How do you design a better polymer?
Polymers rule our lives, quite literally. DNA is a polymer. The long gooey strands of mozzarella cheese that stretch as you take a slice of pizza are a polymer, too. In its simplest terms, a polymer is the bonding of many molecules into long, often-strong chains. If you could stretch it out, a single molecule of DNA would measure 2 meters end-to-end.
Not all polymers come from nature, however. There are countless synthetic polymers that improve our lives, each concocted in laboratories for their many useful physical properties – vulcanized rubber, nylon, polyethylene and polystyrene are but a few of the better-known examples.
For more than 150 years, the race to discover or improve synthetic polymers has been largely one of trial and error. Kevlar, the bullet-stopping fiber, was a laboratory accident, for instance. Great fortunes have been made by opportune polymer discoveries. Many others have been lost in the pursuit of elusive physical traits. Despite the global tire company that bears his name to this day, the inventor of vulcanization, Charles Goodyear, died deeply in debt from years of failed attempts to perfect his product.
Solutions in search of problems
The polymer research process has always followed a similar pattern. Researchers would synthesize a new polymer in the lab and send it off for testing to determine its physical properties – melt temperature, elasticity, tensile strength and so forth. Only then would its creators look for suitable commercial applications. Sometimes the trials paid off; more often, the errors sent them back to the drawing board.
Someday soon, however, all that trial-and-error may go the way of Bakelite – a once-famous-now-obsolete polymer. Engineers, like Stanford assistant professor of chemical engineering, Jian Qin, are turning the process of designing polymers on its head using sophisticated computer applications that allow researchers to custom tailor molecules the way an architect might design a home to suit its new owner’s personal tastes.
“All polymers get their mechanical properties not from chemistry, but from the way that the individual molecules are entangled together,” says Qin, holding two long rubber bands in either hand, twisting them together and pulling them apart to illustrate his point. “We are creating complex computational models that help us understand what’s going on at the molecular level, so we can design better polymers, rather than search for them blindly.”
Take rubber, for instance. Its long individual molecular chains are “crumpled” in appearance, a shape that produces springy elasticity as the molecules stretch and then spring back to their original form. This molecular interaction gives basketballs their bounce and tires their springy resilience even in the heat of summer or cold of winter.
Though Qin has been at Stanford Engineering only a few months, already he is collaborating with a number of fellow researchers. He is, for instance, working with chemical engineer Zhenan Bao on new materials for her groundbreaking flexible computer displays and touch-sensitive, self-healing synthetic skins that might one day be used in robotics or prosthetics.
Sublime science
The sort of knowledge that Qin is imparting to the field is also profoundly important to the multibillion-dollar plastics industry, among others. The manufacture of many well-known products that make up our lives, like a polyethylene water bottle, for instance, requires a complex balance of interrelated molecular stresses and fluid dynamics. This is no easy feat. The maker must create a precise blend of molecules to ensure a uniform and properly formed finished product.
It is knotty stuff, computationally speaking. In fact, Qin and his collaborator Scott Milner at Penn State University use a mathematical principle known as knot invariant theory as the basis of their models of how the molecules are entangled and influence one another. The models were derived from an open-source polymer modeling package called Simpatico that was developed by David Morse at University of Minnesota and Qin. It is freely available for download and use by polymer researchers everywhere.
Qin says that the polymers he is studying display some very interesting properties in their own right. They behave like liquids and solids at the same time. On small scales, they flow like liquids, yet macroscopically, they can sustain shear stress. That is, they don’t break.
“Molecular entanglement creates a sort of fishnet in which the molecules are tangled and mutually restrain each other, giving the material strength,” Qin explains. “Using the knot invariants, we can now count the extent of the entanglement, not unlike the way that that early Incans counted provisions, tax obligations, even their own population using knotted threads known as quipus – sometimes called ‘talking knots.’”
This liquid-solid duality is both intriguing and perplexing to scientists and to plastics manufacturers alike. Liquids, in Qin’s words, are desirable because they can be shaped and molded easily, but they lack strength in their liquid form. Solids, on the other hand, are strong, but they do not flow and are, therefore, hard to shape into useful products.
To understand the processing properties of polymeric liquids – something known as “flow properties” – one must understand the structure and dynamics of the molecular “fishnets,” Qin says. He uses applied knot theory to uncover the complexity and the beauty in entanglement and, in the process, to reveal fundamental mechanisms that give a particular polymer mechanical strength.
“We can carefully tune this dual liquid-solid effect through molecular design, and this is a relatively new development in our field,” Qin explains. “These polymers are viscous and elastic at the same time, often wonderfully so. We can process them as liquids, use them as solids to create great new materials and new applications.”
