Episode 23: Frontal polymerization controls materials properties

In this podcast episode, MRS Bulletin’s Laura Leay interviews Nancy Sottos, the Maybelle Leland Swanlund Endowed Chair and head of the Department of Materials Science and Engineering at the University of Illinois–Urbana Champaign (UIUC), and Justine Paul, a former student at UIUC who now holds a position at DuPont, about their work with frontal polymerization. By mimicking patterns in biological materials such as shells, their research group took a multidisciplinary approach to control crystalline patterning, which ultimately enabled them to control mechanical properties of polymers. By applying heat, they made slight changes in the chemical reactions to achieve specific crystalline patterns. This work was published in a recent issue of Nature.

Transcript

LAURA LEAY: Welcome to MRS Bulletin’s Materials News Podcast, providing breakthrough news & interviews with researchers on hot topics in materials research. My name is Laura Leay. Materials research can draw inspiration from nature. But how do you approach this research when you’re working with complex phenomena and a system that is almost chaotic? The key is to use a multidisciplinary approach. Patterns in biological material such as shells result in enhanced materials properties compared to what is traditionally manufactured by human ingenuity. New research has shown that similar complex patterning can be produced by carefully controlling a process known as frontal polymerization. Professor Nancy Sottos from the University of Illinois explains.

NANCY SOTTOS: There are certain conditions – both chemical, where you change the formulation, or environmental, where you change the boundary conditions – and it can become chaotic. And so the key to this work is not letting it go completely chaotic. Really small changes in the chemistry and the formulation, even just one ligand change in the catalyst, change the patterning and the materials properties. The reason we can do that is we’re working on the edge of equilibrium.

LAURA LEAY: The exothermic reaction is initiated by adding a chemical catalyst and applying heat or UV light. Localized heat generated by the reaction in the liquid monomeric solution initiates polymerization. The reaction then self-propagates. Carefully controlling various parameters, including the ratio of the catalytic initiator to a phosphate inhibitor, results in a polymerization front that is nonuniform.  

JUSTINE PAUL: Essentially it just traversed in the x-plane and then down in the y and so you can think of it as moving a zigzag motion. Or in a test tube you can see it actually spin around, that’s just the heat diffusing to the cold spots.

LAURA LEAY: That was Dr. Justine Paul, who completed this work during her PhD at the University of Illinois, describing the spin mode of the polymerization front in two dimensions. This spin mode is driven by localized heating that arises from the exothermic polymerization reaction which leads to hot and cold zones which is what gives rise to nonuniform crystallization. Both chemistry and the surrounding environment need to be considered to initiate this near-chaotic crystallization process. 

NANCY SOTTOS: Spin modes come because we pull heat out of the system either through chemistry or the environment. There’s this in-between zone where the instabilities occur. If you have lots of heat, it’s hard to make it go unstable like this but there’s a window. We were guided by computations to find this window and in there you’re generating just enough heat to move forward but there’s constant heat being lost to the surroundings and that’s what creates these beautiful modes of propagation.

LAURA LEAY: Numerical modeling work was instrumental in identifying the transition from a uniform polymerization front to a chaotic or unstable one. Coupled reaction-diffusion modeling showed that different chemical catalysts have different effects, and that it must be carefully balanced by the amount of inhibitor; small changes in the chemistry had a big effect on the stability of the polymerization front. The analysis of the kinetics was extremely helpful to understand the influence of chemistry, helping to pinpoint which polymers will produce patterning. If the polymer chains are aligned to the direction in which a load will be applied, the result is higher stiffness and higher strength compared to when the polymer chains are perpendicular. The domain size also has a role; as the domain size increases, the material is more amorphous and the toughness increases. Although patterning has been observed before, this is the first time that is has been precisely controlled in a way that enables the materials properties to be controlled. This controlled near-chaotic polymerization process is a step change in materials fabrication. Usually, to produce patterns an elaborate 3D printing regime would be developed. Controlling the chemistry and thermodynamics leads to a less deterministic, or more natural self-assembly mode on a large scale.

NANCY SOTTOS: There’s obviously a lot of work on self-assembly and how things assemble, but that’s on a very small length scale. These are kind of big, engineering length scales at which these assemble and create these patterns.

LAURA LEAY: The technique can be applied to other systems and could give rise to a new mode of 3D printing where some regions are patterned, applying hierarchical control to the structure in order to tailor the mechanical properties of a material. The numerical modeling and tuning of the chemistry are significant achievements. The results show what is possible through collaboration, as Dr. Paul explains.

JUSTINE PAUL: This work wouldn’t have been possible without a lot of the people who have contributed and collaborated on it. There was a lot of viewpoints and expertise that really allowed this paper to come to its fruition as well as those bigger structure–property relationships. One, first understanding the chemistry space and how that imparts these patterns but then, I guess first going back, how that was guided by numerics. But then really deep-diving in to understanding the polymeric microstructures as well as how those polymer chain orientations influence mechanical properties. That was really driven a lot by collaborations. […] Having different viewpoints on this and expertise allowed for those links to be made a lot faster than we otherwise probably would have.

LAURA LEAY: This work was published in a recent issue of Nature. My name is Laura Leay from the Materials Research Society. For more news, log onto the MRS Bulletin website at mrsbulletin.org and follow us on X, @MRSBulletin. Don’t miss the next episode of MRS Bulletin Materials News – subscribe now. Thank you for listening.