Mechanophores: Unprecedented Insight Into Polymer Crystal Growth Processes

The fluorescence of radicals generated from tetraarylsuccinonitrile would enable visualization of the crystallization. Fluorescence microscopy observations of crystallization-induced mechanofluorescence allow the precise identification of the stress location and in-depth clarification of the polymer crystallization process. Credit: Tokyo Tech

In modern times, manufacturers produce highly specialized materials for a wide array of uses, called polymers. Polymers have a variety of purposes owing to their versatile properties, ranging from being used in construction due to their high tensile strength and resistance to manufacturing plastic bags that require more lightweight, flexible materials, such as nylon or polyethene.

These differences between the properties of different polymers stems from their internal structure. Polymers are made up of long chains of smaller sub-units, called “monomers.” Crystallization occurs when crystalline polymers are melted, then cooled down slowly, which enables the chains to organize themselves into neatly arranged plates.

Depending on the degree and location of crystallization, this process gives valuable properties to the polymers, including flexibility, heat conductivity, and strength. However, if not properly controlled, crystallization can also weaken the material, putting undue stress on the polymer chain. This is especially problematic when polymers are subjected to extreme conditions, such as freezing temperatures or intense pressure.

To guarantee optimal performance, we need to predict how a given polymer will react to mechanical stress and to what degree crystallization contributes to this response. However, scientists know very little about the intricate forces at play during crystallization, having never been able to observe them directly or measure them accurately without destroying the material first.

Based on recent advancements in polymer science, a research group, led by Professor Hideyuki Otsuka of Tokyo Tech, has been working on a method to visualize polymer crystallization in real time. In a recent study published in Nature Communications, they used highly reactive molecules, called radical-type “mechanophores,” embedded in the polymer structures. Radical-type mechanophores are sensitive to mechanical stress and easily break down into two equivalent radical species, which can act as a probe to know when and how stress is applied. In this case, to examine the mechanical forces at play during crystallization, they used a radical-type mechanophore called “TASN,” which breaks down and emits fluorescence when subjected to mechanical stress.

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