An estimated 57 million tons of plastic trash ends up in the environment each year—enough to bury New York's Central Park in a mountain of plastic waste as high as the Empire State Building. The refuse can be found almost anywhere you look—bobbing in rivers and oceans, snagged in the branches of trees, and even absorbed in the organs of our bodies.
The main reason plastics are so omnipresent is that most are nearly impossible to recycle. Wenlin Zhang wants to change that.
"I wish everyone could think about polymers differently," says the assistant professor in the Department of Chemistry. "They're not all bad. Because they're lightweight, they're easier to transport and produce fewer carbon emissions. But we do need to recycle them better and address our growing plastic waste problem."
Zhang studies polyethylene and polypropylene, the two most popular consumer plastics on the planet, at a time when record amounts of single-use plastic are piling up in landfills, littering the landscape, and winding up in the air we breathe when incinerated. The United Nations projects that plastics production will triple in three decades, from 400 million metric tons today to 1.2 million metric tons by 2050.
In November, Zhang received a prestigious National Science Foundation CAREER grant to support his research on ways to enhance the properties of recycled plastics. The five-year grants are the NSF's most prestigious award in support of early-career faculty.
Polyethylene gives plastic bags their gauzy, translucent quality while polypropylene is what makes yogurt containers rigid yet flexible. Both polymer types are made of simple chains of carbon and hydrogen that, depending on their molecular arrangement, impart varying levels of crystallinity, which influence how strong and stiff they are, among other physical properties.
Most polymers are semi-crystalline, meaning some chains are tightly packed and more crystalline, while others are more randomly arranged and amorphous. The challenge in recycling them comes when you try to melt a semi-crystalline polymer blend.
"Like mixing oil and water together, you get phase separation," says Zhang. "The phase separation creates interfaces that degrade the material properties of recycled plastics. This is why businesses prefer to use cheaper virgin plastics."
Zhang wants to better understand phase separation, and the crystallization process that gives polymers their unique structure and properties, to learn how recycled plastics can be improved. If phase separation is inevitable, he says, it may be possible to strengthen the interface to produce higher-quality recycled plastic.
As a computational chemist, Zhang runs most of his experiments on the computer, including at Dartmouth's research computing center. The work involves coming up with mathematical theories and running computer simulations to validate them. Two of the three students in his lab are currently testing how adding different molecules to phase-separated polymer blends can improve their interfacial properties, and thus, improving their quality.
Zhang arrived at Dartmouth during the pandemic, at a time when Wuhan, the once-obscure city where he was born and raised, became known throughout the world. He left China after high school to study at the University of Minnesota and went on to Penn State for a PhD in chemical engineering.
He was unsure of whether he wanted to be a scientist until a required class in statistical mechanics introduced him to polymer physics. "We learned that plastic is simply a long chain of molecules that can be contorted in all kinds of ways" he says. "This gives rise to the many properties of polymers, which can be described very well mathematically."
"The first few weeks were torture," he remembers. "But at some point, we started to write our own code to simulate molecular motions in melts and solutions such as chemical bonds vibrating and twisting as molecules try to escape from each other and phase separate. I saw how powerful statistical mechanics theory could be in allowing us to predict what was happening in the simulations."
Zhang came to Dartmouth after a postdoc fellowship at the University of Michigan, drawn in part by Dartmouth's outsized reputation in polymer science. Walter Stockmayer, a pioneer in the field who co-founded Macromolecules, one of the top journals in polymer science, taught and mentored students at Dartmouth for decades. Associate Dean for the Sciences Jane Lipson, Albert W. Smith Professor of Chemistry, is also a renowned polymer theoretician and served as an editor of the same journal.
"Dartmouth students and faculty alike are fortunate to benefit from Wenlin's innovative research and dedicated mentorship," Lipson says. "He couldn't be more deserving of this prestigious funding from the NSF, and I am excited to see how his research in the coming years moves us towards a more sustainable world."
Though the work that NSF will fund in Zhang's lab focuses heavily on theory, the project will also have a community outreach component. In collaboration with the DALI lab, Zhang and his lab will work with students to create a phone app that can scan plastic containers and other waste to learn about their chemical makeup and recycling potential.
"I want students to learn more about the chemistry and physics of polymers," he says. "They are useful, but very few currently get recycled. We need to work together to improve recycling strategies."
