Professor Giovanni Constantini from the University of Birmingham explains
A new scientific study by the University of Birmingham has uncovered fresh insights into how plastic materials used in electronics are formed, and how hidden flaws in their structure could be limiting performance
Conjugated polymers are a type of plastic that conduct electricity, often used in applications such as optoelectronics, computing, biosensors and power generation. Thanks to their lightweight and low-cost properties, coupled with their ability to be printed in thin layers onto flexible substrates, conjugated polymers are well-suited for use in next-generation technologies. During the study, the University of Birmingham team investigated a popular method for creating such polymers – aldol condensation – which is praised for being versatile, metal-free, environmentally friendly and scalable. Published in the Nature Communications journal, the study’s results reveal that this method of synthesis introduces structural defects that could affect how well the polymer conducts electricity or converts heat into electrical energy in thermoelectric devices.
“The aldol condensation process can create defects in the polymer sequences, like missteps in a molecular dance, which can disrupt the flow of electrons through the material, reducing efficiency and reliability in devices,” explains lead author professor Giovanni Constantini. “Our findings could have wide-reaching implications for the development of high-performance, flexible, low-cost electronics and help reduce reliance on rare or toxic metals in manufacturing.”
ANALYTICAL LIMITATIONS
According to the study, such defects, along with the secondary reaction pathways that produce them, have not been previously considered because conventional analytical techniques are unable to detect them.
“Much of the success of modern synthetic chemistry is due to the availability of well-established and highly reliable analytical techniques such as mass spectrometry (MS), chromatography, X-ray diffraction (XRD) and what is often considered the ‘queen’ of chemical analysis, nuclear magnetic resonance (NMR),” says Constantini. However, for conjugated polymers, virtually all of these techniques are severely limited.
“The main challenge lies in the intrinsic tendency of conjugated polymers to aggregate,” he continues. “This aggregation (also one of the reasons why they perform so well in solid-state thin films – the basis for devices such as organic field-effect transistors and organic photovoltaics) leads to significant analytical complications. For instance, instead of producing sharp, well-defined NMR peaks that allow clear assignment of chemical species, moieties and their environments, conjugated polymers often yield broad, uninformative signals. We jokingly refer to them as ‘potatoes’ because they cannot be assigned to specific chemical environments, essentially nullifying the analytical power of NMR in this context.”
According to Constantini, MS is equally problematic. “Conjugated polymers are inherently polydisperse in length – and therefore in mass – making it essential to determine their molecular weight distribution. Yet MS tends to overrepresent shorter chains (oligomers) resulting in unreliable distributions.”
Chromatography – typically in the form of gel permeation chromatography (GPC) – also falls short. “It requires calibration using standards (most often polystyrene) which has very different hydrodynamic properties from conjugated polymers,” he explains. “As a result, the mass values obtained are known to be inaccurate. Aggregation in solution can further distort the results.
Constantini adds, “X-ray diffraction, finally, is inherently sensitive to crystalline order. Since most conjugated polymers are only semi-crystalline at best, XRD only captures a partial and often unrepresentative view of the material.”
THE RISE OF STM AND ESD
To overcome these limitations, the researchers used a powerful imaging technique called scanning tunnelling microscopy (STM), combined with electrospray deposition (ESD) which allowed them to examine polymers at the molecular level.
“The combination of ESD and STM, which we have developed over the past few years, allows for the characterisation of conjugated polymers at the single-chain level,” Constantini says. “This method provides sufficient resolution to determine, in many cases, the precise monomer sequence of individual polymer chains with sub-repeat-unit accuracy. With this powerful technique, characterising a sample of conjugated polymers becomes conceptually straightforward and intuitive: One can directly inspect the TM images to determine sequence, chain length distribution and assembly behaviour – all through direct visual analysis.”
In practice, however, carrying out these experiments is technically demanding, Constantini acknowledges: “We are currently one of the few groups worldwide with both the necessary equipment and, crucially, the expertise and experience to perform them reliably. The same applies to the analysis of the resulting data, which is often equally complex.
That said, we are actively working to automate various aspects of both image acquisition and analysis, with the long-term goal of transforming what is currently still a rather ‘exotic’ and technically intensive method into a more mainstream analytical technique. It is a highly challenging and ambitious objective, but one we are pursuing rigorously, particularly through recent collaborations with computer science colleagues to integrate machine learning approaches into the analytical workflow.”
ALDOL CONDENSATION DEFECTS
The team studied four different polymers made using aldol condensation, discovering two main types of defects. The first, coupling defects, are like kinks or bends in the polymer chain caused by the building blocks connecting in the wrong orientation or position. The second, sequence defects, occur when the order of the building blocks is incorrect, like having two of the same blocks in a row when they’re supposed to alternate.
However, by adjusting the chemical design and purifying the building blocks before polymerisation, the researchers were able to significantly reduce the number of defects. One approach involved using aldol condensation to create small, well-defined molecules, which were then linked using a different method – producing much cleaner polymer chains.
“By comparing the four conjugate polymers studied in this work, we found that the final polymer we analysed exhibited fewer sequence defects,” Constantini explains. “This was also the only polymer for which the monomers were synthesised via aldol condensation, but the polymer itself was formed using a different coupling strategy, Stille polymerisation. As a result, chromatographic purification methods could be applied to the monomers, which is effective for small molecules but much less so for macromolecules. These purification steps evidently removed a significant portion of defective monomers, thereby reducing the overall defect content – specifically the types of defects introduced during aldol condensation – in the final polymer.”
However, this alternative approach introduced a new class of defects associated with the Stille polymerisation mechanism, Constantini admits, particularly homocoupling defects. These were not observed in the other polymers, which had been synthesised directly via aldol condensation.
“There are indeed strategies to minimise or avoid homocoupling defects in Stille polymerisation, and we have been part of an international team led by professor Wouter Maes at Hasselt University in Belgium that has explored such approached,” Constantini continues. “Unfortunately, these methods are synthetically demanding and not always practical. Moreover, despite its widespread use and reputation as a ‘gold standard’ for conjugated polymer synthesis, Stille polymerisation is far from ideal: It involves multiple synthetic steps and requires organostannane precursors, which are highly toxic and environmentally harmful.”
NEXT STEPS
The team’s immediate goal is to further develop the ESD-STM methodology and apply it to obtain statistically representative sequences of entire populations of conjugated polymers. “This would provide a type of information that has never been available before: A detailed, information-rich, monomer-resolved characterisation of polymer samples at the single-chain level,” Constantini explains. “Such data could offer unprecedented insights into the underlying synthetic processes that generated these samples.”
However, there still remains a large gap between synthesis and meaningful verification of the results: “In most cases, synthetic outcomes are judged solely based on the performance of a device fabricated using the polymer in question. Although, device performance is influenced by numerous factors unrelated to the molecular structure of the polymer batch. As a result, the actual characterisation of the material remains minimal, and progress in the development of conjugated polymers often relies on trial and error rather than rational design.”
The approach taken by Constantini and his team aims to change this paradigm. “By using ESD-TM to acquire highly resolved structural data – and integrating this with quantum mechanical calculations and kinetic modelling – we aim to establish a feedback loop from characterisation back to synthesis,” he says. “This would, for the first time, enable synthetic chemists to refine their approaches based on detailed, sequence-level information about the resulting materials, thereby enabling a true rational design approach.”
