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What makes some plastics stick to metal without any glue? Osaka Metropolitan University scientists have peered into the invisible adhesive zone that forms between certain plastics and metals—one atom at a time—to uncover how chemistry and molecular structure determine whether such bonds bend or break.

Their insights clarify metal–plastic bonding mechanisms and offer guidelines for designing durable, lightweight, and more sustainable hybrid materials for use in transportation.

Combining the strength of metal with the lightness and flexibility of plastic, polymer–metal hybrid structures are emerging as key elements for building lighter, more fuel-efficient vehicles. The technology relies on bonding metals with plastics directly, without adhesives. The success of these hybrids, however, hinges on how well the two materials stick together.

"The molecular-level mechanisms that determine how strongly these materials bond at the interface have remained unclear," said Takuya Kuwahara, lecturer at Osaka Metropolitan University's Graduate School of Engineering and lead author of this study published in Communications Materials.

Zeroing in on the bond, the team used all-atom molecular dynamics simulations to investigate how polyamides (PAs), in this case nylon, adhere to alumina surfaces. The researchers tested two types of PAs, which differ in rigidity: PA6, which has a flexible aliphatic backbone; and PAMXD6, which contains rigid aromatic rings.

They studied them on both hydroxylated (OH-terminated) and non-hydroxylated (non-terminated) alumina surfaces. "Terminated" here refers to how the outermost layer of a material ends: in this case with a functional OH-group or no functional group.

To track molecular behavior at the interface, the researchers first categorized polymer chain segments.

"Surface-adsorbed segments were classified as 'trains,' non-adsorbed segments existing between two trains as 'loops,' and non-adsorbed end segments connected to the PA interior as 'tails,'" Kuwahara explained.

The polymer–alumina interface underwent yielding when subjected to tensile strain. In this context, "yielding" refers to the onset of irreversible atomic rearrangements, where the interface is permanently deformed and atoms can no longer return to their original positions even after the stress is taken away. The researchers analyzed the mechanical response of the polymer–metal interface before and after yielding to evaluate the strength, durability, and reliability of materials at the point where they meet, revealing the strength of the bond.

The simulation results showed that adhesion strength depends on both polymer chemistry and surface termination.

"In the elastic regime, or before the interface yields, the tensile stress is determined by PA chemistry," Kuwahara said. "After yielding, however, the alumina surface termination becomes critical."

Before yielding, the aromatic PAMXD6 is stiffer and resists stretching better than PA6. After yielding, the behavior changes depending on the surface: on hydroxylated surfaces, PAMXD6 detaches, or desorbs, whereas PA6 reorganizes, transforming loops into stretched tails without fully detaching. On non-hydroxylated surfaces, both polymers remain firmly attached through trains and loops.

The findings not only clarify why some metal–plastic pairs bond better than others, but also offer practical design guidelines for selecting surface treatments and polymer types. These insights facilitate theoretical, mechanism-based materials design, reducing reliance on trial-and-error experimentation.

"By understanding how molecular structure and surface chemistry interact, we can design stronger, lighter joints that help reduce vehicle weight and energy use," Kuwahara said. "Ultimately, this work moves us closer to achieving sustainable, carbon-neutral transportation."