Revealing the real picture of soft, self-assembled crystals
AMHERST, Mass. – In Nature, says Greg Grason at the University of Massachusetts Amherst, “Nobody except physics and geometry” tells molecules how to organize into the types of complex soft crystals that form in butterfly wings, for example, allowing them to selectively reflect different wavelengths of light. He and other materials scientists who study such self-assembling structures are often fascinated and inspired by them, he adds, “and we’re starting to learn how and why they are spontaneously form way they do.”
The double gyroid crystal is a lattice network of interconnected tubular struts like the ones that make up butterfly wings, explains Grason, a theoretical polymer scientist. Their colors, which differ among butterfly species, arise from changes in network spacing in the gyroid crystal, which alters the way light reflects off of them.
He adds, “There’s a sense that if we understood the recipe better, we could make our own butterfly wing and replicate its special qualities. Particularly now when synthetic chemistry is so advanced, it can modify polymer building blocks at will and with high precision. If we knew how to direct spontaneous assembly processes, we might be able to, say, make a material to spray on a window to selectively control how light comes in. That’s the sort of thing we want to do, and this new work is a step along the way.”
Grason, an expert in self-assembling complex geometric patterns, is part of a team with others in Taiwan and Greece, plus Ned Thomas at Rice University. Thomas is widely known for his expertise in analyzing experimental shapes of polymer nanostructures and for discovering the double gyroid structure in polymers while at UMass Amherst.
As they describe in an online paper in Nature this week, the team learned that these soft crystals break symmetry “in a particular way that Nature has a reason for adopting,” Grason adds. “We learned by analyzing the strut lengths and angles over a huge array that they are distorted far from the expected cubic pattern. We think it’s because the crystal grains are growing toward each other from different directions. They are born at different times, so when they meet, instead of clashing right at the boundary between grains like atomic crystals, they relax and kind of go with the flow. This skews them, even far away from where the grains meet. We don’t know how exactly, but stress or tension gets distributed over the entire grain.”
He notes, “It may be a new reality, that the way soft crystals grow is different than hard crystals and different than what we expected. Bonds in hard crystals really want to keep the lengths the same. Soft crystals adjust more easily; they don’t mind stretching, but they really don’t want to change their angles. Because we couldn’t see quantitative details over large distances, we couldn’t observe this before now.” Further, “We think different qualities get preserved in different crystal types. We’re not sure how many have this distortion; it’s not all of them, but we think it’s the rule, not the exception.”
He and colleagues used a new type of electron microscopy to observe naturally self-assembling double gyroid block copolymers at a level of detail never before possible, revealing some unexpected findings. The work was supported by the U.S. Department of Energy, part of project between Grason, Thomas and Bryan Coughlin, a synthetic polymer chemist in UMass Amherst’s Polymer Science and Engineering Department.
The authors describe how slice-and-view scanning electron microscopy (SVSEM) can accurately characterize soft crystal “without loss of 3D structural information” of traditional methods. With SVSEM, they see these crystals in a way not possible before, Grason says, “and more importantly, the way they look is not what we expected.”
Most common crystals in Nature are symmetric and regularly spaced arrangements of atoms or small molecules packed into a lattice where the angles and the distances are uniform, he points out. If one knows the lattice’s distances and angles and the location of a single element, one can say where the others should be.
By contrast, the soft crystals they studied are far bigger than atomic crystals, “but still so small that you can’t see them with light microscopy,” Grason notes. They investigated larger polymer molecule assemblies, thousands of atoms chained together, which spontaneously form groups dubbed “mesoatoms,” the basic repeating unit of the soft crystal. In the double gyroid, the molecular tubes form junctions of three struts, he says.
“We had theories saying that the favored network should want to be cubic, with all strut lengths and angles between them the same,” Grason explains. “There is a prevailing assumption that Nature favors symmetry, and you can show with theoretical models that the best states should be symmetric. But the tools we had before over-simplified what was actually happening. The picture turns out to be more complex than we expected when crystals are composed of large numbers of flexible molecules.”
Combining beams from top and side, SVSEM takes a high-resolution 2D surface image. It then slices a few nanometers of the sample at a time. Merging “stacks” of 2D images, one gets a 3D image that has far fewer errors than previous methods, he says. “Now, not only can we see what’s going on in one subunit of the structure, we can use computers to add up thousands of copies to get a higher fidelity image without any missing information,” he notes. “Previous reconstruction algorithms just didn’t have enough information to get this level of detail.”
Grason says that while his team may be among the earliest adopters of SVSEM, materials scientists will be using it more now for other types of soft, nanostructured materials. “This is only the beginning,” he notes. “The techniques existed but there were not a lot of microscopes like this. We think it will become more routine now.”