This graphic shows how the fluorescence of radicals generated from TASN allows polymer crystallization to be visualized. Image: Tokyo Tech.
This graphic shows how the fluorescence of radicals generated from TASN allows polymer crystallization to be visualized. Image: Tokyo Tech.

Due to their versatile properties, polymers are used for a variety of purposes. For example, polymers with high tensile strength and resistance can be used in construction, while polymers that are more lightweight and flexible can be used to manufacture plastic bags.

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

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

Guaranteeing optimal performance requires predicting how a given polymer will react to mechanical stress and to what degree crystallization contributes to this response. But 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 advances in polymer science, a research group led by Hideyuki Otsuka from Tokyo Institute of Technology in Japan has been working on a method to visualize polymer crystallization in real time. As the group reports in a paper in Nature Communications, this method is based on embedding reactive molecules called radical-type 'mechanophores' 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 probes for determining when and how stress is applied. In this case, to examine the mechanical forces at play during crystallization, the researchers used a radical-type mechanophore called tetraarylsuccinonitrile (TASN), which breaks down and emits fluorescence when subjected to mechanical stress.

The team had already used similar molecules to visualize and evaluate the degree of mechanical stress within a polymer material. In the current study, they used a similar method to observe the crystallization of a polymer.

As the crystals form, the mechanical forces cause the mechanophores in the polymer structure to dissociate into smaller, pink-colored radicals with a characteristic yellow fluorescence, allowing the team to observe the crystallization process. By measuring the emitted wavelengths of the fluorescence, the researchers are able to determine the exact rate of crystallization, as well as its extent and precise location within the polymer material.

"The direct visualization of polymer crystallization offers unprecedented insight into crystal growth processes," says Otsuka. This method could now allow manufacturers to test polymer materials for specific mechanical properties during crystallization. The researchers believe their study will permit the industrial optimization of polymer materials by controlling the crystallization process to obtain desired properties. Ultimately, Otsuka concludes, this could "lead to design guidelines for advanced polymer materials".

This story is adapted from material from Tokyo Institute of Technology, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.