Polymer Rheology, Dynamics, Modeling

The out-of-equilibrium problem

Polymers tend to be shaped like spaghetti and it is difficult to nudge them into a crystalline form. Areas of crystallinity are mixed up with areas of disorder and entropy. “What we currently understand is that the way soft materials structure themselves is based on equilibrium principles,” Gomez explains. However, he contends, the structure that is most advantageous for device performance is not always at equilibrium. Many functional soft materials, including polymer semiconductors, are out of equilibrium.

Because the laws of thermodynamics are more fully developed at equilibrium than out of equilibrium: “Suddenly, much of the machinery that Gibbs started working on before the 1900s is lost,” Gomez says. “We’re missing a lot of tools that have been used with great success in commodity polymers.” To solve the out-of-equilibrium problem, his group creates experiments in which they can tease out basic concepts of how structures evolve out of equilibrium. “It’s a challenging task, but that’s what makes our work interesting,” he remarks.

His students are breaking down various pieces of the structure puzzle in various ways: by measuring how long-range order in polymers contributes to charge transfer; by studying the crystallinity of semiconductor polymers using x-ray diffraction; by taking measurements of the interconnectivity between crystals using rheology; and by looking at dynamic processes using dielectrical spectroscopy. They use other advanced tools such as transmission electron microscopes and advanced techniques such as x-ray scattering at synchrotron sources like the one at Berkeley National Laboratory in order to look at the phase separation that occur in mixtures of materials that make up plastic solar cells. They study the dynamics of polymers in confined geometries using block copolymers with nanostructures on the order of 10 to 20 nanometers. What happens when you change the length scale? Do the polymers wiggle a lot or wiggle a little? The answer to such questions could have a bearing on how charges move within a polymer.

Gomez was born in Lima, Peru, and moved to Orlando when he was eleven. He earned a bachelor’s degree in chemical engineering from the University of Florida and a Ph.D., also in chemical engineering, from the University of California at Berkeley. He chose Berkeley, he says, based primarily on that school’s strong emphasis on basic science. He continues to do basic science research, but with the ultimate goal of contributing to making cheap, efficient solar power a reality. He was attracted to Penn State, he says, by the large concentration of good polymer scientists working in departments across the university on all aspects of the science. He collaborates with John Asbury in chemistry and Tom Jackson in electrical engineering.

He calls Jackson one of the “pillars of the field” of organic electronics. In future research, he hopes to collaborate with computational scientists to rationally design organic solar cells. In summary, Gomez asks “Can we improve polymer solar cells enough to be part of the energy portfolio of the nation? That to me is a question of basic science,” Gomez concludes.