This bizarre substance breaks the rules of both glass and plastic

Scientists thought glassy substances had to be either moldable or impact-resistant—but compleximers are both

By Rohini Subrahmanyam edited by Sarah Lewin Frasier

“Compleximers”—materials that can be molded like window glass but that resist impacts like plastic does—shouldn’t exist, researchers say. Nevertheless, a few grams of one such substance sit in a laboratory at Wageningen University in the Netherlands.

In Nature Communications, Wageningen physical chemist Jasper van der Gucht and his team describe what makes compleximers as meltable as glass yet as hard to break as plastic. Someday this paradoxical stuff could make it easier to fashion and fix sturdy protective gear such as helmets.

Window glass, called silica, and most plastics are “glassy” materials—when they cool from their liquid states, they don’t solidify into crystals with neatly arranged atoms like water does when it freezes into ice. Instead they form an amorphous mass that feels like a solid but has randomly arranged atoms like a liquid.

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For decades scientists have thought, based on experimental observations, that the lower a glassy material’s melting rate, the less impact it can bear. Both slow-melting window glass and faster-melting plastic abide by this rule: the former changes state slowly but shatters easily, whereas the latter solidifies and melts abruptly but can better withstand impact. But van der Gucht and his team found that compleximers completely defy this law. The trick could lie in the material’s structure: its long chains of molecules, called polymers, are held together by a far-reaching kind of bond.

The researchers initially created compleximers, a term they coined, as an easily recyclable alternative to a type of plastic called thermoset. Thermosets are made up of polymer chains held tightly together by extremely hard-to-break chemical bonds, which makes them very stable but hard to recycle. The researchers added charged molecules, which made the chains cling to one another using an ionic, “opposites attract” type of bond instead, and they incorporated water-repelling compounds to stop the chains from disintegrating in water. The charged molecules’ ionic interactions—which hold over longer distances than the previous chemical bonds—may help compleximers stay compact rather than rapidly expanding to melt immediately when heated, the team suggests.

Ionic interactions could improve the mechanical properties of glass-forming materials and make them easier to work with, says University of Chicago chemical engineer Matthew Tirrell, who was not involved in the work.

The slow melting also means compleximer-based objects are easier to fix than ones made of thermosets; “just by heating it with a heat gun, you can repair a scratch or a crack,” van der Gucht says.

Both researchers say this rule-defying material could also give physicists a better understanding of how glass forms, a phenomenon called the glass transition. Finding these long-range interactions that make glassy materials melt differently, van der Gucht says, “should help theorists explain the glass transition in a more general sense.”