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Winter 2020

Plastic: Friend or Foe

Plastics have been villainized as the enemy of the planet, but is that the whole story?

Malgorzata (Gosia) Chwatko

In recent news headlines, plastic materials—especially single-use items—have been declared an enemy of the planet. Images of plastic waste covering our beaches or hurting animals are commonly showcased. In response to these stories, politicians are arguing for bans on various plastic items, such as bags or straws. With so much bad press, it is easy to express our outrage and wish for a plastic ban or a world with less plastic, but it is harder to remember why we even started using plastic materials and how much plastic has significantly improved our world.

Let’s think back to the 1950s. Back then, most items were made from wood or metal, making production times longer and transportation expensive due to their weight. Mass production of plastic has promised the global population lighter, less expensive goods. This sparked a revolution to use plastics as a core material in manufacturing of toys, containers, and much more.

Chemical structure of a polymer called polypropylene. Polypropylene is used as material in duffle bags or plastic films around clothing. This structure shows similarities to metal chains due to the repeating units highlighted in blue.

Plastics, however, are only a subset of materials called polymers which make up a large class of materials that can come from either synthetic or natural sources. For example, trees contain a polymer called cellulose, which is a major component of paper. The term polymer describes molecules that have a repeating substructure, commonly described as a molecular chain, made up of many links of a repeating molecular structure. The amazing benefit of these polymers is their versatility in properties from tough durable materials like Kevlar®, which can stop bullets, to lightweight Nylon, which is commonly used as a clothing material. The vast diversity of the properties of these materials is attributed to the underlying polymer chemistry.

Just like water can undergo phase changes from gas to liquid to solid, polymers can change their physical characteristics. Most polymers exhibit a transition from a rigid, glassy state to a liquid-like state at a certain temperature. The glass transition temperature explains why Styrofoam cups, which are made of a polystyrene, are not recommended for holding warm water, as around 100 degrees Celsius the polymer turns from the glassy, sturdy structure and begins to flow. With enough time at this high temperature, the polymer can dissolve into that warm beverage you are consuming, or the cup may eventually mechanically fail and spill the contents. Depending on the specific chemistry of different polymeric materials, they can also melt and crystallize, just like water. These types of polymers are commonly referred to as thermoplastics. Thermoplastics can be easily recycled, since at the end of a product’s life, it can be reheated past this melting temperature and be reshaped.

On the other hand, thermoset polymer materials can only be set once; after the polymer is heated and shaped, upon cooling it will maintain that shape. No amount of heating will ever melt it, only burn or decompose it. These materials commonly undergo a chemical transformation during this initial melting, where polymeric chains connect, forming a nearly indestructible network.

Due to these properties and others, such as strength or flexibility, plastics and rubbers not only replaced materials like wood and glass, but they continuously drive innovation even now. The U.S. Department of Energy's Office of Science supports 46 Energy Frontier Research Centers, of which approximately a third use polymers in order to better understand and transform ideas related to energy.  Examples of polymer-related innovation include work done by the Center for Materials for Water and Energy Systems (M-WET) and the Advanced Materials for Energy-Water Systems (AMEWS). These centers use polymers in the development of materials that come into contact with or transport water.

As an example, water placed on a coffee filter moves through small channels called pores to the other side, while any larger items such as coffee grounds are left behind. Studying how water moves in pores of different shapes and sizes can teach us fundamental knowledge, on which we can eventually build new technology. Manipulation of pore shapes can only be done using a well-controlled system to achieve this goal. M-WET is using polymers which allows researchers to develop different porous architectures with relative ease due to the thermodynamic properties of their specific polymeric materials. This and other research will impact the development of future technologies to purify water that not only ties into water security, but also into energy. One major use of water in the industrial sector is in thermoelectric power plants that boil water to produce steam for creating electricity. Another example of energy-related research is found at the Center for Bio-Inspired Energy Science (CBES), where investigators are using processes found in nature—such as contraction of muscles—to learn how to apply similar principles to soft materials such as polymers to generate and store energy.

Degradation of polymers is possible through the design of special chemical bonds. These bonds can break and degrade the polymer.

Without synthetic polymers, the world would not be able to tackle the next set of challenges facing us, such as switching to more renewable energy sources. Renewable energy sources, such as wind or solar, only produce energy when it is windy or sunny, meaning that storing energy during these times will be required to supplement the non-producing time. Polymers play a key role in the design of new batteries and fuel cells that will better match energy storage needs of renewable technologies. 

The Center for Alkaline-Based Energy Solutions (CABES), Nanostructures for Electrical Energy Storage (NEES), Center for Mesoscale Transport Properties (m2M/t), and Center for Synthetic Control Across Length-scales for Advancing Rechargeables (SCALAR) use polymers in their experimental designs. As an example, one aim of SCALAR is to improve adhesives or glues used in battery manufacturing. Polymers in general make excellent adhesives due to their ease of processing and tunability to change adhesion and cohesion forces. In this case, researchers are improving ion and electrical conduction of their polymeric glues, which is an important part of the electrode, in order to improve performance.  

 Polymer’s positive influence however does not overshadow the negatives of plastics and rubbers. These materials have certainly added a set of challenges we are only beginning to deal with, such as widespread microplastics found in many humans or oceans contaminated with plastic waste. Scientists are already working on proposed solutions, including making polymers from renewable sources or switching to more easily degradable polymers. Some examples of these efforts include using microorganisms like bacteria to produce plastics, or using plants, like corn, to produce degradable polymers.

Supporting this effort, the next round of investment in EFRCs in the coming year includes polymer upcycling as a topical area. This involves converting waste plastics into fuels and other high-value products. Thus, with continuous research on the degradable polymers or upcycling of our single-use and multi-use materials, we can continue to use plastics to innovate while being mindful of our impact on the world.

About the author(s):

  • Malgorzata (Gosia) Chwatko is a postdoctoral fellow at the University of Texas at Austin. She is a member of the Center for Materials for Water and Energy Systems (M-WET) Energy Frontier Research Center. Her research focuses on the development of new polymeric materials for applications such as water purification and energy storage. 

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.