Jun 17 2021
An Army-funded research team was able to quantify the kinetic and mechanical characteristics of individual molecules during polymerization reactions, for the first time. This breakthrough was achieved through a novel technique used for analyzing conjugated polymers.
A new approach to studying conjugated polymers made it possible for an Army-funded research team to measure, for the first time, the individual molecules’ mechanical and kinetic properties during the polymerization reaction. The insights gained could lead to more flexible and robust soft electronic materials, such as health monitors and soft robotics. Image Credit: Udit Chakraborty, Cornell University.
The findings may lead to more robust, flexible and soft electronic materials for applications, like soft robots and health monitors.
Conjugated polymers are actually groups of molecules linked along a backbone that can absorb light and conduct electrons. This makes them ideal for developing soft optoelectronics, like wearable electronic devices. While these polymers are highly flexible, they tend to aggregate and fall out of a solution, making them difficult to study in bulk.
Conjugated polymers are a fascinating class of materials due to their inherent optical and electronic properties which are dictated by their polymer structure. These materials are highly relevant to a number of applications of interest to Army and DoD including portable electronics, wearable devices, sensors, and optical communication systems.
Dr Dawanne Poree, Program Manager, U.S. Army Combat Capabilities Development Command, Army Research Laboratory
“To date, unfortunately, it has been difficult to develop conjugated polymers for targeted applications due to a lack of viable tools to study and correlate their structure-property relationships,” added Dr. Poree.
Equipped with Army funding, Cornell University researchers used a technique they pioneered on other artificial polymers, known as magnetic tweezers, that enabled them to expand and twist individual molecules of the conjugated polymer polyacetylene. The study was published in the Chem journal.
Through the use of novel single-molecule manipulation and imaging approaches, this work provided the first observations of single-chain behaviors in conjugated polymers which lays the foundation for the rational design and processing of these materials to enable widespread application.
Dr Dawanne Poree, Program Manager, U.S. Army Combat Capabilities Development Command, Army Research Laboratory
Previous attempts to improve the solubility of conjugated polymers mostly focused on chemical derivatization, which involves modifying the structures with atom-functional groups. However, this method can impact the inherent characteristics of the polymer.
The conjugated polymer is really a prototype. You always modify it to tailor it for applications. We are hoping everything we measured — the fundamental properties of synthesis kinetics, the mechanical property — become benchmark numbers for people to think about other polymers of the same category.
Dr Peng Chen, Peter J.W. Debye Professor, Chemistry and Chemical Biology, Cornell University
In 2017, Chen’s research team was the first to apply the magnetic tweezers measurement method to examine live polymerization, observing at the single-molecule level. This approach has previously been employed in the biophysics field to investigate proteins and DNA, but no one had succeeded in extending it to the world of synthetic polymers.
In this process, one end of a polymer strand is attached to a glass coverslip, while the other end is fixed to a small magnetic particle. The researchers then applied a magnetic field to exploit the conjugated polymer, expanding or twisting it, and quantifying the reaction of a single polymer chain as it grows. Since the amounts are so tiny, they remain soluble in solution in a way that bulk volumes would not.
The researchers measured the growth of long chains of conjugated polymers, containing hundreds of thousands of monomer units, in real time. They observed that these polymers grow at a relatively faster rate than their nonconjugated counterparts, adding a new monomer every second.
“We found that while growing in real time, this polymer forms conformational entanglements. All polymers we have studied form conformational entanglements, but for this conjugated polymer this conformational entanglement is looser, allowing it to grow faster,” added Chen.
When the researchers pulled and stretched individual conjugated polymers, a technique known as force-extension measurements, they were able to assess the rigidity of these conjugated polymers, and better understand how they bend in various directions while remaining conjugated and retaining electron conductivity.
The team also found that the polymers exhibited different mechanical behaviors from one individual chain to the next — behaviors that had been predicted theoretically but had never been visualized experimentally.
The results demonstrate the uniqueness of conjugated polymers in a variety of applications and also the power of utilizing a single-molecule manipulation and imaging approach on synthetic materials.
“Now we have a new way to study how these conjugated polymers are made chemically and what is the fundamental mechanical property of this type of material. We can study how these fundamental properties change when you start tailoring them for application purposes. Maybe you can make it more mechanically flexible and make the polymer longer or adjust the synthesis condition to either synthesize the polymer in a faster or slower way,” concluded Chen.
Journal Reference:
Baral, S., et al. (2021) Single-chain polymerization dynamics and conformational mechanics of conjugated polymers. Chem. doi.org/10.1016/j.chempr.2021.05.007.