Manufacturers manufacture highly advanced products for a wide variety of applications called polymers in today’s world. Thanks to their versatile properties, polymers have a number of applications, ranging from construction use due to their high tensile strength and toughness to the manufacture of plastic bags that need lighter, more flexible materials such as nylon or polyethylene.
Their internal structure results in these variations between the properties of different polymers. Long chains of smaller subunits called “monomers.” are made up of polymers.
As crystalline polymers are melted and then slowly cooled, crystallization occurs, allowing the chains to organize into neatly ordered sheets.
This process imparts valuable properties to polymers depending on the degree and position of crystallization, including flexibility, thermal conductivity and strength. Crystallization, however, may also weaken the material, if not properly managed, and place undue stress on the polymer chain.
When polymers are subjected to extreme conditions, such as freezing temperatures or severe pressure, this is extremely troublesome.
We need to predict how a given polymer will respond to mechanical stress to ensure optimal efficiency, and the degree to which crystallization contributes to that response. Scientists, however, know very little about the dynamic forces at play during crystallization because without first destroying the substance, it has not been possible to directly observe or accurately measure them.
A research group led by Professor Hideyuki Otsuka of Tokyo Tech has been working on a method to imagine polymer crystallization in real time on the basis of recent advances in polymer science.
They used highly reactive molecules called “radical mechanophores” embedded in the polymer structures in a recent study published in Nature Communications. Radical mechanophores are susceptible to mechanical stress and are easily divided into two radical equivalent species that can act as a probe to decide when and how stress occurs.
In this case, they used a radical mechanophore called “TASN,” which decays and emits fluorescence upon mechanical strain, to study mechanical forces during crystallization.
The team had previously used similar molecules and proved that they could be used inside a polymer material to visualize and determine the degree of mechanical strain.
They used a similar approach in the current study to observe a polymer’s crystallization.
The mechanical forces caused the mechanophores in their configuration to dissociate into smaller, pink radicals with a characteristic yellow fluorescence as the crystals formed, enabling the researchers to observe the process directly.
Thanks to the high visibility of the fluorescence, the researchers were able to quantify the emitted fluorescence wavelengths to determine the exact crystallization rate, as well as its degree and position inside the polymer material, also in three dimensions.
“Direct visualization of polymer crystallization provides unprecedented insight into crystal growth processes.”Direct visualization of polymer crystallization provides unprecedented insight into processes of crystal growth.
Indeed, during crystallization, this process enables manufacturers to test polymer materials for particular mechanical properties.
The researchers believe their study will enable industrial optimization of polymer materials to obtain desired properties by controlling the process of crystallization. Ultimately, Prof. Otsuka concludes, this could “lead to design guidelines for advanced polymer materials.”
Reference: 5 January 2021, Communications in Nature.