Nonequilibrium interfacial behavior dictates adhesive performance

A research group, including Graduate Student Shuji Morita, Assistant Professor Yuma Morimitsu, and Distinguished Professor Keiji Tanaka from the Faculty of Engineering, Professor Satoru Yamamoto from the Center for Polymer Interface and Molecular Adhesion Science at Kyushu University, Dr. Shiho Tanizaki, Assistant Professor Tomohiro Kubo, and Professor Kotaro Satoh from the School of Materials and Chemical Technology at the Institute of Science Tokyo, has used Atomic Force Microscopy (AFM) to directly observe the movement of polymer chains on solid surfaces. The group revealed for the first time that a single interfacial molecular chain contains both thermally activated and thermally suppressed segments (segments whose motion is inhibited by transient adsorption). These segments exhibit nonequilibrium behavior that depends on their interaction with the solid surface. This development allows for an understanding of molecular dynamics that, until now, had only been discussed in terms of "average images."

Tanaka said, "By understanding these principles and rules, we can aid the development of adhesives that bond strongly yet can be easily peeled off. We expect various applications in industrial fields." The results were published in JACS.

Reducing the weight of various modes of mobility, such as cars and aircraft, has the potential to resolve global environmental issues, including the reduction of CO₂ emissions. Lightweighting requires high-performance bonding technology that enables the combination of dissimilar but appropriate materials in the right places. To achieve this, the micro-scale behavior of the interface between the adhesive and the material must be understood.

Tanaka and his colleagues have previously shown that the behavior of polymer chains on the surface of a substrate greatly influences adhesive performance. Although the layer of polymer chains interacting with the bonding interface is extremely thin, only a few nanometers, it was known that their structure and thermal mobility strongly affect adhesive strength and peeling behavior. On the other hand, regarding the structure and thermal motion of polymer chains in contact with a solid (interfacial chains), only the average behavior of multiple interfacial chains had been understood.

In this study, they used AFM to directly observe the motion of polystyrene polymer chains on a solid surface. This method simultaneously achieved high temporal resolution (0.326 seconds) and spatial resolution (approx. 0.4 nm in-plane, less than 0.1 nm in height), succeeding in the quantitative visualization of molecular motions that differ by position within a single polymer chain. Furthermore, by observing the same molecular chain under different temperature conditions (288 K and 308 K), they evaluated the temperature dependence of molecular motion at each position in detail.

As a result, it was found that within a single interfacial molecular chain, two different types of segments coexist: a thermally active segment, whose molecular motion is activated with increasing temperature; and a thermally suppressed segment, whose motion is suppressed by temporary adsorption to the solid surface. In addition, nonequilibrium behavior was observed at many sites, where adsorption and desorption were randomly repeated.

These findings differ significantly from the conventional understanding that interfacial molecular chains exhibit uniform mobility. The frequency and spatial distribution of these behaviors depend strongly on the interfacial interaction (the strength of the interaction between the polymer and the solid surface), showing that molecular motion changes significantly by altering the chemical properties of the interface.

This is a world first achievement in directly observing and quantitatively evaluating the thermal fluctuations of interfacial molecular chains in real space at the molecular level. It clearly demonstrates that molecular motion at the bonding interface essentially behaves as a nonequilibrium system.

The findings obtained in this study will advance the concept of molecular design in heteromaterial adhesives and provide the basis for a new adhesive technology that actively utilizes the structure and physical properties, including the unique nonequilibrium behavior of polymer chains, that exist at the adhesive interface.

By reflecting these findings in molecular design, it will be possible to shift from the conventional design premise of "interfacial polymers frozen in a spatially uniform and quasi-equilibrium state" to a new design concept that actively controls the "spatial distribution, nonequilibrium behavior, and frequency of motion within the interface." Using these findings, performance was improved by adding OH groups to the polystyrene.

Tanaka said, "It has been known empirically that the introduction of OH groups increases adhesive performance, but the molecular mechanism has been unclear. Since we are conducting joint research with many companies through the JST MIRAI Program, I believe this achievement will lead to the strengthening of Japan's industrial competitiveness."

Journal Information
Publication: JACS
Title: Direct Visualization of Segment-Like Dynamics in Isolated Polymer Chains on Solid Surfaces
DOI: 10.1021/jacs.5c23137