An artist's concept depicting block polymers packed together to form, in this case, a new phase of diblock co-polymer. Image: Brian Long.
An artist's concept depicting block polymers packed together to form, in this case, a new phase of diblock co-polymer. Image: Brian Long.

All matter consists of one or more phases – regions of space with uniform structure and physical properties. The common phases of H2O (solid, liquid and gas), also known as ice, water and steam, are well known. Similarly, though less familiar, polymeric materials can also form different solid or liquid phases that determine their properties and ultimate utility. This is especially true of block copolymers, the self-assembling macromolecules created when a polymer chain of one type (‘Block A’) is chemically connected with that of a different type (‘Block B’).

"If you want a block copolymer that has a certain property, you pick the right phase for a given application of interest," explained Chris Bates, an assistant professor of materials in the University of California Santa Barbara (UCSB) College of Engineering. "For the rubber in shoes, you want one phase; to make a membrane, you want a different one."

Only about five phases have been discovered in the simplest block copolymers. Finding a new phase is rare, but Bates and a team of other UCSB researchers have done just that. They report their findings in a paper in the Proceedings of the National Academy of Sciences.

About 12 months ago, Morgan Bates, staff scientist and assistant director for technology at the Dow Materials Institute at UCSB, was doing some experimental work on polymers she had synthesized in the lab. She did this in order "to understand the fundamental parameters that govern self-assembly of block copolymers by examining what happens when you tweak block chemistry."

According to Chris Bates, there are endless possibilities for the chemistry of ‘A’ and ‘B’ blocks. "Modern synthetic chemistry allows us to pick basically any type of A polymer and connect it with a different B block," he said. "Given this vast design space, the real challenge is figuring out the most crucial knobs to turn that control self-assembly."

Morgan Bates was trying to understand that relationship between chemistry and structure.

"I had chemically tweaked a parameter related to what is called ‘conformational asymmetry’, which describes how the two blocks fill space," she recalled of the process that led to the discovery. "We weren't necessarily trying to find a new phase but thought that maybe we'd uncover some new behavior. In this case, the A and B blocks that are covalently tied together fill space very differently, and that seems to be the underlying parameter that gives rise to some unique self-assembly."

After creating the block copolymers, she took them to the Advanced Photon Source at Argonne National Laboratory, where a technique called ‘small-angle X-ray scattering’ was used to characterize them. This process yields a two-dimensional signature of scattered X-rays arranged in concentric rings. The relative placement and intensity of the rings indicates a particular phase. Morgan needed to travel to Argonne because the technique requires X-rays more powerful than can be produced on the UCSB campus.

"Using knowledge of crystallography, you can interpret the scattering data and produce an image as if you were looking at the structure with your eye," explained Chris Bates. "And in this case, the data was of such high quality that we were able to do that unambiguously."

Morgan Bates recalled that when she examined the X-ray pattern, one thing was unmistakably clear: "It looked different. I thought, 'What is that?'"

It was, of course, their newly discovered phase, known as A15. "With these types of AB block copolymers, there are only a handful of phases that people have observed previously, and we've found another one, which adds to the palette of possible options from a design standpoint," Chris Bates said.

"Among the ways of categorizing structures, this phase belongs to a class known as ‘tetrahedrally close-packed’," added Joshua Lequieu, a postdoctoral researcher at UCSB and expert in computer simulations who modeled the phase behavior of the polymers. "The phase we've found in block copolymers was actually first observed in 1931 with an allotrope [or form] of tungsten. But in that case, A15 forms from metal atoms, which create a very small structure at the atomic length scale. Our block copolymers adopt the same structure but at a length scale two orders of magnitude larger, and, of course, no metal atoms are involved.

"If you were to look at both with a microscope, their structures would look the same, but just at different sizes. It's fascinating that nature chooses to use the same structural motifs for completely different materials having entirely unrelated chemistry and physics."

This story is adapted from material from the University of California Santa Barbara, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.