling.
Meanwhile, commercial plastics are being pumped out on the scale of 400 million metric tons a year. And those plastics were intended to last as long as possible, said Jeannette Garcia, an IBM polymer chemist.
“Designing new polymers is going to be absolutely important and absolutely necessary,” Garcia said. But a bigger problem, she said, is learning how to break down legacy polymers of plastic waste in a similar way. “It’s almost like a holy grail challenge.”