From the DNA in living things to the plastics you use, a great many things in this world are made of polymers—individual chemical units strung together to form larger molecules with diverse shapes and unique properties.
For thousands of years, nature was the sole source of polymers that were of industrial utility and commercial interest. Cellulose and lignin, the two most abundant natural polymers, were essential for building and burning, while the silk threads produced by worms clothed royalty and led to the establishment of global trade routes, forever changing the economic trajectory of the world.
But by the 1800s, human civilization was no longer satisfied with what nature could provide. Seeking to enhance the properties of natural polymers or imbue them with new and useful characteristics, scientists and engineers began to make modifications to the materials at hand. By applying sulfur and heat to natural rubber, for instance, American chemist Charles Goodyear produced a stronger and more rigid polymer known as vulcanized rubber.
Then a more radical idea took root—what if we could produce polymers that had never been found in the natural world? The biological systems that produced natural polymers were, in essence, living, breathing vats of chemical reactions, so it may be possible to replicate some of that chemistry independently of life. The seminal work of German chemist Hermann Staudinger proved that possibility beyond doubt and paved the way for a systematic understanding of how synthetic polymers might be manufactured. Since then, polymer science has flourished as a formal research discipline, setting off a ‘Cambrian explosion’ of new polymers for various applications.
Power to the polymers
Scientists at A*STAR are adding to the ever-expanding repertoire of polymers with their research. One team, led by Shuo-Wang Yang at the Institute of High Performance Computing (IHPC), has invented a polymer that can convert changes in temperature into electrical energy—what is known as a thermoelectric (TE) material.
TE materials work by responding to temperature differences, which induce electric charge carriers to flow from the hot to the cold side of the material. An effective TE material needs to have high electrical conductivity, low thermal conductivity and a high ‘Seebeck coefficient’—the voltage generated per degree of temperature difference across the material. However, it is rare for any one material to satisfy all of these conditions, meaning that existing TE materials are limited in efficiency.
“One way of improving TE performance is to use doping, adding certain chemicals to the material to enhance its electrical conductivity by increasing charge carrier concentrations,” Yang explained. “However, doping can also interfere with the materials’ stability and performance, hence finding a dopant that works effectively is challenging. Identifying TE materials that work without doping could transform energy harvesting.”
The team focused their attention on linear-backbone coordination polymers, structures containing metal ions linked by ligands, which can be built in the laboratory to specific designs. These polymers exhibit numerous advantages over conventional inorganic TE materials—they are flexible, have low thermal conductivity and are compatible with biological organisms. However, they have low electrical conductivity—a challenge that Yang and co-workers tried to overcome in their theoretical search.
“Based on first-principle molecular dynamics and structure optimization, we identified a polymer called poly(nickel-ethylenetetrathiolate) and three associated analogs which demonstrate intrinsically metallic behaviors and high electrical conductivity,” said Yang. “This is exciting as it suggests these polymers are potentially dopant-free TE materials.”
The team’s analyses suggested that the metallic behavior stems from the formation of dense, non-bonding molecular interactions between sulfur or selenium atoms within the polymeric structures. These interactions strengthen the forces between the atoms, decreasing electronic band gaps and encouraging the flow of electrical charge.
“Jianwei Xu, Kedar Hippalgaonkar and their teams at the A*STAR Institute of Materials Research and Engineering are now synthesizing these polymers,” Yang told A*STAR Research. “These materials are very promising, particularly in the applications of waste heat recovery and refrigeration near ambient temperature.”
A surprising solution
Although research into polymer properties and products is important, processes to fabricate polymers simply and at scale are just as essential. Often, polymer synthesis involves the use of organic solvents or harsh conditions such as high temperatures or vacuum. This is in contrast to polymerization in nature, which takes place under ambient and aerobic conditions.
But now, a team led by Satyasankar Jana at A*STAR’s Institute of Chemical and Engineering Sciences (ICES) has discovered a technique that allows them to grow polymer coatings made of zwitterions in water, at room temperature and in the presence of air. Zwitterions refer to molecules with both negative and positive charges, with a net charge of zero, that can assemble into long chains.
“It was a serendipitous discovery,” Jana quipped. His team had been attempting to grow zwitterionic polymer coatings using a popular synthesis method called atom transfer radical polymerization, when they realized some reactions were not yielding the expected products. An amine, acting as a ligand on the catalyst used in the reaction, was unexpectedly found attached to the end of the polymer chains. “It took some time and a series of experiments to unfold the mystery of how it got there,” Jana said.
Reaction kinetics observations, nuclear magnetic resonance spectroscopy and other analyses suggested that the amine kick-started the polymerization reaction via an anionic mechanism. These so-called anionic polymerizations are notoriously intolerant to water, methanol and air, but Jana’s polymers were growing in the presence of all three, making the team doubt their findings. They eventually relied on computer models to understand what was going on.
“Density functional theory calculation results confirmed the proposed anionic polymerization mechanism,” he said. “This is the first-ever example of an anionic solution polymerization of a vinyl monomer in aqueous media at ambient aerobic conditions.”
His team has now used this approach to synthesize polymer coatings from four zwitterionic monomers and some other anion initiators, some of which are not amines. “In the future, we will use this methodology to generate anti-biofouling polymer coatings on large surface areas using a spray or dipping method,” Jana noted, adding that such coatings could prevent harmful bacteria from attaching to medical devices, or inhibit mussels from adhering to ship hulls.
Delivering on a promise
Although many polymers are used outside the body, recent research has unveiled classes of polymers that are biocompatible, which means that they are non-toxic and do not trigger adverse reactions when infused into living organisms. Improved synthesis methods have also allowed researchers to precisely control the size, shape, stability and function of such polymers. Alexander van Herk from A*STAR’s ICES and Atsushi Goto from Nanyang Technological University are experts in this domain.
Together, they have created hollow polymer nanostructures that could serve as delivery systems for personal care products, drugs and chemicals used in agriculture. There are two steps in their synthesis method, the first of which involves the polymerization of methylacrylic acid in the presence of iodine to make alkyl iodides. The poly(methacrylic acid) (PMAA) then initiates the polymerization of methyl methacrylate and the resultant formation of a block co-polymer comprising both PMAA and poly(methyl methacrylate) (PMMA).
Sodium iodide is the catalyst in both stages. Since PMAA is hydrophilic and PMMA is hydrophobic, the co-polymer self-assembles into hollow nanostructures in polar solvents such as ethanol and water. By varying the polymer proportions, van Herk and Goto could determine the shape of the polymer nanostructures. The lowest ratio of PMMA to PMAA gave nanoparticles; boosting the PMMA content led to nanocylinders, then nanocapsules. The size and dimensions of the nanostructures could also be tweaked by varying the lengths of the two polymers.
The team also found that they could stabilize the nanostructures by including ethylene glycol dimethacrylate during the second polymerization step. “Crosslinking is important to ‘freeze’ the structure, to ensure it doesn’t change during further handling,” van Herk explained.
Going forward, the researchers plan to load their polymer nanostructures with active compounds such as vitamins and drugs, as well as test whether the nanostructures can deliver those compounds in a controlled manner. Their findings could pave the way for more effective treatments with fewer side effects.
Hence, like the individual chemical units that link up to give rise to polymers, new discoveries are constantly being added to the vast body of knowledge that is polymer science. With each new discovery, further possibilities and opportunities arise, waiting to be leveraged to improve lives and fuel economic growth.
The A*STAR-affiliated researchers contributing to this research are from the Institute of High Performance Computing (IHPC), the Institute of Materials Research and Engineering and the Institute of Chemical and Engineering Sciences (ICES).